PHYSICS NEWS BULLETIN OF THE INDIAN PHYSICS ASSOCIATION July– September 2020 Vol. 50 No. 3 ISSN: 0253 – 7583 www.tifr.res.in/~ipa1970

Confocal and Gravitational multi-photon Wave Interferometry microscopy

Adaptive Extreme optics intensity light

Extreme UV Bose-Einstein Lithography Condensation

Celebrating 60 years of the laser INDIAN PHYSICS ASSOCIATION was founded in 1970 with the following aims and objectives: a) to help the advancement, dissemination and application of the knowledge of physics b) to promote active interaction among all persons, bodies, institutions (private and/or state owned) and industries interested achieving the advancement, dissemination and application of the knowledge of physics c) to disseminate information in the field of physics by publication of bulletins, reports, newsletters, journals incorporating research and teaching ideas, reviews, new developments, announcements regarding meetings, seminars, etc., and also by arranging special programmes for students or establishing student cadres d) to arrange seminars, lectures, debates, panel discussions, conferences and film shows on current research topics and other topics of national and local interest pertaining to research and teaching in physics e) to undertake and execute all other acts as mentioned in the constitution of IPA

President Members Dr. A. K. Mohanty [email protected] Dr. D. Kanjilal [email protected] Vice President Dr. S. M. Yusuf Dr. N. K. Sahoo [email protected] [email protected] General Secretary Dr. Bivash R. Behera Prof. Vandana Nanal [email protected] [email protected] Dr. Sudhakar Panda Joint Secretary [email protected] Dr. Pawan Kumar Kulriya [email protected] Prof. Srubabati Goswami [email protected] Treasurer Dr. D. V. Udupa Prof. Vijay Singh [email protected] [email protected]

Room No. 4, PRIP Shed Room No. 103 B, NIUS Building Bhabha Atomic Research Centre Homi Bhabha Centre for Science Education Trombay V. N. Purav Marg, Mankhurd Mumbai – 400 085 Mumbai – 400 088 Tel.: +91 22 25505138 / 25595369 Tel: +91 22 25072531

E-mail : [email protected] Website : www.tifr.res.in/~ipa1970

PHYSICS NEWS Vol. 50 No. 3 July – September 2020

Contents Editorial 2 From the President’s Desk 3 Articles

Sixty Years of Lasers 4 R. Vijaya Multiphoton Excited Fluorescence Lights Up Neurobiology 11 Sudipta Maiti Staying neutral in an intense plasma 15 M. Krishnamurthy Nonlinear Optics: Looking back – looking forward! 21 Kailash Rustagi Raman Spectroscopy – A Potential Tool for Biomedical Diagnosis 25 Rashmi Shrivastava and Shovan Kumar Majumdar Understanding Features of Weakly Bound Nuclei 31 Vivek Vijay Parkar On the method of Effective Field Theory 37 Joydeep Chakrabortty News & Events Vigyan Vidushi 2020 42 Book Review C. V. Raman and the Press: Science Reporting and Image building 45 Meet the Physicists 47 Backscatter The charge of the mask brigade 48

The opinions expressed in the articles in this issue are those of the authors and do not necessarily reflect the opinion of the Physics News or IPA

Image Credits: Front cover - collage showing key areas impacted by lasers: images courtesy Sudipta Maiti (TIFR); Laurie Hatch, Lick Observatory, spie.org, asml.com and wikimedia.org. Back cover - composite of images showing the recently achieved Bose-Einstein Condensate at the International Space Station, used with permission (Robert J. Thompson, JPL/Caltech, www.nasa.gov/feature/jpl/the-coolest-experiment-in-the-universe/) Physics News

PHYSICS NEWS (ISSN : 0253-7583) July – September 2020

Vol. 50 No. 3 Editorial

[email protected] The unabated spread of the COVID-19 pandemic affects us all in many ways, and compels this issue of Physics News to once again be an online-only edition. In the EDITORS midst of the gloom of the pandemic, one bright aspect is the celebration of 60 years Arnab Bhattacharya of the laser. Locally, the IPA turns 50, which will also be marked with a series of online lectures over the course of the next few months. Vandana Nanal Today, the laser is no longer “a solution looking for a problem”, but most critically Aradhana Shrivastava empowers our everyday lives. We’re of course aware of the lasers we see as we

scan barcodes while shopping, and also the unseen tiny lasers providing the blips CONSULTING EDITORS of light that power the internet. However the impact of lasers on science and in Dipan K. Ghosh enabling the technologies of the future has been phenomenal. This issue focuses on a few of areas where the power of light has been a game changer. R. Vijaya reviews S. Kailas the key developments of the past 60 years, and discusses fiber lasers and photonic crystal lasers. Sudipta Maiti discusses how multi-photon microscopy has

revolutionized biology, allowing imaging of neurotransmitters in the brain. S. Kailas M. Krishnamurthy delves into the world of extreme intensity laser light that allows table top particle accelerators to be envisaged. Kailash Rustagi takes us down a

memory lane with his personal reminiscences of the progress in non-linear optics. Rashmi Shrivastava and Shovan Majumder look at medical diagnostics using Raman spectroscopy, not surprisingly featuring lasers. Apart from the focus articles, we have articles by IPA awardees Vivek Parkar discussing weakly bound nuclei, and by Joydeep Chakrabortty on effective field theory. Our news and events section covers the Vigyan Vidushi summer series – a unique nurture programme aimed at women MSc physics students, book reviews and announcements, and in keeping with our theme, the profiles page features physicists with a laser connection. Our last page feature, Backscatter, highlights the role of physics in our latest clothing accessory – the face mask! We hope you enjoy the different flavours of articles in this issue. As always, we look forward to hear from you with feedback/suggestions and wish you good health. Happy reading!

Editorial Board

Production Assistance: Ghnashyam Gupta PHYSICS NEWS is funded by a grant from the Board of Research in Nuclear Sciences (BRNS) of the department of Atomic Energy, Government of India. PHYSICS NEWS is published quarterly and is the official bulletin of Indian Physics Association, IIT Bombay, Mumbai – 400 076 PHYSICS NEWS is mailed free to all members and is available for purchase at the rate of ₹150 per copy. Correspondence regarding subscription and other matters should be addressed to General Secretary, IPA - [email protected]

Vol.50(3) 2 Physics News

From the President’s Desk

We still are in the midst of the COVID-19 pandemic and it has impacted the physics community as a whole in many ways. A significant amount of teaching and majority of meetings/conferences have moved to an online format. The crisis therefore has forced us to reinvent programmes and find new avenues. While this has also opened up opportunities, in a country like India it is important to find ways to include all - particularly those from rural and remote areas, where connectivity issues may exist.

The global physics community has also recognized several challenges arising due to the COVID-19 pandemic and recently a round table discussion of physics societies of different nations was held. The present situation has implications for the whole research community - students and researchers, as work that can be completed without access to facilities is limited. Due to prevailing constraints our IPA50 event has been converted to a webinar series during Sept. - Dec. 2020. The “New horizons in Physics” webinar series will host a special lecture every Saturday at 5 pm, which will be live streamed on our YouTube channel. We look forward to a much wider participation from all over India in this event and in other IPA activities.

A.K. Mohanty

3 Vol.50(3) Physics News

Sixty Years of Lasers

R.Vijaya Department of Physics and Centre for Lasers and Photonics, Indian Institute of Technology Kanpur, Kanpur 208016, India. E-mail: [email protected] Vijaya is an experimental physicist working in Optics and Photonics. Formerly associated with IIT Bombay, currently she is the Head of the Department of Physics at IIT Kanpur. Recently, she also headed the Centre for Lasers and Photonics at IIT Kanpur. Her research work includes nonlinear optics, fiber optics, nanophotonics, optical devices, miniature lasers, microstrip patch antennas and metasurfaces, with an emphasis on applications. Physics- and optics-related outreach effort for school and undergraduate college education is one of her passions.

Abstract In May 2020, the Optics research fraternity across the world celebrated the sixty-year milestone of the first laboratory demonstration of the laser in the USA. Starting out as a novelty and a hugely expensive source of light, the laser has reached a position of being indispensable today in a host of instruments ranging from daily-use products such as DVD players and laser printers, to highly specialized medical equipment and military weapons. This article traces the history from the early days to the recent years, covering some important milestones and the leap in ideas, and concludes with what is driving the multi-billion- dollar industry associated with lasers.

Introduction India’s research strength in spectroscopic techniques was one of the prime reasons for a quick adaptation to laser research in The process of stimulated emission of electromagnetic the academia in the ‘60s and ‘70s. The research in lasers was waves was proposed by Albert Einstein in 1917. This was the accorded a pride of place in Government laboratories in India. precursor that led to the possibility of Light Amplification by The Centre for Advanced Technology in Indore (later renamed Stimulated Emission of Radiation (referred to by the acronym as Raja Ramanna Centre for Advanced Technology) was LASER). The process of stimulated emission generated a lot founded in 1984 for non-nuclear frontline areas such as lasers, of interest because it promised such qualities to light as and the Defence Science Centre in Delhi (later renamed as unknown at that time, and also because it appeared feasible Laser Science and Technology Centre) was made responsible under the right conditions. The theoretical concept remained for laser research in 1986 [2]. The topic of lasers was in the limelight for a few years but was not realized in the introduced into the educational curriculum in India in the ‘80s laboratory. Then there was a flurry of activity in ‘50s when the and ‘90s. Today, India has a handful of industries and a sizable process of stimulated emission was proposed in the range of manpower trained in laser research. In the following pages, we microwave frequency, and was experimentally realized soon will describe the fundamentals, the first laser, the early after. In 1960, the same effect was demonstrated in the progress, the related contributing factors, and the mind- laboratory conditions for light in the visible range [1]. Today, boggling reach of lasers. the laser is not viewed as an acronym of a process, but as a powerful instrument with a host of applications. What does a laser consist of? It is worthwhile to know what drives the interest in lasers. This The laser is a source of light made from a medium which has will be discussed in more detail later on, but to be brief, the very well-defined energy levels that allow one or more lasers are at the heart of instruments used in military, medical, radiative transitions with the emission wavelength in the range industrial, environmental, and daily use products. The lasers of visible or near-infrared wavelength. Thus, it requires an have enabled extremely innovative applications (such as in ‘active’ medium (which decides the name of the laser), kept in medical surgery) and in some cases, replaced an earlier non- an enclosure named the ‘cavity’ or ‘resonator’, and a method laser source of light for a better and value-added performance of ‘pumping’ to provide input energy to the system [3]. Thus, (such as in microscopes). Either way, lasers have progressed the laser is the optical analogue of the electronic oscillator, as from ‘a solution looking for a problem’ to ‘is it possible to it requires an input (pump), an amplifier (medium with build a laser for my application?’. This enviable position was appropriate energy levels) and positive feedback (cavity). The reached due to several serendipitous events, contributions of pumping method ensures that the population of the different visionaries, related advancements in other fields of science energy levels are modified when the ground state population and engineering, and the ever-present promise it offers for is re-distributed to the other levels. Each level will eventually military dominance. reach its unpumped population level by radiative or non-

Vol.50(3) 4 Physics News radiative downward transitions if the pumping is switched off. ‘absorption’ continue during the passage through the medium The radiative transitions initiate the ‘spontaneous emission’ with increasing number of round-trips, the pumping process due to the finite lifetime of all the excited states. ensures that the populations get re-distributed over the different levels and the spontaneous emission gets amplified. When the pumping is present, the population in different When this process continues further, it is possible to achieve a energy levels will be governed by the pumping rate and the larger population in the higher-energy level (N ) than that of lifetime of the levels. The cavity also plays an important role. 3 the lower-energy level (N ) if the pumping rate is high and the In simple terms, the cavity is a combination of two reflectors 2 lifetime of the upper level is long. This is the condition of kept at the two ends of the active medium, thus increasing its ‘population inversion’ with N >N . In this condition, the light effective length. The light emitted from the radiative 3 2 emitted by the lasing transition will not be absorbed but will transitions is made to pass through the medium multiple times trigger further emission, termed as ‘stimulated emission’ due to this cavity, and gets ‘absorbed’ in the initial stages by (Fig.1). Thus, the probability of absorption and stimulated the corresponding energy levels, instead of being lost to the emission at frequency ν between a set of energy levels is surroundings (Fig.1). As this ‘spontaneous emission’ and governed by which level has the larger population.

N ≫ N ≫ N ≫ N N ≫ N > N > N N > N > N > N N > N ˂ N > N 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 Pumping initiates Spontaneously emitted ν is Amplification of Population Inversion and spontaneous emission at ν absorbed by E -E transition spontaneous emission stimulated emission at ν 2 3 Figure 1: Depiction of how the process of pumping modifies the population of the energy levels in the active medium, and spontaneous emission converts to stimulated emission. E1, E2, E3, E4 are four lowest energy levels in the medium and N1, N2, N3, N4 are their populations which are changed due to the pumping process. The thick red horizontal line indicates the passage of time during the propagation of light in the active medium (and thus an increasing number of round-trips). While multiple downward transitions are possible and thus the threshold for lasing (corresponding to the dominance of spontaneous emission often contains a broad spectrum, the stimulated over spontaneous emission) is crossed, the cavity aids in wavelength selectivity and narrowing of the spectrum will show narrowing at a specific wavelength. spectrum. This can be aided further if the reflectors have an Threshold behaviour in the dependence of output power with appropriately chosen high reflection wavelength. It is essential input power is another signature of lasing. The stimulated to have the length of the cavity to be equal to an integral emission will have high coherence characteristics and will multiple of the required wavelength of lasing. Depending on emerge as a highly directional beam if the cavity is built as a the gain provided by the active medium, the wavelength linear arrangement. selection requires the spontaneously emitted light to go through a large number of round-trips in the cavity as shown in Fig.2, which is from the simulation of a fiber laser. Only a selected transition will experience the gain. This chosen transition will eventually be the ‘laser transition’ providing monochromatic nature to the emitted light. The narrowing of spectral width when spontaneous emission evolves into stimulated emission (Fig.2) is an important signature of the lasing process, and can be clearly seen in an experiment. Of course, it is not possible to monitor the spectrum of light Figure 2: Spontaneous emission (shown at the left) evolves during the round-trips. But one can start with a poor cavity into stimulated emission (towards the right), with increase in (say, by misaligning the mirrors) and record the spectrum of the number of round-trips in the cavity/resonator. spontaneous emission. Then the cavity can be re-aligned well Early foundations to see the narrow spectrum associated with the stimulated emission. Or, the spectrum can be monitored while the In 1900, Max Planck published the work that showed the pumping fluence is being increased in slow steps. When the relationship between energy and frequency of radiation,

5 Vol.50(3) Physics News emphasizing that electromagnetic energy could be absorbed or studying the suitability for their spectroscopic features in emitted only in discrete quanta. In 1905, Albert Einstein terms of their energy levels, lifetimes of levels involved and explained the experimentally observed photoelectric effect, the emission wavelengths, as well as their fabrication where light delivers its energy in discrete quanta (photons). methods, cost advantages, and robustness in dissipating heat. Subsequently, in 1917, Albert Einstein proposed the idea of Fluorescence is the natural initiating process of radiative stimulated emission, paving the way for eventual laser action. transitions from energy levels with long lifetimes. Thus, the Giving a boost to the topic, and huge popularity as well, was study of fluorescence and the associated time-domain the Nobel Prize awarded to Max Planck in 1918 for his measurements got a boost. In less than 20 years lasers had discovery of the quanta and the Nobel Prize to Albert Einstein been studied in all states of matter including solids, liquids, in 1921 for his explanation of the photoelectric effect. gases and plasma. Along with the materials research, there was tremendous progress in the technological aspects as well. It was understood very early on that population inversion, wherein the population of a higher-energy level is more than Pulsed lasers using the techniques of Q-switching and mode- the population of a lower-energy level, was essential for locking, and ultrafast pulse generation with picosecond and stimulated emission to take place. But this was in femtosecond durations (10-12 and 10-15 second respectively) contradiction with the Boltzmann distribution, which dictates became possible. From big table-top laser systems with large that higher the energy of a level above the ground state, lower cooling stations, the lasers became small hand-held systems will be its population at room temperature. Experiments did thanks to semiconductor materials being used as the active not succeed at the fast rate the theoretical concepts were medium. The progress in optical fibers, and the rapidly evolving, since a safe way of reaching population inversion expanding reach of fiber-optic communications pushed the (as well as stimulated emission) at room temperature remained research further. Fiber lasers with rare-earth dopants such as elusive. Then, in 1950s, there was a huge interest in neodymium, erbium, ytterbium, praseodymium, holmium, microwave amplification (MASER). The idea was proposed and thulium in glass fibers are known today. in 1951 by Charles Townes of Columbia University, and Several related advancements were tried and succeeded in the subsequently demonstrated by him in 1954 in ammonia gas subsequent years. Tunable lasers where the emission along with Herbert Zeiger and James Gordon. A new wavelength from the laser could be varied over an impressive pumping method was proposed by Nikolai Basov and range, single longitudinal mode lasers with the shortest of Alexander Prokhorov of P. N. Lebedev Physical Institute in linewidths (and thus the purest of monochromaticity), Moscow in 1955, and the first solid-state maser was dynamically controllable lasers with periodic and chaotic demonstrated by Nikolaas Bloembergen at Harvard pulsed output achievable with the variation of suitable University in 1956. parameters [5], and miniature nanoscale lasers with surface The first one and the next one and the next….. plasmon amplification (spaser) [6] are some amazing examples. The first laser demonstrated in a laboratory was the Ruby laser on 16 May 1960, by Theodore Maiman at Hughes Research Laser research also enabled the related topic of nonlinear labs, USA. This was announced in a press conference on 7 July optics to advance by leaps and bounds. Second harmonic 1960 and published in Nature on 6 August 1960 (after it was generation, third harmonic generation, two-photon absorption, rejected by Physical Review Letters) [4]. Maiman used ruby stimulated Brillouin/Raman scattering, frequency comb as he had experience in making a maser with ruby crystals. generation, and supercontinuum generation are some of the The visible laser beam at a wavelength of 694.3 nm from ruby effects enabled by the advancement in lasers. In fact, the utility is the first reported laser. Soon after, IBM reported Uranium- of lasers increased when certain wavelengths, which were CsF2 laser in November 1960 while Donald Herriott, Ali otherwise not available, were made accessible through these Javan and William Bennett at Bell Labs reported Helium-Neon processes, all of which required a laser to start with. Some of laser in December 1960. Neodymium laser in calcium these processes yielded new wavelengths with ‘laser-like’ tungstate and later in glass was demonstrated in 1961, the red characteristics, so that the new wavelengths could be Helium-Neon laser and the first semiconductor laser were employed exactly in the same way as the original laser, while reported in 1962, while the first ion laser and Nitrogen laser some others resulted in processes very different in their came in 1963. The development of different lasers was so character. An example of the former is the harmonic rapid, their designs so robust, and their applications so many, generation, and an example of the latter is supercontinuum that by 1965, there were many companies set up to develop generation. and sell lasers. It may be worthwhile to mention that the first Two types of lasers in more detail reported use of the laser in eye surgery was in November 1961 and it used a Ruby laser, while the first reported sale of a laser With a vast variety of lasers being studied with widely was by Spectra-Physics in 1962 for the IR Helium-Neon laser. different characteristics, there have been attempts to scrutinize the definition of the laser [7]. The standard definition of Milestones in progress narrow linewidth with pure modal features does not apply in Research in lasers progressed impressively in the following all cases. Hence, one identifies the lasing properties with the years. Initial work was in finding new materials for the lasing help of co-existing multiple factors, such as spectral linewidth medium, and to obtain lasing at more wavelengths. This led to narrowing at higher pumping, threshold effect with increase in a plethora of research articles on materials, both in terms of pump power, directionality of emission as dictated by the

Vol.50(3) 6 Physics News chosen cavity, and the spatial/temporal coherence be possible to extract several tens of laser wavelengths characteristics. We will discuss two widely different types of simultaneously from a single set-up depending on the lasers in some detail to bring out the variety possible in them. broadband range. The spatial coherence of the multi- wavelength output is assuredly high as it is generated and The first type we will discuss is the fiber laser. It has the transmitted in the fundamental mode of a single-mode fiber. advantages of compact size, high gain, and continuous wave The extent of temporal coherence has to be ascertained. lasing possibility for long durations at room temperature, without the need for cooling. The active medium in the form The spectral linewidth of a laser is used to estimate its of an optical fiber gets pumped with another fiber-coupled temporal coherence. But in our case, the linewidth of the peaks laser. By combining the concepts of standard laser design with is decided by the characteristics of the demux. Thus, we need nonlinear optical effects (using specialty fibers which have to quantify the coherence of light output at each channel. We high nonlinear coefficients), it is possible to obtain ‘broadband use a fiber-optic Mach-Zehnder interferometer (MZI) lasers’ and/or ‘multi-wavelength lasers’. The active medium (Fig.4(a)) for this test. A sharp power difference between the in our work is 8m length of erbium-doped fiber (EDF) which two ports of MZI is an indicator of high levels of interference, is pumped with a semiconductor laser at 980nm (Fig.3(a)). and thus of temporal coherence. But the MZI used by us has a The two wavelength division multiplexers (WDM) help in fixed path length. Therefore, it will not give the same combining/separating the wavelengths of pump laser and fiber magnitude or the sign for the power difference between the laser at the input/output. The isolator ensures unidirectional two ports at all the channel wavelengths as the phase propagation of light in the ring cavity and the coupler helps to difference is also dependent on the wavelength. tap out a small portion of the intra-cavity field to test its In Fig.4(b), the normalised difference in power between the wavelength on the optical spectrum analyzer (OSA). two ports of MZI is shown for the different channels, when the By inserting a specialty fiber (1 km of highly nonlinear fiber) output of the broadband EDFRL is passed through the demux. in the ring cavity of the erbium-doped fiber ring laser For comparison, Fig. 4(c) depicts the normalised difference in (EDFRL), it is possible to obtain a broadband laser output as power between the two ports measured using a tunable laser shown in Fig.3(b) spanning a wide wavelength range. Spectral (Yenista OSICS-TLS-AG) precisely set at the wavelengths of broadening is enabled by nonlinear wave mixing between the the demux (without using a demux). This laser is commonly multiple longitudinal modes in the specialty fiber. By using a used in fiber-optic communications. The wavelengths wavelength demultiplexer (Demux) with an inter-channel extracted from the broadband EDFRL possess significant spacing of 0.8 nm, output at different wavelengths are temporal coherence at a few channels, but definitely not as obtained as shown in Fig.3(b) [8] on the OSA. The number on high as that of the commercial laser. The reason for this is the the peaks shown in this figure refers to the channel number of noise level introduced by the demux which can deteriorate the 16-channel demux used in this experiment. It is also their coherence. This is clear in Fig. 4(d) for the results possible to extract multiple wavelengths from the broadband obtained when the EDFRL with an intracavity-filter is used EDFRL using a wavelength filter inside the laser cavity. A set (without a demux) at the input of MZI. It shows extremely of sharp peaks are obtained at the output of the EDFRL sharp variation in the power difference at the two ports of (Fig.3(c)) when we use an interference filter. MZI, and thus its temporal coherence is comparable to that of the standard tunable laser. Thus, it is possible to obtain a multi-wavelength laser for a specific application by suitably modifying the laser design. An interesting question about the broadband EDFRL and the possibility of extracting tens of laser-like outputs from it is if it shows any correlation between the wavelengths from the different channels as they are all generated by the same laser cavity synchronously. To analyze this, two wavelengths (from two different channels) can be given as two inputs to the same MZI, and the output power difference between the two ports can be monitored. In Fig.5, we show the measured results from the broadband EDFRL and the commercial tunable laser (with two laser modules), and they are comparable. While the wavelength (shown for channel-14) extracted from the broadband EDFRL has correlation with adjacent wavelengths on either side, the commercial laser modules (used in fiber- Figure 3: The schematic of the EDFRL is shown in (a) and it optic communications) show correlation with wavelengths has two possibilities; if a demux is used at the output, the even farther off, which can lead to cross-talk problems. It is broadband spectrum from the laser can be extracted as possible to convert the continuous-wave EDFRL (without the narrow spectral lines (b); if an interference filter is used inside specialty fiber) into a pulsed laser by including an amplitude the cavity, sharp spectral lines can be obtained (c) on a broad modulator in the cavity and using the process of active mode- background. locking. Due to their versatility of being used as continuous- One may note the interesting fact that each peak shown in wave lasers and pulsed lasers, fiber lasers are finding Fig.3(b) can act like a continuous-wave laser, and thus it may application as sources of illumination in microscopes.

7 Vol.50(3) Physics News

lasers without any complex equipment and entirely with non- polluting organic materials. The active medium will be present in a structure which has a periodic variation of refractive index (and which acts as a natural cavity without the need for additional reflectors), and pumped by a suitable source. The lasing wavelength is decided by the gain but also by the density of states dictated by the crystalline arrangement. The experimental result from one such design is shown in Fig.6 [9]. The sample is made of Rhodamine B dye doped polystyrene colloids self-assembled into a photonic crystal grown in a face-centred cubic lattice (inset in Fig. 6). The three-dimensional stacking of the crystalline arrangement leads to a stopband for light (at normal incidence from the top in the inset) lying in the wavelength range of 590 nm to 640 nm. The stopband will shift to lower wavelengths at oblique incidence. The stopband range is decided by the size and refractive index of the colloids, and their crystalline lattice arrangement.

Figure 6: The spontaneous emission is less at low pump fluence (P1), which amplifies initially as a broad spectrum (at pump P2), and then converts to stimulated emission with Figure 4: (a) Schematic arrangement for testing temporal spectral narrowing (pump P3 and P4). coherence using a Mach-Zehnder interferometer (MZI). Normalised power difference at the two ports of MZI is shown When pumped at 532 nm within the absorption spectrum of for (b) broadband EDFRL with demux, (c) tunable laser set at the dye, it emits a broad spectrum of spontaneous emission at individual wavelengths (without demux) and (d) broadband low pump fluence (P1 in Fig. 6). When the pump fluence is EDFRL with intra-cavity filter (without demux). increased (P2), the broad spectrum gets amplified uniformly. For higher pump fluences (P3 and P4), spectral narrowing is observed and is centred at 587 nm, along with two step-like features near 570 nm and 610 nm, corresponding to the band edges of the photonic stopband of the crystal. The spectral narrowing is thus obtained within the stopband. This, along with the threshold-effect of this process, is obtained only at a specific angle (22o in this work), along with a directional beam of light at a centre wavelength of 587 nm [9]. The versatility of photonic crystals for lasing applications Figure 5: Normalised power difference at the two ports of arises from the variety of cavity designs that can be supported. MZI for single- and dual-input to verify inter-channel In heterostructures, photonic crystals can be combined to correlations: (a) broadband EDFRL, (b) commercial laser. possess multiple periodicities, such as one-, two- or three- dimensional (1-D, 2-D, or 3-D) periodic stacking of the The second type of laser is the photonic crystal laser. This refractive index. In Fig.7, four possible designs are shown for laser is very small in size, with cavity thickness of the order of heterostructures. In Fig. 7(a), the simplest design of the laser- a few microns. Unlike semiconductor lasers, which are also active medium present as a thin-film defect layer in 1-D small, it is possible to make some designs of photonic crystal multilayer stack is shown. In Fig. 7(b), the active medium is

Vol.50(3) 8 Physics News in a 2-D arrangement of colloids as a defect layer sandwiched local effects can also enhance the emission significantly if between 1-D multilayer stacks. In Fig. 7(c), the active medium their spectral contributions are maximized [9]. is in a 3-D arrangement of colloids as a defect layer with 1-D multilayer stack on either side, while in Fig. 7(d), the active Path-breaking ideas medium in a 3-D crystal has a passive defect layer in its midst. A very important factor in laser development is its efficiency, Such combinations of 1-D, 2-D and 3-D arrangements of the and realistic ways of enhancing it. In this pursuit, the work led active medium help in modification of the stopband to the possibility of ‘low-threshold’ laser. The threshold of a characteristics. The thickness of the individual layers of the 1- laser is decided by the condition at which stimulated emission D multilayer stack is t1 and t2, while the effective dielectric sets in. This, in turn, is decided by the population inversion constant of the active medium (shown in purple in Figs. 7(a)- condition. In specialized structures, such as photonic crystal 7(d) is . The location of the active medium, its absorption / lasers, the threshold is proportional to the group velocity of emission spectral ranges, the stopband features such as its light in the medium, which can be modified using band central wavelength and bandwidth, and the distribution of engineering in the photonic domain. The choice of the density of states as decided by the periodic arrangement refractive indices of the materials, their periodicity and their govern the stimulated emission from the active medium. lattice arrangement are crucial factors that can be utilized in photonic band engineering. An interesting trend in the 21st century has been to challenge the role and thus understand the most basic of optical processes that are associated with lasers. For example, laser designs without a confining cavity led to random lasers, laser cavity designed with lossy metal could demonstrate the plasmonic contributions, and a miniature cavity in three dimensions (which can confine all the emitted light) enabled nanophotonic and quantum photonic principles to be tested. The research in lasers went from the technological aspects back into fundamental studies. Needless to mention that some of the latest developments in quantum optics have benefitted enormously by the necessary lasers available for research. Most of the reasons for the importance of lasers stem from its extremely narrow wavelength band, its low divergence angle, its ability to produce light in continuous or pulsed nature, its coherence, and its increasingly compact size. A near- monochromatic nature ensures that only the medium that Figure 7: Different heterostructure designs for photonic absorbs that wavelength will get heated when the laser beam crystal laser: (a) 1-D stack with thin film defect, (b) 1-D stack passes through it, its low divergence enables it to be directed with 2-D crystal defect, (c) 1-D stack with 3-D crystal defect at highly specific objects in ranging experiments, and pulsed and (d) 3-D stack with thin film defect. lasers have produced extremely small-duration bursts with very high peak powers enabling extremely complex processes These designs help us to understand fundamental processes. to be studied. The small size of lasers has expanded their range 1-D, 2-D and 3-D periodic photonic crystals possess of applications in consumer goods such as in general-purpose stopbands in specific directions, which result in high reflection laser printers and laser pointers. It is interesting to note that of a certain wavelength range. Any defect in the periodic research in lasers has been both a curiosity-driven enterprise arrangement leads to a ‘defect mode’ which enables highly as well as a commercial target-driven industry. It is very selective wavelength transmission. For example, in Fig. 7(a), unlikely that there are many other fields which have the only benefit is the localization of the electric field of light progressed so well at such speeds on both the fronts. in the defect layer due to the reflection from the 1-D stack. In Fig. 7(b), the 2-D photonic crystal has an in-plane stopband Applications while the 1-D stack has an out-of-plane stopband. In addition, Big-budget commercial applications of specialty lasers are in there is a defect field as well. In Fig. 7(c), it is possible to have materials processing, healthcare, and in aeronautic / defence the 1-D stack only on one side or on both sides of the 3-D sectors. In materials processing, high-power lasers are used in arrangement of colloids, and it is possible to choose the size welding, cutting and drilling with minimal wastage. There is a of colloids and the layer thickness of 1-D structure for booming industry in laser-based additive manufacturing [10] perfectly or partially overlapping individual stopbands of 1-D and micromachining of precision units in a host of and 3-D arrangements. As a result, it was possible to obtain applications. One of the biggest buyers of lasers in India is the highly enhanced emission at the band edges with the help of diamond-cutting and processing industry, specifically for laser such spectrally engineered designs [9]. In the design of marking of diamonds. Fig.7(d), the active medium is present everywhere except in the passive defect layer. With this design, we could One of the most advanced applications is in light detection and demonstrate that it is not always necessary to maximize the ranging (LIDAR). This comprises of a laser to illuminate an field at the spatial location of the active medium, since non- object, and a detector for time-domain measurement of its

9 Vol.50(3) Physics News reflected power and thus to extract the distance of the object. stages. This pursuit has gone far ahead in the sixty years, and The hand-held speed-measuring device used in traffic lasers are used today in multiple defence platforms, and enforcement is the simplest example, while LIDARS are also prominently as dazzlers and interceptor-cum-destroyers used in aerial measurements and mapping of huge areas of aboard warplanes and tanks. land and water. One of the earliest experiments in laser ranging (which is in use even today) is the monitoring of the Summary earth-to-moon distance by sending a laser pulse to the moon The search for new lasers continues due to the rapidly which returns after reflection from one of the corner-cube emerging demands on their features and their applications. reflectors placed on the moon by the first visitors in 1969. Simultaneously, this has led to a deeper understanding of the While the advantage of using lasers in ranging applications is subject and has brought together physicists, chemists, due to its narrow beam (specificity), speed of propagation engineers and doctors on the same platform towards shared (speed), and the choice of wavelength (infrared to make it goals. While the laser beams are being used to map the remote invisible), the disadvantage is that it cannot detect objects areas on earth and even on far-away planets in ambitious hidden from sight. projects, they are also enabling the common man on our planet The most rewarding application of lasers, and the one seeking to access the far-corners of the globe remotely through the the highest precision, is likely to be in the medical field, as it widespread use of fiber-optic internet. This article gave a mere provides an excellent tool to the already-dextrous surgeon’s glimpse into the exciting topic. It gives us hope that many hands. The wavelength requirement is critical for some more milestones will be crossed in the years to come. applications in medicine, and hence it is one of the fields that Acknowledgments pushes the boundaries of laser research. While the use of lasers in ophthalmology and cosmetic medical procedures has been This article is dedicated to all my teachers and my students around for many years [11], non-surgical medical applications who stimulated and sustained my interest in laser research such as low-level laser therapy for alleviating pain is a wide- over three decades. open field. References: What is the latest? 1. J. Hecht, Opt. Eng., 49, 091002 (2010) The lasers normally operate in the visible and near-IR ranges 2. www.rrcat.gov.in; drdo.gov.in/labs-and-establishments 3. Lasers by Anthony E. Siegman (University Science Books, of wavelength. It is difficult to get lasing at lower wavelengths USA, 1986) in the ultraviolet (UV) range because the ratio of the rate of 4. T. H. Maiman, Nature 187, 493 (1960) stimulated emission to the rate of spontaneous emission 5. G. Kumar and R. Vijaya, J. Opt. Soc. Am. B, 34, 574 (2017) depends inversely on ν3. It is thus easier to get stimulated 6. D. J. Bergman and M. I. Stockman, Phys. Rev. Lett. 90, 027402 emission to dominate over spontaneous emission at lower (2003) frequencies. Most of the laser-based applications in the UV 7. I. D. W. Samuel et al., Nat. Phot. 3, 546 (2009); editorial Nat. range today use indirect methods, such as harmonic generation Phot. 11, 139 (2017) or inter-modal four wave mixing in optical fibers [12]. Thus, 8. D. Venkitesh and R. Vijaya, J. Appl. Phys. 104, 053104 (2008); the search for new laser materials, new laser wavelengths and Suchita and R. Vijaya, IEEE J. Quant. Electr. 54, 1600508 (2018) new laser designs [13] is far from over. Do we know of 9. M. S. Reddy et al., IEEE Phot. J. 5, 4700409 (2013); G. Kumar ‘natural lasers’? Yes; maser and laser action in the cosmos and R.Vijaya, J. Opt. Soc. Am. B, 35, 61 (2018); D. Rout et al., were reported back in 1965 and 1996 respectively [14], while J. Nanophot. 13, 046005 (2019) another major milestone was the demonstration of air lasing in 10. A. J. Pinkerton, Opt. Laser Technol. 78, 25 (2016) 2014 [15]. In recent years, petawatt (1015 Watt) power levels 11. Peng et al., Rep. Prog. Phys. 71, 056701 (2008) in lasers are targeted, and have been achieved in China, 12. S. K. Chatterjee and R. Vijaya, Opt. Commun. 458, 124816 Europe [16], Japan and South Korea, while USA is aiming to (2020) reach 3 PW capability through ZEUS. Such power levels are 13. D. Kottilil et al., Adv. Funct. Mater. 30, 2003294 (2020) to enable fundamental experiments in quantum 14. H. Weaver et al., Nature 208, 29 (1965); V.Strelnitski et al., Science 272, 1459 (1996) electrodynamics as well as generate proton beams for cancer 15. A. Laurain et al., Phys. Rev. Lett. 113, 253901 (2014) therapy. No discussion on lasers can end without a heads-up 16. F. Lureau et al., Proc. SPIE, 11259, 112591J (2020) on the laser weapons. This ‘death-ray’ possibility was the foremost driver of funding towards laser research in its early

Vol.50(3) 10 Physics News

Multiphoton Excited Fluorescence Lights Up Neurobiology*

Sudipta Maiti Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India E-mail: [email protected] Sudipta is currently a Professor in the Department of Chemical Sciences at the Tata Institute of Fundamental Research. His research interests span areas of protein aggregation, monoaminergic neurotransmission, membrane interactions, and the development of optical techniques for studying biological phenomena. He is the co-founder and President of the Fluorescence Society (India), and President of the International Society on Optics Within Life Sciences (Germany). He is a Fellow of the Indian Academy of Sciences.

Abstract

Lasers have changed microscopy, in ways neither the laser physicist nor the microscopist could have imagined. In recent years, lasers have helped break the barriers of optical resolution and have made ultraviolet excitation relatively benign for living things. The latter has been achieved via a technique called multiphoton microscopy (MPM), which has opened new windows into the inner workings of the brain. I will describe some of the advancements in this area made in our lab.

Introduction perform label-free imaging, using the native fluorescence of the molecule. Many of the body's constituents are indeed Light microscopy in biology fluorescent, albeit in the ultraviolet. That brought us back to the same problem: UV irradiation is incompatible with live For four centuries since Zacharias Janssen built one, the biology. light microscope has remained an indispensable tool of the biologist. No other tool lets one look at a subject with so much A large number of very significant problems had remained out detail but with so little damage. However, fundamental of reach due to these limitations of microscopy. Over the last physics had set limits to the resolution and contrast achievable three decades, these limits have been overcome, ushering in a with the light microscope. No matter how good the new era in visualizing living cells. These traditional limits microscope was, it seemed that the resolution was forever have been creatively tackled to yield fluorescence images at doomed to stay on the larger side of 200 nm. This had to do sub-10 nanometer scales (e.g. using STED, i.e. STimulated with the nature of light (diffraction) and the long visible Emission Dumping microscopy) or to produce fluorescence wavelengths that light microscope used. The latter was limited images of UV fluorescent biomolecules using only benign by the damage that ultraviolet light caused to living systems. infrared radiation. The latter depended on the ingenuity and Unfortunately, with a molecular view of biology emerging in foresight of Maria Göppert-Mayer, a graduate student the last half a century, these limitations had become real producing a Ph.D. on “two-photon quantum transitions” in hurdles in understanding how life works. 1931 [1], based on which the technique of ‘multiphoton microscopy’ was developed nearly 60 years later [2]. Of A separate problem with microscopes had been that of contrast course, it was the invention of lasers sixty years ago that made - the inability to see just the molecular species of interest in a all of these experimentally possible. specimen. The molecule of interest is always crowded in a live cell with everything that absorbs or scatters light in a similar One such limitation that I will talk about here is one that we wavelength. Development of the fluorescent proteins of the have successfully worked on for nearly two decades. This GFP family has been a boon, and now we can see the species problem is the visualization of messenger “neurotransmitter” of our interest, as long as we are able to fuse it with an molecules in the brain. We have developed various forms of appropriate fluorescent protein. However, there is always a multiphoton microscopy to make a series of the most nagging worry that tagging a GFP molecule would severely important neurotransmitters visible in living systems, a impair the functioning of that molecule. One possibility is to glimpse of which is given below.

* A part of this article appeared in Garai and Maiti, IANCAS Bull. iii (4) (2004) 327. Adapted with permission

11 Vol.50(3) Physics News

Multiphoton microscopy subsequent exocytosis form the basis of chemical neurotransmission in all higher organisms. Very little If a fluorophore absorbs in the ultraviolet, but not at lower quantitative information exists about the intracellular events wavelengths, we understand that its first excited state is controlling these processes. Neurotransmitters come in many separated from the ground state by an energy equivalent to that varieties, but serotonin is a special one. Serotonin (5- available from an ultraviolet photon. Normally, a photon hydroxytryptamine (5-HT)) containing neurons in the brain having a longer wavelength (say twice) would not have have been implicated in a wide range of phenomena such as enough energy and would not be absorbed by this molecule. cognition, affect, pain, emesis, sex, neuroendocrine functions, However, if somehow the energies of two of such lower learning and memory tasks, regulation of sleep, blood energy photons could be combined, the molecule in principle pressure, sensory responsiveness and suicidal behaviour. can be excited, and can subsequently emit a fluorescent photon Psychiatric disorders involving the 5-HT system result from from this state by a normal mechanism. Multiphoton an alteration in the content, distribution and flux of the excitation achieves exactly that. Soon after the birth of modern neurotransmitter. Drugs acting on the serotonergic system quantum mechanics, Maria Goppert-Meyer recognized that have been found to be effective in the treatment of depression. perturbation theory allows such a simultaneous interaction of a molecule with two photons, in effect helping it to end up in Quantitative characterization of the serotonin vesicles is the excited state [1]. However, such second order effects critical for understanding these processes. We need to know would have very small probability of occurrence, unless the how many vesicles are there in each cell, where they are impinging light field were very intense. It took three decades located, how much neurotransmitter is contained in each and the invention of the laser to verify this remarkable vesicle, and finally how this pool responds to signals for prediction [3]. Another three decades later, Watt Webb and exocytosis and to drugs of abuse such as amphetamines. In coworkers focused a femtosecond laser source (to keep the short, we need to have a detailed single neuron view of many instantaneous intensity high while the average intensity low) of the processes that are understood at the level of the through a microscope objective lens into a cell, scanned each organism. It is difficult to quantify serotonin distribution in a point of the cell and collected the resultant fluorescence from living neuron in a microscopic scale, as most methods of each point to generate a multiphoton fluorescent image of the identifying serotonin, such as single photon emission cell [2]. This constituted the invention of the multiphoton computed tomography (SPECT) and positron emission microscope. tomography (PET) do not quantify the level of endogenous serotonin. Direct imaging of the auto-fluorescence of the The Challenge: Visualizing Neurotransmitters neurotransmitters can potentially determine all these It was recognized a few years later that the technique of two- quantities. While it is possible to image the auto-fluorescence photon microscopy can be extended to perform three-photon from serotonergic and dopaminergic neurons with ultraviolet microscopy and to image ultraviolet molecules in live cells excitation, these images have not resolved individual [4]. The specific challenge was to observe the neurotransmitter vesicles, and appear to be toxic to the cells. neurotransmitter molecules in live cells. A better alternative appears to be multiphoton excitation, which uses infrared wavelengths to probe ultraviolet Neurotransmitters are small molecules used by one neuron to fluorophores with high three-dimensional resolution. So we communicate with another neuron across the synaptic decided to excite serotonin with a three-photon excitation junction. Some of these molecules known as monoamines mechanism, using 740 nm, 100 fs laser pulses. The ultrafast (e.g. serotonin) are involved in most of the major functions laser pulses were required to keep the average power low, that we value our brains for: learning, pleasure, sensation of while enabling high peak powers for the three-photon reward etc. Of course the major drugs of abuse, such as excitation. Of course the detection system had to be cocaine and meth, also work on these neurotransmitters. They reengineered for detecting the emitted near-UV radiation at are packaged into small intracellular vesicles at a high around 350 nm, but that was not a very difficult modification. ++ concentration. A Ca mediated signal induces these vesicles Detection of deeper UV emitting fluorophores (such as to fuse with the plasma membrane and unload their content dopamine, which emits around 300 nm) was more difficult, into the extracellular space. Since none of the neurotransmitter and required a completely different detection scheme. molecules are fluorescent in the visible region, and since it is not feasible to fuse them with fluorescent proteins while It was shown that three-photon microscopy can keeping their function intact, no neurotransmitter has ever simultaneously image all the serotonergic vesicles inside a cell been directly imaged in a live cell. In this context, the and quantify their content [Fig.1]. The first success came in monoamine neurotransmitters are of particular interest as they imaging large serotonin vesicles in a model cell line [4], but are related to various psychopathologies. Two of these are the smaller neuronal vesicles could not be resolved. However, serotonin and dopamine. These molecules are structurally now the efforts have been successful in imaging endogenous related to the amino acids tryptophan and tyrosine neurotransmitter vesicles in live neuronal cells [5] and in respectively, and are consequently fluorescent, though only living rat brain slices [6]. We could visualize even the under ultraviolet excitation. Multiphoton microscopy uses dynamics of serotonin vesicles moving along the femtosecond pulses of longer wavelength (visible or infrared) microtubules, in neurons busy with the business of sending photons to achieve ultraviolet excitation, and it is possible to messages to the next one [7]. The last few years in this series record the first images of serotonin vesicles in live neurons. of successes has made dopamine visible [8], which is the most Neurotransmitter synthesis, sequestration into vesicles, and ultraviolet of molecules (excitation 270 nm, emission 305 nm)

Vol.50(3) 12 Physics News to have been successfully imaged in a living system so far. Conclusions Also, serotonin has been quantitatively imaged, using label- Biophotonics depends on a combination of modern laser free ratiometric imaging, in live cells [9]. To aid biologists technology and fruitful ideas for tackling biological problems. who do not have access to a multiphoton microscope, we have The quick commercialization of these new ideas is testament also developed a single-photon techniques to visualize to the potential of this field (MPM had become commercial serotonin, but of course, with a chemical labelling technique. product within the a few years of its invention). The ability to Fortunately, this technique is not very toxic, and a comparison build one's own instrument is also crucial, as the with direct multiphoton imaging shows that this method is neurotransmitter imaging described here would have been reliable [10]. possible with commercial instruments during the time they were attempted. In addition to the technique of MPM described here, and the STED microscopy I mentioned, other significant laser based microscopy technologies such as fluorescence correlation spectroscopy, optical tweezers, laser microsurgery, single molecule imaging and near-field scanning optical microscopy have emerged in the last few years. As long as laser experts keep tinkering with problems of modern biology, this field will keep flourishing with novel inventions. References 1. M. Göppert-Mayer, Ann. Phys. 9, 273–295 (1931) 2. W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73–76 (1990) 3. W. Kaiser and C. G. B. Garrett, Phys. Rev. Lett. 7, 229 (1961) 4. S. Maiti, J. B. Shear, R. M. Williams, W. R. Zipfel, W. W. Webb, Science 275, 530-532 (1997) 5. J. Balaji, R. Desai, S.K. Kaushalya, M.J. Eaton, and S. Maiti, S., J Neurochem. 95(5) 1217 (2005) 6. S.K. Kaushalya, R. Desai, S. Arumugam, H. Ghosh, J. Balaji, S. Maiti, J Neurosci. Res. 86(15), 3469-80 (2008) 7. B. Sarkar, A. K. Das, S. Arumugam, S. K. Kaushalya, A. Bandyopadhyay, J. Balaji and S. Maiti, Front. Physiol., 3(414), 1-13 (2012) 8. B. Sarkar, A. Banerjee, A.K. Das, S. Nag, S.K. Kaushalya, U. Tripathy, M. Shameem, S. Shukla and S. Maiti, Neurosci. 5(5) 329–334 (2014) 9. A.K. Das, B.K. Maity, D. Surendran, U. Tripathy, and S. Maiti, Label-free Ratiometric 8(11), 2369-2373 (2017) 10. K. Bera, A.K. Das, A. Rakshit, B. Sarkar, A. Rawat, B.K. Maity, Figure 1: Three-photon microscopic images of serotonin and S. Maiti, ACS Chemical Neuroscience 9, 469-474 (2018) vesicles (false colour coded) in cultured neuronal cells. (A) Unperturbed vesicles (bright spots in the cell images) (B) after induced loading of extra serotonin. [adapted with permission from Ref. 9]

13 Vol.50(3) T h e M u l t i - photon Connection

DONNA STRICKLAND The “Laser Jock” (As Nobel laureate Donna Strickland calls herself)

Image: University of Waterloo Donna Strickland and Gérard Mourou invented the technique of chirped pulse amplification, to stretch out each laser pulse both spectrally and in time before amplifying it and then compressed each pulse back to its original duration. This helped in generating ultrashort optical pulses of high intensity (terawatt to petawatt) and facilitated building of compact table-top terawatt lasers. In her 2018 Nobel prize acceptance speech, she referred to the seminal work of Maria Goeppert- Mayer, a theoretical physicist, where the idea of multi-photon physics was proposed.

MARIA GOEPPERT-MAYER

The effect predicted by Maria Goeppert Mayer in her thesis work was experimentally observed after 30 years. Maria Goeppert Mayer was awarded the Nobel Prize in 1963 for her work on nuclear shell model and became the 2nd woman Nobel laureate in physics. Its an interesting coincidence that multi-photon physics connects these two Nobel laureates

Did you know? The unit for two photon absorption cross-sections is called a Goeppert-Mayer (1 GM = 10-50 cm4 s photons-1)

Original paper published in 1931 https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.19314010303 , (English translation) 10.1002/andp.200910358 something else in common ? Both say “Doing physics is fun” Physics News

Staying neutral in an intense plasma

M. Krishnamurthy Department of Nuclear and Atomic Physics, TIFR, Mumbai, India TIFR Hyderabad, Hyderabad, India E-mail: [email protected] M. Krishnamurthy is a Professor at the Tata Institute of Fundamental Research. He has been working on experiments related to atomic, molecular and plasma science using intense ultra-short laser fields for more than 20 years. Recently he was instrumental in setting up new laser laboratories and an Extreme Photonics Innovation Center (EPIC) at TIFR Hyderabad. He is a fellow of the Indian Academy of Sciences and the Indian National Science Academy.

Abstract Laser pulses at sufficient intensity can ionize any matter and covert it to a hot dense plasma. A plasma temperature as high as an MeV, can be generated even with a mJ pulse of 20 fs duration. Even inert gas atoms like Ar are charged to 14+ and set up acceleration fields of TeV/m. Surprisingly such strongly ionizing laser fields can be manipulated to be an efficient charge reducer. Ions, such as Ar14+, can be converted to Ar without losing its kinetic energy. In such a case, the laser plasma becomes a neutral atom accelerator. The present article is a summary of recent work where it is amply demonstrated that atoms can and do remain neutral even with the hottest plasma. Thus novel plasma accelerators can thus be manipulated to be neutral and negative ion accelerators.

Introduction At low intensities (< MWcm-2), light is a mere perturbation. Only when the light of the right frequency arrives, does the Lasers have turned 60 years young! From the initial days matter gets ‘excited’ about it (pun intended). Else, it would of an invention trying to find a purpose, today at an age just scatter the light. Of course, the scatter cannot be taken where humans retire, it is going leaps and bounds and is lightly, as the most spectacular things we see every day are omnipresent in every modern science and application. The due to this phenomenon. Done well, the scatter can be used recent 2018 Nobel prize in Physics relates to laser as a tool to cool the atoms to incredibly low temperatures. development/applications and yet again reiterates its advancement and importance. It is too large a subject and Intensity is an incredible knob that can tune the interaction of today to cover its vast scope or reach in any small overview the light with matter. Before we turn to what intensity can do, or article, is a near impossibility. This article is to pay my first let ask the question of importance: How intense is own personal tribute to the laser@60 and the sub branch of Intense? The first scale to consider is in the atomic intense laser physics@30. I summarise some of our recent interaction. The electric field (EH) experienced by the work on using intense lasers for doing some novel atomic electron in a Hydrogen atom is taken and the intensity of the physics experiments. electromagnetic radiation with its peak electric field amplitude same as (E ) is calculated. This intensity, 3.5 x Ever since the first lasers were made, the quest for higher H 1016 Wcm-2 is also the atomic unit of intensity. So, anything intensities was a dream. Intensity refers to the number of close to this is considered ‘intense’, since in the presence of photons per square cm per second and is measured in the such field, the atomic potential is countered by the electric units of Wcm-2. A magnifying lens that we used as children, field of the light and the ‘stability’ of the atom is in jeopardy. can focus the light to less than a mm and so even few watts A second scale used for laser intensities is ‘relativistic of light properly focused can become kWcm-2. Light, as a intensity’. A free electron oscillates in the electro-magnetic coherent laser beam is much better suited to reduce the beam waist. Coherent laser beams can also be pulsed very field. If I is the light intensity of wavelength , m and e are effectively. Even by the time lasers were teenaged, pulse the mass and charge of the electron, c the light speed, then widths were reduced down to a ps. A 1mJ laser, with a pulse the oscillation energy of the electrons in the light field is given to be: width of 1 ps and focal spot of 100 m, means a focused light intensity of 1013 Wcm-2. Today, lasers routinely operate e2 Il 2 at 20 fs though specialised systems can work down to <100 U = (1) P 2 2 attoseconds. Also, commercial lasers in the regime of 16c p me Petawatts (20 J in 20 fs) can be procured. Ten times more When the oscillation energy of the electron, reaches the rest powerful lasers are being built in European consortiums. mass energy of the electrons, it is relativistic intensity.

15 Vol.50(3) Physics News

Typically, this is in the regime of 1018 Wcm-2 for the infrared with the electron temperature of a plasma and a density of light used in most intense laser experiments. solid. This is a newer variety of plasma than generally what anybody might have come across in a non-laser world. In the The physics of laser matter interaction changes dramatically following, I take my own personal favourite form of matter, with intensity [1]. Intensity regime of 1010 Wcm-2, is nano-solids, to paint the scenario laid above. sufficient for multi-photon excitation/emission in most systems. At higher intensities, in the regime of 1012 W/cm2, Super-sonic jet cooling methods can be used to make clusters the number of photons absorbed in the multi-photon scheme of atoms [2]. Inert gas atoms like Ar or Xe form nanoclusters is large enough that the electron leaves the bound potential of (typically 1-100 nm) that have solid like intensity. Typically the atom and is ionized. Also, at this point, the light field is the number of atoms in such a cluster varies from few large enough that perturbative physics way of accounting the thousand to a few million. A 2 nm Argon nano-solid with laser-matter interaction becomes increasingly difficult. about 40,000 atoms is strongly ionized by 800 nm light Beyond 1014 Wcm-2, the intensity is large enough that most pulses of 1016 Wcm-2 intensity. The average charge/atom matter will be multiply ionized. Given that most lasers at becomes more than 7 [3]. The electron temperature in this these intensities operate at infra-red wavelengths, in this nano-plasma is about a keV. Such a strongly ionised plasma, regime the transient distortion of the atomic field (see Fig.1) leads to the coulomb explosion of ions. The atoms on the by the laser field spans enough time for the electrons to surface are accelerated to 1 MeV. Can the electrons tunnel out of the atoms and thus tunnel ionization becomes recombine with the ions after the laser pulse is gone in such more dominant channel of ionization. By about 1015 Wcm-2, strongly ionized plasma? To understand this question let us the potential distortion is large enough that the electron does look into the details of electron-ion recombination [4]. not even have to tunnel and the ionization is over-the-barrier ionization where the ionization probability is close to 1 (see Fig.1). Beyond this intensity quantum mechanics of the bound states of the electron becomes less important, the physics of the interaction of the free electron with the light in the presence of the atomic ion becomes increasingly dominant.

Figure 2: An atomic ion either captures a free electron (electron-ion recombination) or an electron bound to another atom M (charge transfer) An ion can either pick up a free electron or an electron bound to another atom (see Fig.2). A collisional transfer of electron from an atom/molecule to the ion, is called charge transfer. Figure 1: Atomic ionization at different intensities: Multi- The cross section of this depends on the binding energy of photon, tunnel and over the barrier ionization the electron in the target atom and the projectile ion, the At the atomic unit of intensity with 800nm light, the center of mass energy of the system etc. Typically the cross oscillation energy, as given in eq.1, is about 2 keV. A free sections are in the range in sq. Angstroms (10-16cm2) [5]. A electron released early in the laser pulse (by the time free electron is much harder to be captured. An ion capturing intensity is 1013-1014 Wcm-2; and this can happen early in a the electrons forms a transient scattering or excited state. The pulse of 1016Wcm-2 peak intensity) therefore gains this excess energy that the electron brings in, has to be released energy when it interacts with the field. In one half of the and momentum has to be conserved for the ion to electro-magnetic field cycle, the electron moves away from accommodate the electron in its bound state. Electron-ion the atom and in the next half of the cycle it returns with such combination comes in three favours [4]: Dielectronic energy. This will be akin to electron-ion impact of 2 keV recombination, photo recombination and the three body electrons with a duty cycle of about 2.5 fs. The cross section recombination. The first two processes release the excess for an electron-impact ionization is generally much larger energy as a photon. Three-body recombination is when an than direct photo-ionization cross section. Collisional ion interacts simultaneously with two electrons. In this, one ionization by laser driven electron impact, therefore, electron is captured and the other is scattered out, conserving dominates very soon. Any system therefore becomes a the momentum. All the three processes need electron energy strongly ionized plasma and this transition (from normal to be low [4], for efficient recombination and the three body matter to plasma) is within the pulse time of the laser (<< 20 process also needs large electron density. Most of these or 100 fs). Such a rapid transition to plasma gives little room processes need the electron temperature to be below 2 eV to for the atoms to move. This means that, if the initial phase of be effective and the typical capture times are in few hundred the matter absorbing light is a solid, then we have matter ps [4-6].

Vol.50(3) 16 Physics News

S 9+ several experiments, analysis and simulations, we figured out -V Ar Ar1+ and this is published in several papers. A summary of efforts +V y N x to resolving this puzzle is given here without the details dE 150 eV [5,6,8,9]. D L 10 100 d B Photo peak Figure 3: A Thomson Parabola spectrometer and image. Ions up to Ar9+are seen with Ar8+being the most prominent. The un-deflected spot in the center is marked the photo peak. In laser produced plasmas, the electron temperature is very much larger than 2 eV and the fast ions escape the plasma in a few ps. So the fast ions don't capture electrons and one only talks about either high energy electrons or high energy ions, when it comes to intense laser matter studies. In some recent Figure 4: A) The time resolved measurement of the photo experiments, we built a new ion diagnostics instrument, shows that a large fraction of the signal is due to atoms. called Thomson Parabola spectrometer [3,7]. In its basic B) Measurement of the atom to ion fraction showed that near design, the instrument is quite simple. A well collimated 100% neutralisation is possible with cluster densities of beam of ions, say travelling on the z axis, are deflected by 1013 cm-3. Normal charge transfer (n=1) is inadequate to parallel electric and magnetic fields. If the electric field explain. deflects the ion in the x- direction, the magnetic fields are set When the ultra-short pulse is focused, there are about few to deflect the ion in the y-direction. After the ions pass thousand clusters in the focal volume. By the laser through the field, they strike on a charge particle detector interaction, strong ionization results and large number of which gives the information as to how far along the x and y electrons leave the focal volume (in 100-200 fs) much before the particle is deflected from the z- axis. The ion splat the ions move (clusters marked red in Fig.5). This dense position depends on the charge of the ion and the energy of burst of electrons collide with the clusters that are present the ion. An uncharged particle, like a photon or a neutral beyond the focal volume (clusters marked green in Fig.5). A atom would continue to traverse in the z direction (un- keV electron impinging on an nm cluster, will be stopped in deflected). Generally, whenever there are hot electrons and the cluster. The energy deposited is expended in the form of atoms, there will be photo-emission. Hot plasmas emit light ionization and excitation of the Ar atoms. When a solid that extends to x-ray regime depending on the electron density nanoparticle is ionized, the free electron can be temperature. Most of the peers in this field and in all the captured by the atoms in the cluster and lead to clusters with years of work, made a major assumption that the un- excited atoms. In Rydberg excited states, electrons are in the deflected spot is largely due to the photons. We too assumed higher, n principal quantum number states. Their atom size the signal on the un-deflected spot to be due to photons but 2 wanted to check to ensure that we understand the newly built (r) scales as n . Since charge transfer cross-section () scales as r2, the charge transfer with the Rydberg excites atoms spectrometer well. The strategy we employed was fairly 4 simple. Apply very high deflection fields (E and B in Fig. 3) scales as n . This means that, if an Ar atom is excited to so that all the charged particles are deflected to distances n=10, the charge transfer cross section scales as 10,000[10]. beyond the size of the detector. The only signal then on the Our elaborate experiments with electron emission spectra, detector would be due to the un-deflected particles. An photo emission spectra [11] of the atoms in the focal volume arrival time of the particles should covey what this provided strong evidence that more than 30% of the clusters uncharged particles are. Photons should arrive in few ns and in the immediate vicinity of the laser focal volume are if it was atoms, they would take a micro second to reach (see excited at a mean n=9. This Rydberg excited sheath, can Fig. 4a). To our surprise, we found that the signal at the truly enhance charge transfer. An ion that traverses this center is dominantly due to the atoms and their energy Rydberg excited sheath, under goes very efficient charge extended to same as ions, upto 1 MeV. If the fast atom transfer. It is no wonder that all the 7 or 8 electron that the fraction (compared to ions) was few percent it was not ion can capture are available and ions are converted to difficult to understand by normal simple charge transfer neutral atoms in a fraction of cm depending on the cluster reactions mentioned above. But the fraction ranged from density. We coined this the Enhanced Charge Transfer by 40% to almost 100% depending on the experimental Rydberg Excited Clusters (ECTREC) [5]. conditions [5]. This was out of normal. In short what we We showed that the Rydberg excited sheath can be observed was, the short pulse laser impinges on Ar40000 influenced by laser polarisation [8]. Since electrons are 280000+ clusters and ionizes it to about (Ar40000) in about a ps. driven strongly along the laser polarisation direction, electron The ions explode under the strong Coulomb forces. As the emission is asymmetric and this should bring in an ions move out, with an energy/atom that extends to about 1 asymmetry in the Rydberg excited sheath. This also means MeV, more than 90% of the Ar7+ ions, take back all the that the neutral atoms emission should be asymmetric, with seven electrons and become neutral Ar atom with no loss of the maxima along the laser polarisation and should be kinetic energy. How is the laser plasma bringing about this tuneable with the laser polarisation. Experiments, that tremendous sequence of electron-ion recombination? After measured the electron, ion and neutral atom anisotropy

17 Vol.50(3) Physics News confirmed all the expectations and proved the validity of ahead of the peak of the pulse. This is strong enough to ECTREC in generating the neutral atoms. This showed that ionize and the plasma formed by this pulse contrast in called laser plasma provided a strong tool for doing some novel a pre-plasma. The main pulse actually interacts on the matter atomic physics experiments. Ultrashort burst of electrons that modified by the pre-plasma. We found that the pulse contrast lasers brings out, can be used in well manipulated systems to plays a very big role in the neutralisation along with the laser convert laser plasma ion accelerator to energetic neutral atom focus (see Fig.6). By controlling the laser focus, the pre- acceleration schemes. plasma can be controlled and the temperature of the pre- plasma can be varied. Suitably lowering the electron temperature of the pre-plasma, has a very strong influence on C- dB the electron-ion recombination. A loose focus, will decrease O- the pre-pulse intensity and subsequently lowers the electron density. This decreases the electron-ion recombination rate. Tight focus will increase the electron density but also Photo peak increases the electron temperature. So, an optimum laser focus is important to keep the electron temperature low, dE electron density high and also efficient for ion acceleration. Experimentally, we see that under the optimal conditions, the Figure 5: Schematic of the Enhanced Charge Transfer by fraction of neutralisation can be made as large as 80% (see Rydberg Excited Clusters (ECTREC) that explains the Fig.6). The computations corroborate the measurements well observations. [12]. But this strategy works only for the converting heavier ions like Cu, C or O to neutrals. Can we play with the We further questioned if a fast atom can be further converted electron ion recombination physics to also influence proton to negative ions using such strong electron-ion neutralisation? recombination processes? This would also prove the whole scheme. Inert gas atoms like Ar have no positive electron affinity, but one can make clusters with atoms that have positive electron affinity. To test, this we generated CO2 clusters. As anticipated, the positive ions of C and O could be converted to C- and O- [9] (see Fig.5b). Their ion energies and emission angles directly correlated with that of positive ions, in conformity with the ECTREC scheme. Are such novel charge reduction physics unique to special systems like nano-clusters? Can the conventional laser plasma experiments, that have been done over the past 30 years with solid slab targets, also show such schemes? We did find a method in which neutral atoms can be accelerated efficiently with solid substrates [12]. Unlike in the clusters experiments, there are no unionized atoms in the neighbourhood of the laser focus for the charge transfer physics to be used. We can only use the electron-ion recombination physics to convert the ions to fast neural atoms. Generally, the electron temperature in laser interaction with matter become as large as tens of keV. Even the bulk temperature of the coldest electrons raises to a few hundred eV and remains that high for a few picoseconds after the laser incidence. That is why the electron-ion recombination is weak and nobody saw neutral atom emission in the laser-solid interaction. We however found a route to the electron-ion recombination. After considerable experimentation, we found that the laser focus has an answer. Before, we discuss this path, let me introduce the concept of laser contrast. When laser pulses are measured and reported, as say 20 fs, we refer to the full- width-at-half-maxima of a Gaussian shaped peak. But, how far the intensity raises in the very initial portion of the pulse is important especially when the peak intensity is so high. Figure 6: a) Neutralisation of Cu ions from solids can be For example, how low is the intensity of the pulse 10 ps enhanced with a change in laser focal waist and is shown for away from the peak of the pulse? Is it 10-5 or 10-8 of the peak experiments done at two different laser energies. A decrease intensity? When the peak intensity is say 1018 Wcm-2, a 10-5 in electron temperature (b) and increase in electron density contrast pulse will have an intensity of 1013 Wcm-2 at 10 ps scale length (c), enables neutralisation to reach 80%.

Vol.50(3) 18 Physics News

Since proton acceleration is much better with high contrast the electrostatic potential at the target surface comes down pulses, we switched the experiments to high contrast pulses by the movement of the protons (see Fig.8). Everybody (10-9 intensity in the pre-pulse) and with high peak intensity, thought that was the end of the story. Electrons that leave in 1018 Wcm-2. But such high contrast and high intensity pulses the beginning are the fast electrons, protons that follow are pose an experimental problem with the time-of-flight the fast ions and this explains the ion acceleration. One technique. These pulses produce a strong electro-magnetic conjecture that could explain the electron-ion recombination pulse (EMP) noise that lasts for about 100ns, which is larger was that some low energy electrons move out with the ions. A fraction of these electrons have the same velocity as the Gate on No gate H+ H+ - ions and they co-propagate to the detector. The fact that the H low energy electrons take longer to escape the surface fields C3+ C2+ give the ions head start for these low energy electrons to O2+ C+ catch up with the ions. In the co-propagation frame, the O+ H0 electron-ion collision temperature is low and also that the ions have a long time of interaction, not just the picoseconds that the ion take to leave the plasma focal area. How do we Figure 7: A gate voltage on Thompson parabola see if the protons are indeed getting neutralised mm away measurement can selectively measure ions and neutral of a from the target? particular species. The measurement increases signal-to- noise ratio and a weak H- signal is clearly visible. than the time taken for the protons to reach the detector. So arrival time measurements, cannot be used for the neutral atom detection. Even if the noise is managed, how to know that we are only detecting protons or Hydrogen atoms? This is important since the high contrast high intensity pulse can also accelerate Carbon and Oxygen ions. We developed a new detection scheme, where we pulse the detector with high voltage pulse so that the detector is ON only in a desired time window (see Fig.7). Since the detector is a Micro-channel plates (MCP) coupled to a phosphor, particles that strike the detector appear as a space resolved scintillation detected by a camera. The scintillation is for ms and the camera can detect it easily without any influence of the EMP noise. Once the detection scheme is made [7], we looked for the neutral Hydrogen atoms. Surprisingly copious emission of high energy Hydrogen atoms was clearly visible even without any additional efforts [13]. The experimental conditions are same as what anybody else has done over the past 30 years, just that we made the diagnostic better for neutral Hydrogen atom. Even more surprisingly, the Hydrogen atom fraction was about 60 to 80% of the fraction of protons, in the spectral range of 10 to 100 keV (see Fig.8). Everybody had wrongly thought that the signal was due to the scatted or x- ray light when it was actually dominated by neutral atoms.

Of course, the question is, why are the fast neutral atoms Figure 8: Neutralisation of protons in solids with high generated? The conditions are not at all favourable to any of intensity and high contrast pulse. The schematic above shows the charge reduction reactions. Charge transfer by the that low energy electrons co-propagate with the high energy background accounts to only 10% of the neutral fraction protons and undergo electron-ion recombination. seen. 90% of the signal came by electron-ion recombination. But the electron temperature in the plasma is too high. So We devised an ion pin hole imaging technique to see this. where is the charge transfer happening? If one looks into the The detector is set to image the laser focus with an aperture details of ion acceleration, the dominant mechanism is by at suitable magnification. The neutral atoms reaching the target normal sheath acceleration. When the light is incident, detector will coincide with the laser focal spot. Now, we if electrons that are easy to move out of surface are first driven we apply a magnetic field along the path to the detector, a out of the target into the vacuum. This result is a strong neutral atom formed in the magnetic field would not follow a quasi-static electric field and the atoms at the surface of the straight path to the detector (see Fig.9). The protons are bent target are pulled out of the target surface along the direction at some small angle by the magnetic field. If the protons are of the electron motion. Hydrogen atoms being the lightest are converted to the Hydrogen atom after the protons enter the the first to go. Following the protons, some of the electrons magnetic field, they will continue to move in the bent that are rescattering back on the target surface also escape as direction rather than normal to the target. In the pinhole

19 Vol.50(3) Physics News image of the neutral, these Hydrogen atoms formed inside Fast neutral atoms are very important for lithography, plasma the magnetic field would appear deflected (see Fig.9). We heating and diagnostics in Tokomaks etc. Normal methods can simulate the ion motion and assuming that neutralisation lead to excessive beam loss due to straggling in collisional occurs as at an arbitrary distances in the magnetic field, we detachment technique. The laser plasma method is novel new can simulate neutral atom trajectory as seen by the detector alternative that is compact and efficient. The ion emission in [13]. solids are quasi-directional low emittance beams normal to the target. Since neutrals are generated from the ions they share the same emittance characteristic and so it should be better for some of these applications. Summary The intense laser field is still in its infancy even though the laser is 60! The ability of create short burst of energy poses very new features of interaction physics. With the intensity ladder, we are now well into the electromagnetic interaction

in the relativistic regime. Newer scales to reach include Figure 9: Pin-hole imaging of neutrals. Neutrals formed in regimes where protons become relativistic and vacuum the external magnetic fields appear away from the center and breaks down to reveal QED effects. Every new intensity are seen as a distortion in the neutral spot. The distortion regime, nature of the matter which interacts have shown varies smoothly with the magnetic field and can be surprises in physics and advances in technology. GeV simulated. acceleration in a mm or ability to generate tunable, tens of We measure the distortion in the neutral spot as a function of MeV coherent laser beams are demonstrated. Advances in the field and correlate that with the simulations. Thus we can laser technologies are threatening to make these prove that a substantial fraction of the neutralisation is demonstration into a robust high repetition rate, easily occurring far away from the target and that the con- operable and controllable systems. Clearly there are miles to propagating scheme is the method for electron-ion go and heights to reach, hopefully we will be able to keep up recombination. the strides in India and the support for this activity continues. We can estimate that such a scheme should also produce Acknowledgements - reasonable fraction of fast H ions. We indeed see the I thank Rajeev, M. Dalui and S. Tata who worked with me - anticipated H with our improved scheme of pin hole imaging for their doctoral theses on these topics. I also thank all the gated ion spectrometry (see Fig.7). As it turns out, this is the present and past group members of UPHILL and TIFR first laboratory experiment where a direct gas phase Hyderabad and many collaborators who have co-authored the sequential two electron recombination is seen to turn a papers on these topics. Lastly thanks to TIFR and the Dept. - proton to H . So, indeed novel atomic physics experiments of Atomic Energy for supporting this activity. on the electron-ion recombination or charge transfer physics occurs in the laser produced plasmas, where short bright References burst of low energy electrons are generated. This quest has 1. Atoms in Intense Laser Fields by C. J. Joachain, N. J. Kylstra, added two more particles to the laser based acceleration R. M. Potvliege, (Cambridge University Press, 2012) schemes: neutral atoms and negative ions. 2. V. Kumarappan, et.al., Phys. Rev. Lett. 87, 085005 (2001) 3. R. Rajeev et. al., Rev. Sci. Instrum. 82, 083303 (2011) These results also impact many experiments in laser plasma 4. V.P. Krainov, et.al., J. Exp. Theor. Phys. 103, 35 (2006) interaction carried out thus far. If it is presumed that 5. R. Rajeev et. al., Nature Physics. 9 185 (2013) electrons are the only negatively charged particles emanating 6. T. Madhu Trivikram et. al., Phys. Rev. Lett. 111, 143401 from the laser plasma, it would be problematic in electron (2013) spectrometry. Electron spectroscopy is done by bending the 7. Sheroy Tata, et. al., Rev. Sci. Instrum. 88, 083305 (2017) negatively charged particle by a magnet and the angle of 8. R. Rajeev et. al., New. J. Phys. 17 230033 (2015) bending or the position at which the electron come in the 9. R. Rajeev et. al., New J. Phys. 15 043036 (2013) dispersion plane is generally taken to measure the energy. 10. K. Harth, et. al., Z. Phys. D 14, 149 (1989) But a 10 keV H- would come at the same position as 10 MeV 11. R. Rajeev et. al., Phy. Rev. A 87, 053201 (2013) electron. This would lead to a wrong diagnosis of the 12. Malay Dalui et. al., Sci. Reports 7 3871 (2017) 13. Sheroy Tata, et. al., Phys. Rev. Lett, 121 134801 (2018) electron spectrum [14], especially in the high energy cut-off 14. Angana Mondal et.al., AIP Advances, 9, 025115 (2019) region.

Vol.50(3) 20 Physics News

Nonlinear Optics: Looking back - looking forward!

Kailash Rustagi E-mail: [email protected]

Kailash Rustagi, presently a retired scientist, has been affiliated with TIFR, BARC, RRCAT and IIT Bombay in the past. His research interests span interaction of light with matter in all forms and properties of semiconductors. He was awarded "Distinguished alumni award " of the TIFR Alumni Association. He has been closely associated with various IPA activities. He is passionate about modernizing optics education and writes for popular Science journals.

Abstract This article summarizes my personal journey through the field of non-linear optics since the 1960s, highlighting the progress in the field, and the influences from developments in materials science and other related areas.

Everyone knows that the first laser lased on 16 May 1960 American handshake symbol on that laser possibly in 1969. just over 60 years ago. The first experiment [1] on nonlinear For my Ph.D. thesis, I calculated nonlinear optical response of optics was done a year after and surprisingly it used a semiconductors. One of the key results that I found was that a commercial ruby laser. Triggered by this experiment, new resonances can appear in nonlinear response when Bloembergen and his team produced a classic paper [2] laying relaxation processes are included because relaxation terms can the foundations of nonlinear optics in 1962! So far reaching thwart a cancellation between different contributions to a was the impact of this paper that even now it is cited over process [5]. A resonance was predicted in four wave mixing hundred times each year! About 3 years later I joined TIFR, generating 2− from  and  as the frequency fresh from atomic energy training school. I got my first difference − goes to zero. This was actually observed introduction to nonlinear optics by Prof. Jha to whom I had [6,7] several years later and they constructed independently a been directed by Prof. Udgaonkar for a possible placement in theory which was an approximate version of my calculation! I the Theory Group. This article presents a personal view of the also calculated second order susceptibility of In Sb for which progress of the field since then with a view to show case how Kane’s k.p model was an excellent representation of the band the field has grown and how it keeps expanding its reach! structure at the fundamental gap and found that the Included in this narrative are some comments on influences by nonlinearity would vanish unless spin orbit coupling was fully progress in material science and information technology. included! So clearly nonlinear spectroscopy could provide new information about the band structure (in this case about The first papers I read were Prof. Jha’s pioneering works [3] the linear k dispersion at the zone center.) The experiment was on nonlinear optical properties of metals wherein he calculated however, difficult and information it gave did not seem much both the surface and the volume terms in reflected second sought after at that time. harmonic from a metal surface. The surface contribution later became an important tool in surface science. My perspective Before moving further, I should note that Bloembergen’s those days was that the dielectric function gives us famous book [8] on nonlinear optics came to TIFR a year or information about the band structure of solids so nonlinear two after I joined. In one of the papers I saw a reference to susceptibility will give more information. A very nice review some lecture notes by PN Butcher [9]. So, I sent a reprint of optical properties of solids had just appeared [4]. It is request card! I was delighted when I received by airmail a perhaps worth recalling that it was not clear to me as to how nicely packed copy of Butcher’s book! The joy was slightly one would do any experiments without a tunable laser. Dye dimmed when I got a bill for USD 6 or 7 about half for the lasers became common only later and even that would be book and half for airmail. This was not small money then – it useless because they worked only in the visible or near IR. was the cost of 6 restaurant meals! Anyway, in TIFR at that time there were no lasers so the This time in nonlinear optics was the exploratory age. connection to experiments was only through literature. The Operating even the few commercial lasers then available was first laser in India was a homemade semiconductor laser an expert’s job. Most experiments in nonlinear optics operating at liquid N2 temperature at BARC but that power happened in labs who built their lasers. The surprisingly large was much too small to do any nonlinear optics. IIT Kanpur response for four wave mixing mentioned above was for had a ruby laser which was most likely gifted to it under some example seen first by CKN Patel and group and showcased Indo American plan, because I remember seeing Indian their CO2 laser [10].

21 Vol.50(3) Physics News

My first interaction with experimentalists in nonlinear optics Stokes Raman Scattering), HORSES (Higher‐order Raman came with my first post doc at Orsay in Jacques Ducuing’s Spectral Excitation Studies) etc. Optical Parametric group and it came with a bang – I was the only theoretician in Oscillators were developed to hugely expand the frequency the group. Going from a group dominated by high energy spectrum covered by coherent optical sources, and Laser theorists to a group entirely composed of experimentalists was plasma studies started with the aim of obtaining controlled actually quite exciting. For a few months I shared office with fusion energy. George Bret who was then in the process of forming his now In January 1974, I got to attend a Gordon research conference famous company called Quantel! When he left, Andre at Santa Barbara. (Fig. 1). It was thrilling to present our work Mysyrowicz joined me in the office and introduced me to to people like Bloembergen, Armstrong, Lax, and Wolff. excitons! Andre and Francois (Pradere) were measuring two- Many of my contemporaries who have made very important photon absorption spectra in Cu2O with one fixed frequency contributions were also present among whom I recall source from a laser and the other, a tunable one, from a flash Yablonovitz, Alfano, Shapiro, Clair Max, Penzkofer, lamp and monochromator combination. In a clever Levenson, and my friend Mysyrowicz. I shared my room with arrangement they exploited the fact that the flash lamp pulse Mark Levenson who was then a postdoc with Bloembergen. has a much longer duration than the Q switched laser. So, it Apart from very interesting talks in the conference I heard was easy to measure the loss of flash lamp photons caused by for the first time about how Doppler broadening can be simultaneous absorption of one photon from flashlamp and eliminated by two Photon spectroscopy by making the two one from the laser. During our lunch discussions I learnt many photons come from the opposite direction so that the net things that experimentalists worry about. At that time the new momentum transfer is zero. The initial idea came from the thing was time-resolved spectroscopy made possible by the Soviet Union [12] and its important was soon demonstrated by progress in picosecond lasers. Most people made their own 3 Phys. Rev. Letters in the same issue [13-15]. Hansch and because I think the commercial one became available only Shallow’s paper [16] on laser cooling of a gas of atoms later. With the short pulse lasers research in nonlinear optical appeared soon after. materials also changed course. Since it was realised that large On my return to India, I joined the laser section of BARC with nonlinearities could also come from orientation of polar the hope of maximizing my interaction with experimentalists. molecules but that would not work for shorter pulses. To have Because theorists can move more easily from one problem to nonlinear optical materials that would work for even shorter the other compared to experimentalist I would have preferred pulses it was important to look for materials which have large to remain unattached and free to interact with all groups. nonlinearities of electronic origin. It was in this background However, wiser advice about the local attitudes persuaded me that Ducuing suggested that we look for optical nonlinearities to start a nonlinear optics experimental group with two in organic molecules. We extended a simple model of  brilliant youngsters Shrikant Mehendale and Pradeep Kumar electrons in conjugated organic molecules which treated them Gupta. My choice of theoretical problems now changed to as free electrons in a box. Kuhn [11] had shown that such those problems which experimentalists can also do. One of simple models explain linear optical spectra of the conjugated these yielded a very interesting result. We found that with molecules rather well. With a simple calculation we were able nitrogen substitution in some organic dyes one could enhance to show that optical nonlinearities in conjugated molecules the optical nonlinearity by up to two orders of magnitude. I increased very rapidly with the conjugation length. The tried very hard to persuade some of the chemists to synthesize calculation was so simple and the results so startling that these molecules which we could then try to measure. Ducuing decided to wait for an experimental verification Unfortunately, my persuasion was inadequate! Disappointed, before publishing. With available resources it was important we decided to publish it and move on [17]. Experimental work to find a material which was available in a solid (high was actually done about a decade later by the Dupont group, concentration of molecules) form with good optical quality. who verified our prediction in detail [18]. Pushed by a survival instinct, I started looking for materials in which the experiments could be done. With hardly any Research on second order nonlinear effects had by then moved background in organic materials, I looked for books on to making more and more efficient devices. In early 1980’s, organic crystals and found an excellent book on organic the huge lasers used in fusion research were converted to the semiconductors that listed many conjugated organic third harmonic using seriously large KDP crystals stacked molecules from which I selected a few that looked promising. together! One of the new challenges now is to convert Then for each molecule I went to chemical abstracts on recent relatively low power diode lasers to second harmonic. One of activities and found that beta carotene could be made into a the key contributors to this is an idea already clearly stated in transparent glass. To form the glass one had to just melt beta the famous ABDP paper. Idea was to compensate for phase carotene at 184 degrees. Armed with this knowledge Jean-Paul mismatch due to dispersion by periodically changing the sign Hermann was able to form the first sample of the beautiful of nonlinear coupling. This quasi phase matching technique orange colored glass almost immediately using his great skill lends itself admirably to making compact nonlinear optical and melting beta carotene powder between two microscope waveguide devices which combine virtues of large interaction slides heated by a Bunsen burner! length and large nonlinear coupling coefficient [19] Meanwhile elsewhere exciting things were happening. Optical During a sabbatical at École Polytechnique in 1983 Christos bistability, self-focusing and optical solitons made their debut, Flytzanis, gave me a book on composite materials -electrical nonlinear optical spectroscopy was expanding fast with a and optical properties with an excellent review of the two rapidly expanding list of acronyms CARS (Coherent Anti- complementary effective medium theories of the optical

Vol.50(3) 22 Physics News

Figure 1: Participants of the 1974 Gordon Research Conference on Non Linear Optics and Lasers response. The main idea was that if the inhomogeneity scale Stuttgart where I got an intense 2 day tutorial on clusters and is much smaller than the wavelength of light, scattering can be small particles from an old friend T Patrick Martin who had neglected and the material behaves effectively like made seminal contributions to the physics of clusters and homogeneous material but with effective dielectric function small particles. This was extremely useful because it filled me calculated in an ”average” manner. The two theories differ in with all kinds of information about what happened so far with the kind of structure they assume. Our intention was to extend a fat collection of reprints for further study. Remember e-mail these theories to the nonlinear regime. I made a simple and internet were still in far future! extension of the Maxwell Garnett theory to a case in which the I should also mention here that there was a great buzz around dielectric constant of one of the constituents was nonlinear and optical phase conjugation or time reversal of wavefront by dependent on local intensity. To see how much difference it degenerate four wave mixing at that time. It was thought that would make I needed a concrete example. A quick search in it would play an important role in laser technology as well as the Physics Abstracts for optical properties of composite in optical computing. While it has remained an important materials gave me the paper of Jain and Lind [20] which phenomenon in material investigations the original appear only a year ago. Looking through all the references and expectations have not quite materialized. through citations of some of the references I formed a fairly comprehensive view of the then present knowledge. Next Back in BARC, we started looking at the problem of excitons came analysis of the experimental results in light of our theory in small particles. It was then that Selvakumar Nair joined us and we planned some experiments to answer some of the as the first theoretician in the group and we progressed with a questions raised. Samples used by Jain and Lind’s work were calculation of quantum size effects in nanocrystals. commercial colour glass filters with CdSSe nanoparticles Experimental results were, however, slow in coming because made by Corning while most labs in Europe used similar we needed first to make a picosecond laser which could Schott glass filters. With careful experiments Daniel Ricard operate trouble-free for a longer spell of time, and since we found that response of the samples that he used was much were using Nd: glass rather than Nd:YAG which rods were slower than that measured by Jain and Lind. It was only after not being supplied to us, the rep rate of the laser was painfully we could get a sample of corning glass filters and all the slow. In those early days in RRCAT, our library was also experiments were repeated by Roussignol and Ricard, that the rather limited and so it was in TIFR that I saw several papers picture became clear and showed that there was much that on the buckyball or C60. I decided to explore this because this needed further investigation [21]. was a conjugate  electron system without any C-H bonds. So far we had assumed that the properties of the two materials From the nonlinear optical perspective it seemed interesting remains same as in bulk. But of course, as the size of because one of the problems with organic nonlinear materials inclusions reduces quantum size effects have to kick in. Since was that the residual absorption below the HOMO-LUMO gap I had decided to explore these, I made a rather hectic trip to was attributed phonon overtones which should reduce

23 Vol.50(3) Physics News substantially if the highest phonon frequency available was The next buzz is surely coming from the increasing reach and small which meant that hydrogen should be avoided. We sophistication of nonlinear spectroscopy, and quantum synthesized C60 and purified it to conduct our first experiments nonlinear optics in the context of quantum computing and on nonlinear optics trying to measure harmonic generation quantum sensing. Integrated quantum photonics will most from a surface coated with thin film of C60.With the available likely result in development of components and devices which resources this was not successful and our theoretical work will find many uses, some foreseen, many unexpected. later showed C molecule did not have a high non-linearity 60 To conclude, nonlinear optics looks forward to its 60th because of its 3D character. Lesson learnt was that lower birthday as healthy, vigorous and growing and shows no sign dimensional  electron systems were still more interesting. of getting tired anytime soon! But, C60 was a promising material for optical limiting and we obtained many interesting results on that. Acknowledgements Another great buzz in nonlinear optics around mid 80’s was My sincere thanks to Prof. Sudhanshu Jha for initiating me related to the self-electrooptic effect device using quantum into this field and to all my collaborators and friends for many wells which showed optical bistable behavior at modest shared efforts and joys! intensities and raised hope of realizing optical computers with References advantages of parallel computing architectures and faster speeds. Although optical computers using spatial light 1. C. Franken EP, Hill AE, Peters CE, Weinreich G. Phys. Rev. modulators were demonstrated, it is fair to say that despite Lett. 7, 118 (1961) sporadic successes the field did not make a headway simply 2. Armstrong JA, Bloembergen N, Ducuing J, Pershan PS. Phys because it could not compete with the progress in electronic Rev. 127, 1918 (1962) digital devices. But as they say, good work never goes waste, 3. Jha SS. Phys. Rev. Lett. 15, 412 (1965) spatial light modulators are much in use for many applications 4. Phillips JC, Solid State Physics Vol. 18, pp. 55-164 (1966). Academic Press. and with artificial intelligence a revival of optical computing 5. Rustagi KC. Phys. Rev. B. 2, 4053 (1970) techniques seems possible.[22-23] We were also very keen at 6. Yuen SY, Wolff PA. Appl. Phys. Lett. 40, 457 (1982) this time to start a serious activity on nonlinearities and lasing 7. Rustagi KC. Appl. Phys. Lett. 44, 1121 (1984) in quantum wells. Unfortunately, because of subcritical 8. N. Bloembergen, Nonlinear Optics, Benjamin (1965) funding available, we had to postpone that for several years. 9. P. N. Butcher: Nonlinear Optical Phenomena, Ohio State With some money we started a pulsed laser deposition activity University Engineering, Columbus 1965 for growing epitaxial layers of ZnSe on GaAs. This was later 10. Patel CKN, Slusher RE, Fleury PA. Phys. Rev. Lett. 17, 1011 used very effectively for a large number of important (1966) contributions on ZnO- a really promising wide gap material. 11. Kuhn H. J. Chem. Phys. 17, 1198 (1949) 12. Vasilenko LS, Chebotaev VP, Shishaev AV, J. Exp Theor. Phys. With the advent of ultrashort pulses nonlinear optics became Lett. 12, 113 (1970) a central part of laser technology because one needed 13. Biraben F, Cagnac B, Grynberg G Phys. Rev. Lett. 32, 643 nonlinear optical effects to produce as well as characterize the (1974) pulses. With fs-pulses several of these techniques like 14. Pritchard D, Apt J, Ducas, TW, Phys. Rev. Lett. 32, 641 (1974) frequency-resolved optical gating (FROG) and spectral phase 15. Levenson MD, Bloembergen N. Phys. Rev. Lett. 32, 645 (1974) interferometry for direct electric-field reconstruction 16. Hänsch TW, Schawlow AL, Optics Communications. 13, 68- (SPIDER) became regular tools in the lab. (Looking at the 69 (1975) various acronyms, the NLO community seems to have many 17. Mehendale SC, Rustagi KC. Opt. Comm. 28, 359, (1979) wildlife lovers!) 18. Stevenson SH, Donald DS, Meredith GR. MRS Online Proceedings Library Archive 109, 103 (1987) The last 30 years have been seen great progress in the field. 19. For a recent review see, Rustagi KC in Guided wave optics and Two of the Nobel prizes have been awarded for work related photonic devices eds S Bhadra and A Ghatak, CRC press (2013) to femtosecond pulses. These important stories are not 20. Jain RK, Lind RC. JOSA. 73(5), 647-653 (1983) repeated here. Laser cooling of atoms and ions have shown 21. Roussignol P, Ricard D, Rustagi KC, Flytzanis C. Optics that “there is a strong interplay between the dynamics of communications. 55(2), 143-148 (1985) internal and external degrees of freedom” as Cohen-Tannoudji 22. G W Burr, Nature 569, 199-200 (2019) said in his Nobel Prize lecture. Now there are several 23. Feldmann J, Youngblood N, Wright CD, Bhaskaran H, Pernice competent labs in the country doing coherent nonlinear optics WH. Nature. 569, 208-214 (2019). experiments on laser cooled atoms.

Vol.50(3) 24 Physics News

Raman Spectroscopy- A Potential Tool for Biomedical Diagnosis

Rashmi Shrivastava and Shovan Kumar Majumder

Laser Biomedical Applications Division, Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India E-mail: [email protected] Dr. Rashmi Shrivastava received her PhD degree in Biotechnology from Indian Institute of Technology-Kharagpur (IIT-KGP), Kharagpur, India. She joined Raja Ramanna Centre for Advanced Technology (RRCAT), Indore as K.S. Krishnan Research Associate and was subsequently appointed as Scientific Officer. Her current research interests include spectroscopy for in vitro diagnostics, nanoparticle based theranostics and optogenetics.

Dr. Shovan Kumar Majumder heads the Laser Biomedical Applications Division at Raja Ramanna Centre for Advanced Technology (RRCAT). His group primarily focuses on development and evaluation of photonics based technologies for biomedical diagnoses. Extensive research from his group not only led to the development of a variety of new techniques required for advancing the applications of optical spectroscopy and imaging for biomedical diagnosis, but also made it possible to perform non-invasive screening of oral neoplasia and automated diagnosis of tuberculosis in resource-limited clinical settings.

Abstract Recently, optical spectroscopy has garnered significant attention in the field of biomedical diagnosis. Among various optical spectroscopic techniques, Raman spectroscopy being a highly molecular specific and sensitive technique has inherent potential to enable identification of diseased states based on tissue and body fluid analysis. However, weak Raman signals from biological samples and a large fluorescence background has posed a major roadblock in successful implementation of the technique in real clinical situation. Our experience of in-lab experiments with ex-vivo tissue samples led to development of a field-deployable portable Raman spectroscopy system along with an artifact free Raman probe for tissue analysis. Successful clinical studies for in vivo diagnosis of oral cavity neoplasia established Raman spectroscopy as a rapid diagnostic tool. Development of various depth sensitive analysis approaches enhanced the applicability of Raman spectroscopy for tissue analysis. Further, a novel sensitive technique was developed for in-vitro detection of disease specific trace biomarkers in body fluids. A glimpse of these developments undertaken at RRCAT is presented in this article.

Introduction major limitations are, inherently weak Raman signals from biological samples and a large fluorescence background Optical spectroscopy, in recent times has gained great originating from it. significance in the field of biomedical applications [1,2]. Raman spectroscopy among other spectroscopic techniques Over the years, different Raman spectroscopic techniques like fluorescence and diffuse reflectance, posses a leading have been developed to overcome these limitations and make edge because of its intrinsic ability to identify molecular it more suitable for on-field biomedical applications. The specific signatures with high sensitivity [3]. earliest of these is Fourier transform (FT)-Raman spectroscopy [4], that measures Raman spectra with high Raman spectroscopy basically, probes the vibrational energy signal-to-noise ratio (S/N) and minimal fluorescence levels of the molecules which are reflected as specific interference and has been applied on many ex vivo samples. wavenumber shifts (peaks) in the Raman spectrum of the However, practical in vivo applicability is mostly hindered chemical bonds or bond groups comprising the molecule. typically because of long integration times and bulky Several diseases are associated with intrinsic biochemical instrumentation. Another approach to enhance Raman signals changes in cells or body fluids. Identifying these specific is by using Ultraviolet resonance Raman (UVRR) changes through Raman spectroscopy has significant potential spectroscopy to target specific bio- molecules by selecting in disease diagnosis as well as prognosis. excitation wavelength at their resonance [4]. However, the Despite holding significant potential, Raman spectroscopy is high excitation intensities and mutagenicity of UV light make associated with some limitations, especially when samplesare it unsuitable for in vivo use. Considering all the above facts, of biological origin, whether cells, tissues or body fluids. Two Near-infrared (NIR) dispersive Raman spectroscopy, in which

25 Vol.50(3) Physics News

NIR excitation minimizes fluorescence and absorption by scattered light from the sample is transmitted by a proper tissue, has been the technique of choice for several in vivo combination of optics to the distal end of the collection arm applications [4]. for detection. The technology of the developed Raman probe has been transferred to the industry. Instrumentation

Early attempts at measuring Raman spectra of biological tissue were difficult because of the fluorescent nature of tissue and the limitations of sources and detectors. Over the past several years, stable diode lasers emitting high power NIR light have been developed. These lasers have been proven to be ideal sources for clinical use because of their portable nature and availability at wavelengths in the “optical window” of tissue, such as 785 nm and 830 nm, at which they generate minimal background fluorescence while penetrating fairly deeply into tissue. In the case of detectors, spectrographs designed specifically for NIR-Raman spectroscopy are now readily available. Silicon-based CCD detectors have made great strides as well. Back-illuminated, deep-depletion CCDs that avoid etaloning, allow high-resolution Raman spectra to be recorded with short (<5s) integration times. Recent developments in cooling technology have led to the Figure 1: The Raman probe development of thermoelectrically cooled, sensitive detectors Raman Signal Processing that can be operated at temperatures below –80 °C. Another, significant component of a Raman system intended for non- Post acquisition of raw spectra from tissues and other invasive in-situ tissue analysis is a suitable Raman probe. The biological samples, a significant challenge is the proper fiber-optic probes that generally make use of fused silica- extraction of the Raman signals from it. Raman signal based optical fibers have assisted in development of portable extraction is mostly achieved using system calibration, Raman spectroscopy systems. However, the fibers themselves fluorescence background subtraction, and noise smoothing. have a Raman signal, so this signal must be minimized using Over the years several mathematical approaches have evolved appropriate filters. To mitigate this issue an alternate approach for background correction and extraction of rather weak tissue employing a free space Raman probe was developed. Raman signals from the acquired raw spectra. Polynomial fitting [5-11] is the method of choice mostly in extraction of Raman Probe tissue Raman signatures. Single polynomial fitting and multi- The extremely weak Raman intensity that is associated with polynomial fitting (ModPoly) are a couple of approaches the order of magnitude larger Rayleigh background swamping explored initially. Further, Zhao et al.[10] improved the the measured Raman signal makes it difficult to develop an algorithm by addition of peak removal steps during iterative appropriate tissue Raman probe. The commercially available fitting. This considerably improved the background Raman probes are not well suited for applications related to correction, especially in cases of spectra with high noise. tissue analysis, because they introduce several artifacts which These algorithms are simple and highly effective in interfere with the Raman signatures appearing in the background reduction but have one major shortcoming. These fingerprint region of the tissue Raman spectra. This may lead methods are sensitive to the choice of the spectral region used to confusion and erroneous interpretation of the tissue Raman in the fit. To obliviate this concern we have developed a novel signature. To overcome these hindrances and acquire good background subtraction algorithm, called Range Independent quality Raman signal, a hand-held Raman probe capable of Algorithm (RIA) [12]. RIA is based upon iterative smoothing measuring artifact-free tissue Raman spectra was designed of the measured raw Raman spectrum. The method uses a and developed. The design minimizes the fiber and optics- model based on modified iterative smoothing of the measured generated Raman artifacts, optimizes collection efficiency, Raman spectrum in such a manner that the high-frequency and has resulted in highly efficient Raman probe that is Raman peaks are gradually eliminated finally leaving the capable of collecting high-quality spectral data with an underlying broad baseline which can be subtracted from the acquisition time of only few seconds (Figure 1). The raw spectrum to yield the true Raman signal. Briefly, the performance of the probe was found to be better than the two isolated region of interest of the raw spectra is extended commercially available Raman probes (Make: Visionex and linearly on both ends and two Gaussian peaks are added to the InPhotonics) in measuring the artifact free Raman signal from extended linear part on the two sides, the whole of the biological samples. The capability of the probe for measuring extended spectrum is then subjected to modified iterative artifact-free Raman spectra is evident from the comparable smoothing based on moving point averaging (MPA), which is quality of the measured Raman signal to that of measured with a zero-order Savitzky-Golay filter. It is equivalent to a low- an open-air bench-top Raman system. The developed probe pass filter which tends to filter out the high-frequency comprises of two arms at right angle to each other (Figure1). components of a signal leaving the low-frequency baseline One arm is for excitation light and the other for collection of intact. The iterative smoothening continues until the Raman signals from the tissue sample. The collected Raman amplitudes of the Gaussian peaks added to the raw data are

Vol.50(3) 26 Physics News recovered on subtracting the intermediate background (at that deliver excitation light (~80 mW) to the target tissue. The light round of iteration) from the raw data (Figure 2). The range- was delivered to the tissue and the Raman signal collected independence of RIA is due to two reasons: first, it takes care back using a Raman probe. An imaging spectrograph coupled of the end points of a spectrum by tending to anchor it to their with a thermoelectrically cooled (-70ºC), back-illuminated, original position with the use of linear extrapolation combined deep-depletion charge-coupled-device (CCD) camera was with MPA. used to acquire signal. A comprehensive evaluation of the efficacy of in-vivo Raman spectroscopy for differential detection of oral lesions in the oral cavity was performed. The developed system was deployed at Tata Memorial Hospital (TMH) for collection of Raman spectral data from oral cavity, that included both normal and abnormal regions. The in- vivo clinical study was performed on the patients undergoing routine medical examination of the oral cavity at the Out Patient Department (OPD) of TMH. The patients included in the study had no history of malignancy or dysplasia. Visual examination and interpretation of the suspected areas by an experienced physician examining the patient was the basis of choosing areas of spectral acquisition in the oral cavity. Patients having gone through any prior treatment like surgery, chemotherapy or radio therapy for earlier cancers or with recurrences were excluded from the study.

Diagnostic algorithm Figure 2: Pictorial representation of the Range independent algorithm (RIA). (a) The raw Raman spectrum (red )truncated spectrum in the range of interest (green), (offset) truncated spectrum with linear extension (magenta),data with added Gaussian peaks(blue);(b) the input data (blue), intermediate background (magenta), intermediate recovered peaks (green), recovery threshold of Gaussian peaks (dash); (c) Input data (blue), final background (red), final recovered peaks (green) along with threshold peak intensity of added Gaussian peaks achieved (light blue dash)(d) the recovered Raman spectrum in region of interest. Second, since it is based on iterative smoothing which keeps Figure 3: A schematic diagram of the portable Raman on flattening the high frequency Raman bands riding a broad spectroscopy set-up for in vivo applications envelop to finally mimic the low-frequency contour of this Successful implementation of Raman spectroscopy for tissue envelop, the slope of the computed background curves diagnostic applications essentially requires a suitable remains the same in the common spectral region irrespective diagnostic algorithm to discriminate between various of the fitting range used for background subtraction. On the categories of tissues based on the recorded Raman spectra of basis of thus devleoped tools, we explored the potential of tissues. A probability based multivariate statistical algorithm Raman spectroscopy for in vivo diagnosis. capable of simultaneously classifying spectral data into Diagnostic Applications multiple classes was employed [13]. The algorithm firstly extracts diagnostically relevant spectral information through One of the major advantages of using Raman spectroscopy for nonlinear maximum representation and discrimination feature in vivo diagnosis is inherent in its ability to provide a non (MRDF) [14] and then perform probabilistic classification via invasive as well as non-destructive procedure for rapid sparse multinomial logistic regression (SMLR) [15].The diagnosis. Oral cancer is a major public health problem in MRDF reduces dimensionally of the high-dimensional India, as it is the most prevalent type of cancer in the country. spectral data and results in a set of few non-linear output Large scale screening and rapid diagnosis forms an essential features that contained the maximum class discriminatory tool in its management. We have explored Raman information. The optimal number of these features are decided spectroscopy as a diagnostic tool for oral cancer. by employing a cross-validation procedure and are identified Oral Cancer Diagnosis as the values that yields the least misclassification error using nearest mean classifier [13]. The optimal number of features Driven by the immense need for a non-invasive tool for usually varied from 15 to 20 depending on the classification screening of oral cavity for suspected neoplasia, Raman task. This optimal number of output nonlinear features was spectroscopy was explored. A portable Near Infra-red Raman used as input to the SMLR for subsequent classification. The spectroscopy set up was developed for such applications in SMLR classifier is trained with different values of the clinical settings (Figure 3). A 785 nm diode laser was used to regularization parameter λ (varying between 1 and 1e-5) and

27 Vol.50(3) Physics News the optimum value of λ is chosen to be the one that gives the stage is to be moved towards the objective to probe different highest classification accuracy. The major advantage of using depths, this is a major limitation in its application in field. To the the nonlinear MRDF-SMLR based diagnostic algorithm overcome this shortcoming another approach of off-confocal [13] is its ability to both compress large amount of data detection was employed. The off-confocal detection is obtained with each Raman spectrum as well as retain only the achieved by moving the tip of the detector fiber from the focus diagnostically relevant portions of the spectra in this of the Raman collection objective by moving it along the axis compression. of the objective [23]. This approach enabled signal collection from probing depths that are beyond the reach of conventional Performance in oral cancer diagnosis confocal Raman. In addition, the approach eliminates the need In a clinical situation, requirement may be of two broad types, for any adjustment at sample arm and the illumination light is i) Screening and ii) Screening and diagnosis. The developed fixed at the sample surface. Off-confocal Raman spectroscopy probability based multi-class diagnostic algorithm was could achieve a probing depth of around 800 µm. Further, a applied on the set of Raman spectra corresponding to the novel scheme was developed to enhance the probing depths different oral tissue sites. With respect to histology as the gold beyond 800 µm. Here, 3 identical axicons are used for standard, the diagnostic algorithm was found to provide leave- excitation of the target in form of a hollow conic section. one-subject-out cross validation accuracy of up to ~89% in Raman scattered light is collected through same conic section classifying the oral tissue spectra into the different tissue (solid). Since, a cone-shell excitation and a cone detection is categories. In case of binary classification, the algorithm used, the technique is named cone-shell Raman spectroscopy resulted in a sensitivity and a specificity of 94% each in (CSRS) [24]. Probing depths achieved using CSRS is around delineating the normal from all the abnormal oral tissue 2-3 mm. Probing depths in the system can be varied by varying spectra belonging to oral squamous cell carcinoma (OSCC), the separation between the axicons in the excitation arm. The oral submucosal fibrosis (OSMF) and leukoplakia (OLK) system provides advantage of non-contact depth sensitive pooled together [16].Thus, Raman spectroscopy along with an measurements in tissues. Despite several advantages of CSRS, appropriate diagnostic algorithm proved to provide real-time, a major limitation is the poorer collection efficiency of an non-invasive diagnosis of malignant and potentially malignant axicon as compared to a lens or microscope objective at the lesions of oral cavity in a clinical setting. collection arm. Inverse-spatially offset Raman (i- SORS) was then developed to probe millimeter depths and with better Depth sensitive Raman spectroscopy signal to noise ratio than CSRS. The technique is a variant of Conventional Raman spectroscopy has proved to be important spatially-offset Raman spectroscopy (SORS) where Raman tool for non-invasive in-vivo disease diagnosis. However, signals are collected from regions away from the point of there lies an inherent shortcoming that the information illumination. In case of i-SORS, the illumination is in the form provided by such analysis is limited to the surface of the tissue. of concentric rings of variable diameter to probe different A biological tissue is a multilayered structure with different depths and collection is always done at the centre of these sub-surface layers possessing different biochemical and rings [25]. Here, axicon is used for only illumination but not morphological information. As a tissue transforms from for collection and thereby eliminating the shortcomings of normal to diseased state, the morphological and biochemical CSRS. i-SORS was found to be advantageous in deeper changes that each of the sub-surface layers undergo may be probing of samples where subsurface layer is low Raman quite different from that of the superficial layers. active and is surrounded by a scattering matrix of high Raman Conventional Raman spectroscopy misses out these layer active material. The i-SORS was used to isolate signals of specific changes, as the Raman signal at a given point on the formalin fixed tissue embedded in paraffin blocks. In view of surface of an interrogated tissue is volume integrated over the the advantages provided by i-SORS, a clinically deployable sub-surface depths. A depth sensitive Raman spectroscopy, compact system was envisioned [25]. However, non- thus, could provide a better picture and identification of availability of small size axicons posed hindrance in disease specific changes corresponding to various layers in the implementation of this technique in clinical settings. To tissue [17-19]. Over the years, various approaches have been overcome this limitation an axicon free approach was explored to untangle the Raman signatures from different explored. In this approach, ring illumination and point depths in a tissue. Of these confocal and spatially-offset collection is achieved through use of multi-mode optical Raman detection techniques are found to be most appropriate fibers. The diameter of the illumination is varied by varying for depth sensitive Raman signal acquisition. While the former the angle of optical fiber with respect to the axis of the measures the un-scattered or ballistic Raman photons near the focusing lens. Backscattered Raman light from sample is point of illumination, the later measures the diffused Raman collected through a central fiber. The system could achieve a photons generated away from the point of illumination. penetration depth of ~3-4 mm [26]. Confocal detection is found suitable for analysis of shallow Multimodal Approach depths (ranging from few tens to hundreds of micrometers) and spatially-offset Raman detection allows deeper probing i) Raman spectroscopy and optical coherence depths upto several millimeters. In recent times, confocal tomography (OCT) Raman spectroscopy has been reported to distinguish cancerous tissues from normal ones [20-22]. However, a Recently, there is increasing interest in combining Raman clinically acceptable confocal Raman system was still lacking. spectroscopy (RS) with optical coherence tomography (OCT) Generally, in confocal Raman spectroscopy set-up the sample for tissue analysis. This multimodal approach has benefit of

Vol.50(3) 28 Physics News providing morphological information using OCT as well as analytes. Surface enhanced Raman spectroscopy (SERS) is biochemical information from RS. Most of the dual- mode RS- one such approach which has been immensely pursued by the OCT systems reported in literature suffered from a major scientific community [34-36]. However, clinical application drawback that though OCT provides information from various of SERS is limited by lack of reproducibility as well as large sub-surface depths, Raman signal, that is co-registered with spatial variability due to random distribution of ‘hot spots’ OCT image from a point, is a volume integrated signal. To [37,38]. Optimization of SERS substrates has been the major overcome this drawback, a depth sensitive Raman system was combined with OCT. In this scheme the Raman beam from a confocal based depth sensitive Raman system was co-aligned with the OCT beam in the sample arm of a time –domain OCT (TD-OCT) system. Thus developed RS-OCT system could distinguish epithelial and stromal layers of resected mucosal tissue samples both morphological and biochemical properties [27]. ii) Optical trapping- Raman spectroscopy Besides diagnostic applications, as described above, Raman spectroscopy based basic research is also being pursued. Major focus here lies on single cell optical trapping- Raman spectroscopy (OT-RS). Integration of Raman spectroscopy with optical trapping has opened a new frontier of analysis of Figure 4: Schematic illustration of the mechanism of Nano- molecular changes at single cell level. Raman spectra from a Trap Enhanced Raman Spectroscopy (NTERS) with time, trapped single cell can be collected from specific cellular showing the process of formation of the nanoparticle regions like nucleus, membrane, cytoplasm etc. to provide aggregates and corresponding enhancement in Raman signal. even organelle specific information. Such spatially resolved Raman spectroscopy of an optically trapped red blood cells focus of most of the efforts to address these issues. At (RBC) using 785 nm Raman excitation beam and 1064 nm RRCAT, a novel technique called Nano-trap enhanced Raman dual traps was used to decipher the mechanism of laser power spectroscopy (NTERS) was developed which addresses the dependent de-oxygenation of haemoglobin [28]. Further, long long-standing challenges of SERS. In NTERS, the ‘hot spot’ term hypoxia was shown to result in decrease in oxygen is formed by drying up a microlitre drop of liquid containing affinity of haemoglobin in RBC [29] and a shape the analyte and the gold nanoparticle colloid in the focal transformation to echinocytic RBC resulted in increase in volume of the Raman excitation laser [39]. Raman signal is oxygen affinity of haemoglobin [30]. Polarised Raman then collected from these hot spots containing the analyte spectroscopy of trapped RBCs suggested that the haemoglobin molecules trapped within the nanoparticle aggregates (Figure molecules are orderly arranged such that the heme planes are 4). NTERS shows around 2 orders of magnitude higher signal preferentially orientated parallel to the equatorial plane of the enhancement as well as better reproducibility as compared to RBCs [31]. OT-RS was used as a tool for label free SERS. NTERS is a simple technique that does not require any identification of eukaryotic cell-cycle stages based on specialized substrate. characteristic Raman peak of DNA at 783 cm-1 [32] Interestingly, the effect of commonly used organophosphate Other technological developments pesticide (chloropyrifos) on RBCs was also studied using OT- RS. The study suggested that chloropyrifos results in suppression of the cellular anti-oxidants and enhances the oxidative stress in cells [33]. Raman spectroscopy for trace detection of analytes in body fluids Besides tissue analysis for diagnosis, RS has tremendous application in in- vitro disease diagnosis based on analysis of body fluids. Generally, the diseased condition is reflected in the compositional changes of the analytes present in the body fluids like blood and urine. Some of the current diagnostic methods involve tedious sample preparations along with being chemical and skilled labor intensive. Being molecular specific, RS has immense potential in detection and quantification of analytes in body fluids. However, weak Raman signals from analytes of biological importance as well as presence of analytes in traces poses major challenge in application of RS for body fluid analysis. Several approaches Figure 5: a) OncoDiagnoScope, b) OncoVision and c) were reported for enhancement of Raman signals from these TuBerculoScope developed at RRCAT

29 Vol.50(3) Physics News

Apart from the above mentioned studies, we explored 5. C. A. Lieber and A. Mahadevan-Jansen, Appl. Spectrosc. 57, diagnosis on the basis of other optical spectroscopy and 1363 (2003) imaging techniques. On the basis of these studies, we have 6. A. Mahadevan-Jansen et al., Photochem. Photobiol. 68, 123 developed optical spectroscopy and imaging based point-of- (1998) 7. V. Mazet et al., Chemom. Intell. Lab. Syst. 76, 121(2005) care diagnostic devices. OncoDiagnoScope (Figure 5) is a 8. G. Schulze et al., Appl. Spectrosc. 59, 545 (2005) point-of-care optical device for non-invasive screening of oral 9. T. J. Vickers et al., Appl. Spectrosc. 55, 389 (2001) cavity for abnormalities leading to cancer. It provides 10. J. Zhao et al., Appl. Spectrosc. 61, 1225 (2007) advantage of rapid and non-invasive screening of oral cavity. 11. M. N. Leger and A. G. Ryder, Appl. Spectrosc, 60, 182(2006) It performs fluorescence and diffuse reflectance spectroscopy 12. H. Krishna et al., J. Raman Spec. 43,1884 (2012) based characterization of oral tissue into normal and abnormal 13. S. K. Majumder et al., Appl. Spectrosc. 61, 548(2007) types (tissues transforming towards cancer development). 14. A. Talukdar, PhD Thesis, Carnegie Mellon University, Pennsylvania (1999) Another imaging device ‘OncoVision’ is developed to identify 15. B. Krishnapuram et al., IEEE Trans. Pattern Anal. Mach. Intell. oral tissue transforming towards malignancy based on the 27, 957(2005) differences in the fluorescence emission characteristics from 16. H. Krishna et al., J.Biophotonics 7, 690 (2014) an abnormal and normal tissue type. A device for diagnosis of 17. C. Zhu et al. J. Biomed. Opt. 8, 237 (2003) ubiquitous disease tuberculosis was also developed. 18. R. A. Schwarz et al., Opt. Lett. 30,1159 (2005) TuBerculoScope is a portable and affordable fluorescence 19. R. A. Schwarz et al., Appl.Opt. 47, 825 (2008) 20. J. Choi et al., Bioploymers 77, 264 (2005) imaging device that is developed for rapid detection of 21. C. A. Lieber et al., J. Biomed. Opt. 13, 024013 (2008) tuberculosis causing bacteria in dye-stained sputum smears 22. C. A. Lieber et al., Lasers Surg. Med. 40, 461 (2008) (Figure 5). These technologies are transferred to the industry 23. K. M. Khan et al., J. Opt. 18, 095301 (2016) and are available for the on-field applications. 24. K. M. Khan et al., J. Biophotonics 8, 889 (2015) 25. K. M. Khan et al., J. Biophotonics 9, 879 (2016) Acknowledgement 26. K. M. Khan et al., J. Biophotonics 12, 0140 (2019) The authors would like to thank all the concerned members of 27. K. M. Khan et al., J. Biophotonics 7, 77 (2014) the Laser Biomedical Applications Division who have active 28. S. Ahlawat et al., Appl. Phys. Lett. 103, 183704 (2013) 29. A. Chowdhury and R. Dasgupta, Appl. Opt. 56, 439 (2017) and significant contributions in the various activities described 30. A. Chowdhury et al., J. Biomed. Opt. 22, 105006 (2017) in this article. 31. S. Ahlawat et al., J. Biomed. Opt. 19, 087002 (2014) References 32. S. Ahlawat et al., Analyst 141, 1339 (2016) 33. Y. Singh et al., J. Biophotonics 12, e201800246 (2019) 1. Q. S. Wan et al., Tumor Biol. 1 (2017) 34. W. R. Premasiri et al., J. Phys. Chem. B. 116, 9376 (2012) DOI:10.1177/1010428317717984 35. A. Bonifacio et al., Anal. Bioanal. Chem. 407, 8265 (2015) 2. H. Ahm et al., Appl. Spect. Rev. 53,264 (2017) 36. Y. S. Huh et al., Microfluid. Nanofluid. 6, 285 (2009) 3. S. K. Majumder et al., Biomed. Opt. 13, 054009 (2008) 37. Z. Liu et al., Adv. Mater. 26, 2431 (2014) 4. A. Mahadevan-Jansen, Raman spectroscopy: from benchtop to 38. Y. J. Oh et al., Adv. Mater. 24, 2234 (2012) bedside, Biomedical Photonics Handbookby T. Vo-Dinh (CRC 39. S. B. Dutta et al., Anal. Chem. 91, 3555 (2019) Press, Washington DC, 2003)

Vol.50(3) 30 Physics News

Understanding Features of Weakly Bound Nuclei

Vivek Vijay Parkar Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai – 400085, India E-mail: [email protected] Vivek Vijay Parkar is presently working as a Scientific Officer in Nuclear Physics Division, BARC, Mumbai. His research interests are in the area of experimental nuclear physics in general and reaction dynamics in weakly bound nuclei in particular. He is a recipient of INSA Young Scientist Award and is an associate member of INYAS and MASc. For his research contributions with weakly bound projectiles, he was awarded the IPA Aswini Kumar Rath Memorial Award in Nuclear Physics. .

Abstract Weakly bound nuclei are characterized by predominant cluster structures and low separation energies. These nuclei can break very easily in the field of another nucleus and hence alter the reaction mechanism. Various reaction mechanisms like elastic scattering, transfer / breakup and fusion around Coulomb barrier energies with stable weakly bound 6Li, 7Li and 9Be projectiles on several targets have been studied. In this article, highlights of fusion studies with these stable weakly bound projectiles have been given. The fusion can be sub divided into two parts: complete and incomplete fusion. It is observed that the loss (suppression) in complete fusion is correlated with measured incomplete fusion. Systematic comparisons of incomplete fusion cross sections between weakly and strongly bound projectiles and a strong correlation with  binding energy has been established. These studies have also highlighted the strong role of incomplete fusion on inclusive  cross sections.

Introduction can break into two or more constituent nuclei / clusters with very low separation energies around 3 MeV or less. Few of the Basic nuclear physics research aims at understanding the stable weakly bound nuclei that I have used for the study are properties of nuclei and their interaction. The thirst for 6Li, 7Li and 9Be having lowest separation energies as 1.47, producing new elements away from the line of stability and understand their structural properties remains the major 2.47 and 1.57 MeV respectively and also all having +x motivation in this area. Line of stability is shown in black cluster structure. Another distinctive feature is the strongly squares in Fig. 1, which gives the configuration of protons and bound nuclei have few bound states below the continuum neutrons in a stable nucleus which exists in nature [1-3]. Away while in weakly bound nuclei, continuum is very close to the from the line of stability, neutron rich or neutron deficient ground state. Fig. 2 shows the difference between the two nuclei till the neutron and proton drip lines (defined as the categories. lines on the chart of nuclides that delineate the limits of how many protons and neutrons can be bound within the atomic nucleus i.e. beyond which the binding energies will become negative) are predicted, some of which were experimentally observed. The super heavy element production is also one of the very important milestone under investigation. There are also wide range of applications of these radioactive nuclei in healthcare, industry, agriculture, safety etc. With the advent of new technologies, the worldwide nuclear physics community is involved in producing and accelerating new species of nuclei in which there are big technological challenges. With the new species of nuclei, many novel features hitherto unexplored can be studied through the study of nuclear reactions and spectroscopy. Strongly bound vs Weakly bound nuclei There can be two classes of nuclei: Strongly bound and weakly bound. Typically, strongly bound nuclei are having separation energies around 8 MeV. It means that on the average 8 MeV Figure 1: Nuclear Landscape (Original picture taken from : is required to break this nucleus. While weakly bound nuclei https://www.physics.uoguelph.ca/nuclear-structure).

31 Vol.50(3) Physics News

Typical nuclear reactions with weakly bound projectile can cause them to break up before reaching the fusion barrier. 7Li having +t cluster structure on 208Pb target around Thus, the breakup process may reduce the fusion cross Coulomb barrier energies are shown in Fig. 3. As can be seen, sections, making it difficult in super heavy element formation. variety of nuclear reactions are possible. We find the Alternatively, the extended structure of weakly bound nuclei probability of each process with respect to bombarding energy could in principle induce a large enhancement of fusion. and study the role of projectile cluster structure on various Observations of these effects on fusion have been reaction mechanisms. controversial [5].

Figure 4: (A) Borromean structure and (B) Large size of weakly bound unstable nuclei. In recent years, there has been a tremendous interest, worldwide, in the study of exotic nuclei, principally as the Figure 2: Schematic distinction between strongly and weakly result of the developments of radioactive ion beam (RIB) bound nuclei. facilities [6]. At present, as intensities and beam resolution of RIBs are limited, it is difficult to carry out precision measurements using RIBs. Hence in the last two decades, Mumbai group have given special attention for the studies with stable weakly bound projectiles (WBP) like 6,7Li and 9Be on various targets. Since projectiles with good intensities are available, we could able to do the precise inclusive and exclusive (coincidence) breakup and fusion measurements and understand the role of breakup in reaction dynamics. These studies will built the foundation for next generation RIB induced reactions as similar effect of breakup will be prominent there. We have also extensively studied incomplete fusion (ICF) reactions, in which part of the projectile fuses with the target. There have been similar studies using strongly bound (12,13C, 14N, 16O, 19F, 20Ne) projectiles (SBP). We have reviewed [7] all the literature data recently and built the systematics of ICF with these two classes of nuclei. Experimental Observables

14UD BARC-TIFR Pelletron-Linac accelerator facility, Figure 3: Schematic of typical nuclear reactions with weakly 7 208 Mumbai has been utilised for all the measurements reported bound projectile Li on Pb target. here [8-10]. Pelletron accelerator was installed [8] in the year Special attention in weakly bound nuclei 1988 and subsequently superconducting linac was indigenously developed [9] few years later. This facility can Besides low binding energy and cluster structure, there exists give stable ion beams with very good intensities and is used a long tail in the density distribution of some of these weakly for various basic and applied research studies. The details of bound nuclei in comparison to the strongly bound nuclei. This this facility are given in Ref. [10]. long density tail corresponds to anomalously large size which may be interpreted in terms of halo and borromean structures In order to understand the role of weakly bound projectile on (nuclei comprising three bound components in which any reaction mechanism, we have performed measurements that subsystem of two components is unbound) [4], as shown in can be divided in two categories: (i) Projectile Like Fragments Fig. 4. As can be seen from the figure that the size of 11Li is as (PLF) measurements and (ii) Evaporation Residue (ER) - - comparable to that of 208Pb nucleus owing to the large density ray measurements. 11 distribution in Li. For the study of reaction dynamics, fusion Projectile Like Fragment (PLF) measurements involving such weakly bound nuclei is of interest for super heavy element formation, nucleosynthesis in astrophysics and In a typical inclusive measurement, Projectile Like Fragments energy production in stars. For weakly bound nuclei, the (light charged particles) are measured over a wide angular fusion process is affected by their low binding energy, which range. Silicon surface barrier detector telescopes are generally

Vol.50(3) 32 Physics News used for the charged particle angular distribution shown in Fig. 6. Different bands like elastic 7Li, 6Li, 6He, 4He, measurements. Now days with the availability of highly t, d, p are labelled. For the exclusive (coincidence) study, one segmented large area Si-strip detectors, more detailed singles can choose the particle type (e.g. ) in one detector and see and coincidence measurements are possible [11]. The the correlated particle (e.g. d/t) in other strip/detector segment. experimental setup having both surface barrier detector telescopes and Si strip detector telescopes is shown in Fig. 5. Evaporation Residue (ER) - -ray measurements The signals from the detectors were fed to multichannel pre- The method of ER detection directly or through the γ -rays amplifiers. Pre-amplifier channels were connected to the rest from its deexcitation is widely used in the fusion of the electronics to get the timing and energy information measurements. In nucleus–nucleus collisions at relatively low which were fed to analog to digital converters (ADCs). energies, the residual nuclei formed through CF and/or ICF processes dissipate energy and angular momentum by emitting particles or γ -rays. Therefore, fusion studies have been performed in a large number of experiments using the γ -ray spectroscopy method using both online and/or offline methods. By identifying the γ -lines emanating from the de- excitation of corresponding unstable residual nuclei and from the yield corresponding to these γ -lines, both CF and ICF cross sections can be determined separately. Prompt -ray transitions were detected using the Compton suppressed high purity germanium (HPGe) clover detector array [14] as shown in Fig. 7. Particle--ray coincidence measurements were also performed with this setup with few Si surface barrier detectors inside the chamber.

Figure 5: Si surface barrier and Si-strip detector telescopes used for the charged particle angular distribution measurements.

Figure 7: Compton suppressed clover detector array for online -ray measurements.

Figure 6: Typical 2D particle identification spectra obtained with strip detector telescope for 7Li+112Sn system (reprinted with permission from Ref. 13). The timing information is generally used to generate the `master gate' which triggers the ADCs. We have used VME based data acquisition system which currently handles nearly Figure 8: Low background set-up for offline -ray 500 parameters. Indigenously developed LAMPS (Linux measurements. Advanced MultiParameter System) software [12] was used to process and store the data. Particles were identified using In the offline -ray measurements, target was first irradiated energy loss information in the E and E detectors of for sufficient time and then taken out from the irradiation telescopes. Typical 2D particle identification spectra obtained chamber and placed in front of the low background detection with strip detector telescope for 7Li+112Sn system [13] is set-up consisting of Cu, Cd and Pb shielding to attenuate room

33 Vol.50(3) Physics News

background as shown in Fig. 8. All the  lines associated with A comparative study of available ICF cross sections ICF for various residues were identified following different half-lives. various projectile- target systems involving SBP and WBP as The energy calibration and absolute efficiency of the HPGe a function of incident beam energy in reduced scale is plotted detector and clover array were measured by using a set of in Fig. 11. calibrated radioactive 152Eu, 133Ba, and 241Am sources placed at the same geometry as the target. Typical -ray spectrum obtained in 7Li+124Sn system [15] is shown in Fig. 9. All the -lines corresponding to different residues were identified and labelled. Salient features of fusion measurements Complete and Incomplete Fusion The ER cross sections were compared with the statistical model code PACE2 [16] to get the fraction of measured residues and hence to estimate the CF and ICF cross sections. In general, it was observed that CF cross sections with these weakly bound projectiles are suppressed above and enhanced below the barrier w.r.t. one dimensional barrier penetration model (1DBPM) calculations for total fusion cross sections. The amount of suppression commensurate with the measured ICF cross sections. The suppression of CF represents an indirect method to study about the ICF contribution.

푠푢푝 Figure 10: (Top) Systematics of 퐹퐶퐹 as a function of ZP.ZT Figure 9: Typical -ray spectra obtained in the low for different systems involving 6,7Li, 9Be, and 10,11B projectiles. 7 124 푠푢푝 background set-up for Li+ Sn system (reprinted with The lines are guide to an eye. It is observed that 퐹퐶퐹 remains permission from Ref. 15). constant for a particular projectile and its magnitude increases with decreasing breakup threshold. (Bottom) ICF Systematics of CF suppression and ICF fraction fraction (퐹퐼퐶퐹) for various projectiles as a function of  Mumbai group have done lot of measurements with WBP on separation energy, S. The dashed line is an exponential fit to various targets [15,17-24] looking for the suppression factor the extracted 퐹퐼퐶퐹 showing systematic behaviour with S 푠푢푝 푠푢푝 (퐹퐶퐹 ) and is found that for a particular projectile, 퐹퐶퐹 is (reprinted with permission from Ref. 7). similar; viz., it is around 30% for 6Li and 9Be, 25% for 7Li, It is seen that  for the WBP systems is higher than that for 15% for 10B, and 7% for 11B. Also as all WBP are  cluster ICF 푠푢푝 the SBP systems. The 1DBPM calculations for total fusion nuclei, there is a strong correlation of 퐹퐶퐹 with their  푠푢푝 shown by dotted and solid lines confirm nearly 30% and 10% separation energies (S). In Fig. 10 (Top), 퐹퐶퐹 values are fraction of total fusion for WBP and SBP respectively is plotted as a function of charge product of projectile and target coming from ICF cross sections. It is also seen that the onset (ZP.ZT) [7]. It can be seen from the figure that the CF of ICF in the SBP case occurs at relatively higher energy than suppression for a given projectile is almost independent of 푠푢푝 ICF in WBP systems. More details are given in Ref. [7]. target atomic number. Further, 퐹퐶퐹 values have been found to be correlated with  separation energies of the projectiles Role of ICF in  production and these increase with decreasing S values [7] as shown in From the particle spectra as shown in Fig. 6, we can identify Fig. 10 (Bottom) which also includes data from strongly different particles and also measure their angular distributions bound projectiles having  cluster. and get the integrated production cross sections. Here we

Vol.50(3) 34 Physics News

푁퐶퐹 7 specifically discuss the ICF role in production of  particles. The plot of  푖푛푐푙 with reduced energy for Li projectile In the case of both SBP and WBP systems, it has been systems is shown in Fig. 12. It shows universal behaviour in observed that  production dominates than other particles. We  production for all the targets. The plot also includes the t- have done a systematic survey of this large  production on ICF data which almost explains the  production with 7Li various targets. projectile. Similar plots of non-CF inclusive  cross sections along with ICF cross sections for 6Li, 9Be also show universal behaviour [7]. The limited data set for exclusive breakup measurements [7] suggest that their contribution in inclusive  is very small (around 10-20 %). Comparative studies for 6,7Li projectiles have shown that due to low breakup threshold, higher non-CF inclusive  production is obtained in 6Li induced reactions. Further similar non-CF inclusive  cross sections were also plotted for SBP and RIB induced reactions in Ref. [7]. It is seen that there 퐍퐂퐅 is a characteristic difference observed in  퐢퐧퐜퐥 for these projectile systems, where larger values are seen for RIB compared to the values for stable WBP, which are in turn larger than the values for SBP. Applications of ICF There are large number of applications of ICF process. It Figure 11: ICF cross sections as a function of reduced energy offers access to states at relatively high angular momentum in for systems involving WBP and SBP. Dashed and solid lines heavy-ion induced reactions. Hence, ICF has been utilized in are 1DBPM calculations multiplied by factors 0.3 and 0.1 for several -ray spectroscopic measurements where it leads to WBP and SBP, respectively (reprinted with permission from population of higher spin states with greater selectivity. It can Ref. 7). also be used to populate nuclei in the super heavy region, nuclear cross section data information, in particular for the unstable nuclei. It is also useful tool in nuclear astrophysics for nucleosynthesis studies. The details of these applications are given in Ref. [7]. Summary In order to understand the role of weakly bound nuclei in reaction mechanisms, several experiments with these projectiles on various targets have been performed. In the fusion studies it is observed that the CF cross sections are suppressed above barrier energies w.r.t. one dimensional barrier penetration model calculations for total fusion and this amount of suppression commensurate with ICF cross sections. Also the suppression factor is found to remain constant for a particular projectile and it increases with decreasing the  separation energy of the projectile. Extensive and systematics study of ICF cross sections with systems involving strongly and weakly bound projectiles have been performed. The ICF cross sections with WBP systems are higher than that with SBP systems at all the energies. ICF cross sections with WBP increases at below barrier energies, showing the importance of breakup channel. We have also shown strong correlation of Figure 12: Systematical behaviour of inclusive  production ICF with measured inclusive  cross sections. The  cross sections due to non-CF processes with 7Li projectile as production found to be larger for RIB compared to the values a function of reduced energy. The plot also includes the t- for stable WBP, which are in turn larger than the values for capture/t-ICF data (reprinted with permission from Ref. 7). SBP. Many new studies with upcoming RIBs will be vital in extending these systematics. The yield of evaporation  particles due to the CF contribution can be separated out using the statistical model Acknowledgements predictions and get the non-CF  cross sections. The CF part The author acknowledges Dr.(s) S. Kailas, A. Chatterjee, was estimated from the statistical model calculations using A. Shrivastava, K. Mahata, V. Jha, S. K. Pandit, code PACE2 [16] and non-CF inclusive  production cross K. Ramachandran, S. Santra, R. Palit for their guidance, 퐍퐂퐅 sections ( 퐢퐧퐜퐥) were determined. support and encouragement at various stages during this work.

35 Vol.50(3) Physics News

References 12. https://www.tifr.res.in/~pell/lamps.html 1. https://www.nndc.bnl.gov/ 13. D. Chattopadhyay et al., Phys. Rev. C 98, 014609 (2018) 2. E. Arunan, Current Science, 117, 1962 (2019) 14. R. Palit et al., Nucl. Instrum. Methods Phys. Res. A 680, 3. S. Kailas, Physics News 49, 21 (2019) 90 (2012) 4. J.S. Vaagen et al., Phys. Scr. T88, 209 (2000) 15. V.V. Parkar et al., Phys. Rev. C 97, 014607 (2018) 5. L.F. Canto, P. R. S. Gomes, R. Donangelo, J. Lubian, 16. A.Gavron, Phys. Rev. C 21, 230 (1980) M.S. Hussein, Phys. Rep. 596, 1 (2015) 17. V.V. Parkar et al., Phys. Rev. C 82, 054601 (2010) 6. B.R. Fulton, Journal of Physics: Conference Series 312, 18. V.V. Parkar et al., Phys. Rev. C 98, 014601 (2018) 052001 (2011) 19. H. Kumawat et al., Phys. Rev. C 86, 024607 (2012) 7. V. Jha, V. V. Parkar, S. Kailas, Phys. Rep. 845, 1 (2020) 20. P.K. Rath et al., Phys. Rev. C 79, 051601 (2009) 8. S.S. Kapoor, V. A. Hattangadi, M. S. Bhatia, Indian 21. P.K. Rath et al., Nucl. Phys. A 874, 14 (2012) Journal of Pure and Applied Physics, 27, 623 (1989) 22. A. Shrivastava et al., Phys. Rev. Lett. 103, 232702 (2009) 9. R.G. Pillay, Physics News 48, 11 (2018) 23. C.S. Palshetkar et al., Phys. Rev. C 82, 044608 (2010) 10. https://www.tifr.res.in/~pell 24. P.K. Rath et al., Phys. Rev. C 88, 044617 (2013) 11. S.K. Pandit et al., Phys. Rev. C 93, 061602(R) (2016)

Vol.50(3) 36 Physics News

On the methods of Effective Field Theory

Joydeep Chakrabortty Department of Physics, Indian Institute of Technology Kanpur, Kanpur E-mail: [email protected] Dr. Joydeep Chakrabortty is an Associate Professor at the Department of Physics, IIT Kanpur. His specialization is on Effective Field Theory, Unified Field Theory in the arena of theoretical high energy particle physics. He has been awarded IPA Buti Foundation Award 2018. He has been selected as member of Indian National Young Academy of Science (INYAS) for five years and also an Associate of the Indian Academy of Science, Bangalore, India.

Abstract Effective Field Theory (EFT) can connect the physics of different lengths or energy scales. Thus, by its virtue, it is one of the best-known platforms to handle the current situation in particle physics. It is a tool that can smell the presence of new physics, if any, even without knowing its exact Ultra Violet (UV) complete root. We have developed two scientific packages: (i) CoDEx and (ii) GrIP whose working principles are based on Covariant Derivative Expansion and computation Group Invariant Polynomial respectively. These two programs have automatized the effective operator construction either by integrating out heavy fields or using only the low energy degrees of freedom. These allow us to prepare a platform to use the EFT program to address the ``inverse problem" to identify correct nature of new physics, if any, supported by experimental data. Introduction The emergence of effective operators solely depends on the The interesting physical phenomena involving the sub- characteristics of the integrated out heavy field and how it atomic particles occur over a range of smallest length scales. interacts with the lighter fields. On the other hand, the second Unfortunately, not all of these scales are accessible to method relies on low energy symmetries and on-shell DOFs. contemporary high energy experiments. Thus, we require It is possible to construct a complete set of effective operators conceptually neat ways to pinpoint the effects or implications at every mass dimension (> 4). This construction is completely of physics that occur at ultra-high energy scales or infrared unaware of the possible root of these effective operators, scales much below the resolving power of the most ingenious unlike the earlier method. Here, the accompanying WCs are experimental tools. This is where effective theories come into free parameters. Now, we can think of the multiple extensions the picture. of the Standard Model (SM) by different choices of IR DOFs. Effective Field Theory (EFT) provides a platform to realize It is indeed possible that these minimally extended models are the high scale theories in terms of the low energy the effective ones, originating from some UV complete theory. renormalizable Lagrangian accompanied by effective This paves the way to construct the BSM EFT, in the same operators. These effective operators can be computed in two line of thoughts as of the SM EFT. At this point, we can utilize methods: (1) by integrating out the heavy fields from the UV the GI-EFT method to construct the operator basis for any theory - Wilsonian EFT, (2) by constructing the group higher mass dimension, e.g., dimension-6 for all possible invariant operators using the knowledge of low energy extensions we are interested in. This enables us to generalize symmetries and on-shell infrared (IR) degrees of freedom the concept and methodology of SM-EFT. Now we can start (DOF) - Group Invariant EFT (GI-EFT). As we are working with any one of our favourite UV complete models, and some in (3+1) space-time dimension, these effective operators have selective heavy particles can be integrated out to reduce it into mass dimensions greater than 4. These higher mass a BSM EFT scenario. dimensional operators are accompanied by the respective The choice of IR DOFs apart from the SM ones is decided on Wilson Coefficients (WCs). the fact that which UV models we want to engage for the The Wilsonian and Group Invariant EFT methods comparative analysis. Interesting choices of the extra IR DOF complement each other. In the first case, we would be able to can be right-handed neutrino, charged scalar, dark matter reduce a UV complete beyond Standard Model (BSM) into a particles of any spin or even a complete multiplet, e.g., extra relatively low energy theory that contains a fewer IR DOF. Higgs doublet, complex SU(2) triplet scalars, etc. It is This daughter theory will contain effective operators, which interesting to note that, GI-EFT EFT method enables us to are constituted of the IR DOF, apart from the renormalizable write the complete set of operators at a given mass dimension. ones. The WCs that are generated in the process are the Thus, the operators, generated using the Wilsonian EFT functions of the UV model parameters. approach, will always be a subset of that complete basis, and

37 Vol.50(3) Physics News this is true for any BSM EFT at a suitable matching scale. and mass}. We construct the CoDEx -representation of the heavy field using the `defineHeavyFields' 퐂퐨퐃퐄퐱: bridging BSM physics and SMEFT function. CoDEx is a Mathematica package to compute the WCs of 2. Using ϕ,we build the Lagrangian for CoDEx: We effective operators up to mass dimension-6 [1] by integrating only write the SM fields and heavy field interaction out heavy particles from the theory. This program automatize terms, and heavy field self-interaction terms. The SM the Wilsonian approach of EFT. The package integrates out Lagrangian and heavy field kinetic (derivative and propagators of the heavy field(s) from the tree as well as 1- mass) terms are constructed internally. loop diagrams of that BSM theory. It can integrate out heavy 3. Next, we need to define the symmetry generators. We 1 field(s) of spin-0, and 1 (scalars, fermions and gauge name the model `ewrss' (user may define any name). 2 The name is used to identify the associated symmetry bosons). It generates the WCs for two bases: "Warsaw" and generators in the following step. In our case the "SILH". With very basic information (gauge quantum symmetry generators are trivial because the heavy numbers and spin quantum number(s)) about the heavy field is singlet under the SM gauge symmetry. This field(s), CoDEx computes the WCs at the high scale (mass of step can be skipped when user needs the tree-level the heavy field(s)) and is capable of performing the WCs only. renormalisation group evolution (RGEs) of these operators in 4. Next and the final step to compute effective operators the complete "Warsaw" basis, using the anomalous and associated WCs. We write the WCs in the dimension matrix, to the electroweak scale. We, for now, rely Warsaw basis (Table 1): on the status of the present-day precision data and restrict ourselves up to dimension-6 effective operators. Table 1: Real Singlet Scalar -- Warsaw Basis

퐶표퐷퐸푥 is based on the ``Covariant Derivative Expansion" Dimension- Wilson Coefficient method discussed in [2] (and references therein). Each and 6 operators every detail about this package can be found in [3]. The main 3 2 2 3 functions based on which this program works are captured in OH ca μ ca κλϕ ca κ κ 6 − 2 4 − 4 − 2 2 Table 3. Here, we demonstrate the working methodology of 6mϕ 32π mϕ 2mϕ 192π mϕ 2 2 2 2 퐶표퐷퐸푥 with an explicit example. OHD ca μ ca λϕ 2ca caκμ 2 6 − 2 4 − 4 − 2 4 96π mϕ 8π mϕ mϕ 48π mϕ κ2 + 2 2 96π mϕ

These WCs are defined at the heavy field mass scale `m'. These WCs can be runned down to the electro-weak scale, using the 퐶표퐷퐸푥 -function RGFlow (see details in [1]). We can compute the WCs in “SILH" basis as well. We write the WCs in the SILH basis (Table 2).

Table 2: Real Singlet Scalar -- SILH Basis

Dimension- Wilson Coefficient 6 operators 3 2 2 3 O6 푐 휇 푐푎휅휆휙 푐 휅 휅 푎 − − 푎 − 6푚6 32휋2푚4 2푚4 192휋2푚2 Figure 1: Flow chart depicting the working principle of 휙 휙 휙 휙 퐂퐨퐃퐄퐱. 2 2 2 2 OH 푐푎 휇 푐푎 휆휙 푐푎 푐푎휅휇 Models with one BSM field 2 6 + 2 4 + 4 − 2 4 192휋 푚휙 16휋 푚휙 푚휙 96휋 푚휙 Consider extending the Standard Model with a SM gauge 휅2 singlet real scalar (ϕ). We write the most general Lagrangian: + 192휋2푚2 1 2 1 휙 ℒ = ℒ + (휕 휙) − 푚2 휙2 − 푐 |퐻|2휙 ℬ풮ℳ 풮ℳ 2 휇 2 휙 푎 1 1 1 In the current version 퐂퐨퐃퐄퐱 -1.0.0, the package can generate − 휅|퐻|2휙2 − 휇휙3 − 휆 휙4(1) 2 3! 4! 휙 all the WCs resulting from integrating out the heavy field Now we implement the mentioned BSM theory in CoDEx propagators from the tree and 1-loop processes. In future and demonstrate the workflow step-by-step: versions, we shall incorporate contributions from processes 1. We have to provide the properties of the heavy field where the SM fields also appear with heavy fields in the loop first. Format for heavy field properties: {name, no. of resulting in mixed heavy-light WCs (see [4] and references components, the SM gauge quantum numbers, spin therein).

Vol.50(3) 38 Physics News

Table 3: Main functions provided by CoDEx. See orders of the polynomial. Some of the most common Documentation for details [5] intricacies are associated with the non-compact nature of the spacetime symmetry group, the inclusion of the covariant derivative operator as a quantum field thus introducing dependence between the monomials based on the equations of motion of fields and integrations by part. Other issues may be related to the incorporation of some external discrete or global symmetry on part of the field content and demanding their conservation or violation by a specific amount.

GrIP: the road to Lagrangian The field pf particle physics phenomenology is replete with a myriad of models that have a huge variety of degrees of freedom transforming under several non-trivial groups. More specifically, phenomenological models are comprised of quantum fields that describe various particles and these Figure 2: Flowchart illustrating the working mechanism of transform under various representations of the spacetime GrIP. symmetry as well as the internal symmetry of the fields which is described by connected compact groups. High energy Keeping these in mind we have developed an automated experiments seek to understand the dynamic interplay Mathematica package GrIP which computes Group between these fields, i.e., the observations are based around Invariant Polynomials at different mass dimensions for any well-defined interactions of the fields among themselves and model with only the particle content and their transformation with each other. The necessary information of these properties under various symmetry groups as the input. The interactions is encoded in the Lagrangian density of the model. rudimentary details and instructions regarding the installation The Lagrangian density is nothing but a polynomial and usage of the package can be found at [6]. GrIP provides constituted of the quantum fields whose order is determined several functions for manipulating the polynomial output and by an appropriate parameter such as the mass dimension of the to filter out specific operators, e.g. baryon and lepton number fields and each monomial is an invariant under all the violating operators which form a popular research avenue. The symmetries, i.e., the spacetime as well as the internal program is flexible in its use so that one can use it for symmetry of the model. While construction of a small number supersymmetric as well non-supersymmetric model building. of such invariant monomials for a small number of fields and The essential mathematics on which GrIP is based on is quite basic symmetry configurations may not appear to be a related to the linear representation of groups in terms of tall order, the complexity increases multi-fold when we have characters and the orthogonality of those characters which a large number of degrees of freedom and we go to higher allows one to obtain the Hilbert Series [7,8] of the field

39 Vol.50(3) Physics News variables from which we can extract the invariant in Fig. 2 and the important functions have been described in polynomials. GrIP also provides the user with the provision Tables 4 and 5. Table 4: Display functions of GrIP and their details. Let us give an illustration using the fermions of the Standard Model of particle physics. We have listed them in Table 6 and we have highlighted their transformation properties under the internal symmetry group SU(3)C  SU(2)L  U(1)Y. After carefully defining the input file and loading the package by following the instructions given in [9]. We can see what types of output the different functions listed in Table 5 generate. Keeping in mind that 퓓 represents the covariant derivative, we can see that the output of the above command contains only the fermion kinetic terms and a term composed solely of 퓓 but since it is a total derivative term it is usually not included in the Lagrangian. The preceding lines show how to use a GrIP function to search for operators of lowest mass dimension that violate baryon number by +1 unit and lepton number by -1 unit. The search runs from mass dimension 1 up to mass dimension 10.

Table 6: Quantum numbers of single generation SM fermions Table 5: GrIP functions for generating and manipulating under the symmetry groups. Here, l and r denote the chirality, polynomial output. i.e., the left or right handedness of the field.

SM SU(3)C SU(2)L U(1)Y Fermions Ql 3 2 1/6 Ur 3 1 2/3 Dr 3 1 -1/3 Ll 1 2 -1/2 er 1 1 -1 The preceding sequence of code shows how to save the output within a variable and subsequently how to compute the number of independent terms in the polynomial. Summary Effective Field Theory (EFT) paves the direction to adjudge the compatibility of the experimental data and the proposed new theories. As it allows one to bring different new scenarios into same footing, it is indeed possible to perform a comparative analysis to conclude which is the most favored one. Thus EFT provides a better handle to address the so called ``inverse problem". We have developed two automatized programs `` 퐂퐨퐃퐄퐱 " and ``GrIP" that capture the primary essence of EFT. These codes enable us to understand EFT for both top-down and down-top approaches, and thus complement each other. These have also laid the platform to use EFT in a more effective way. Acknowledgement I would like to thank Supratim Das Bakshi, Upalaparna to compute only the group theoretic quantities such as Banerjee, Suraj Prakash, and Shakeel Ur Rahaman who have characters and Haar measures without defining any particle collaborated in these projects. This work is supported by the content. This is implemented through a subroutine known as Science and Engineering Research Board, Government of CHaar which is bundled with GrIP but has no dependency India, under the agreements SERB/PHY/2016348 (Early on it. All the necessary mathematical background as well as Career Research Award) and SERB/PHY/2019501 the parallels between theory and the code are explained in (MATRICS) and Initiation Research Grant, agreement great detail in [9]. We have summarized the working of GrIP number IITK/PHY/2015077, by IIT Kanpur.

Vol.50(3) 40 Physics News

References 1. S. Das Bakshi, J. Chakrabortty and S. K. Patra, Eur. Phys. J. 6. https://TeamGrIP.github.io/GrIP/ C 79, no. 1, 21 (2019) ; doi:10.1140/epjc/s10052-018-6444-2 7. B. Henning, X. Lu, T. Melia and H. Murayama, JHEP 08, 016 [arXiv:1808.04403 [hep-ph]]. (2017); doi:10.1007/JHEP08(2017)016 [arXiv:1512.03433 2. B. Henning, X. Lu and H. Murayama, JHEP 1601, 023 (2016); [hep-ph]]. doi:10.1007/JHEP01(2016)023 [arXiv:1412.1837 [hep-ph]]. 8. A. Hanany and R. Kalveks, JHEP 10, 152 (2014); 3. https://effexteam.github.io/CoDEx doi:10.1007/JHEP10(2014)152 [arXiv:1408.4690 [hep-th]]. 4. S. A. R. Ellis, J. Quevillon, T. You and Z. Zhang, Phys. Lett. B 9. U. Banerjee, J. Chakrabortty, S. Prakash and S. U. Rahaman, 762, 166 (2016); doi:10.1016/j.physletb.2016.09.016 [arXiv:2004.12830 [hep-ph]]. [arXiv:1604.02445 [hep-ph]]. 5. https://effexteam.github.io/CoDEx/html/tutorial/CoDExOvervi ew.html

41 Vol.50(3) News & Events

Vigyan Vidushi 2020 A TIFR advanced programme in physics for women students in first-year M.Sc. Amol Dighe1a, Anwesh Mazumdar2b, Vandana Nanal3c (Conveners- VV2020) 1 Department of Theoretical Physics, Tata Institute of Fundamental Research, Mumbai 400005, India 2 Homi Bhabha Centre for Science Education, Tata Institute of Fundamental Research, Mumbai 400088, India 3Department of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai 400005, India

Gender gap in STEM is a matter of global concern. Recent (over Zoom), while 335 were allowed to participate passively UNESCO data (2014 - 2016) reveal that only around 30% of over YouTube live streaming. The geographical distribution all female students select STEM-related fields in higher of selected participants is shown in Figure 1. Questions by education, and globally, female students’ enrolment is both sets of students were addressed during the lectures as well particularly low (~5%) in natural sciences, mathematics and as later in online Q & A forums. statistics [1,2]. In India, studies have revealed that the problem The programme was held during June 1-20, 2020. Five of gender imbalance in physics is acute [3]. While some physics courses were taught: two long core courses (Quantum programmes have been initiated by various science academies Mechanics, Statistical and Condensed Matter Physics) and and the governmental departments dealing with science and three short topical courses (Introduction to Astronomy and technology, the problem is complex and needs a multipronged Astrophysics, Experimental Techniques, Introduction to approach. The Indian Physics Association has also formed a Nuclear and Particle Physics). A large fraction of each course Gender in Physics Working Group (GIPWG) to work towards was devoted to tutorials and problem solving. There were also achieving a gender parity in Physics research in India. seven Special Lectures by women physicists, with eminent Tata Institute of Fundamental Research (TIFR), a premier names like Bimla Buti and Rohini Godbole among the research institute in India, recently started an initiative named speakers. The success of the public lectures may be gauged by “Vigyan Vidushi” (literally: a learned woman scientist), to the observation that these lectures have an average viewership address this problem at the level of women students about to of about 1000 each on YouTube so far, and the numbers are complete their first-year Masters in physics. These students rising. would be at the threshold of deciding whether to continue research towards their Ph.D., look for other opportunities based on their education, or leave their scientific pursuits altogether. It is the right time to provide them an exposure to advanced physics topics and research opportunities, and encourage them to take up research in physics as a career option. The students in this programme would also get an opportunity to be taught, inspired, and mentored by successful women scientist role models. The Vigyan Vidushi (VV) programme was conceptualized by the physicists in TIFR, Mumbai, and Homi Bhabha Centre for Science Education (HBCSE), a national centre of TIFR that focuses on research in science education and is also the nodal centre for the science and mathematics Olympiad activities in India. Originally planned to be a three-week residential programme in HBCSE, the VV2020 edition had to be converted to an online version at a short notice (due to the

COVID19 pandemic). In this first year of the programme, more than 650 applications from all over Indian were received, Figure 1: Geographical distribution of VV2020 participants out of which 51 students (from 47 institutions spread all over (green and red symbols refer to Zoom and YouTube India) were selected for participating in an interactive manner participants, respectively)

a [email protected], b [email protected] c [email protected]

Vol.50(3) 42 News & Events

Figure 2: Team-VV2020 at a glance

In addition to advanced physics courses during the day, there The experience of holding this summer school should be were sessions in the evenings devoted to special activities. useful in conducting regular teaching programmes online, as These introduced the students to Physics Education Research, may be necessary under the prevalent pandemic situation in and to future opportunities through Career Discussion the country. sessions. The latter involved two interactive workshops Although an online programme cannot replace a residential outlining the opportunities and challenges in research careers programme (and it is planned to continue the VV series in as well as gender-specific issues. The interactive sessions with future years as a residential programme), the response of the mentors were held in smaller groups (6-8 persons) to facilitate students was quite encouraging. There were a lot of physics one-to-one discussions. questions during the lectures and tutorials, and there was Apart from the faculty, postdocs and students from TIFR and strong feedback that the willingness of instructors to answer its Centres, many former women students of TIFR, currently questions was a major factor appreciated by the students, pursuing research elsewhere (in India or abroad), also which is often missing from the usual M.Sc. teaching. Also, a participated as tutors or mentors (see Figures 2 and 3). It must lot of students expressed that the mentoring sessions helped be noted that the fraction of women involved in all the them become more confident. Several of them, who were academic as well as administrative aspects of the programme earlier uncertain about their career after M.Sc., said that the was around 50% or sometimes even more. career discussions with mentors of all ages has made them more inclined towards continuing for their Ph.D. The main feature of this programme was that it was held completely online: 110 Zoom sessions, with 57 live YouTube The feedback from the students, organisers and others streams (including 8 public events). This was perhaps the first suggests the need for continuing, expanding, and time such a summer school in basic sciences was held strengthening the Vigyan Vidushi programme in future. It completely online, at least in India. Therefore, it was essential should be mentioned that almost all the students have to create new online infrastructure and processes. A Moodle expressed their appreciation for the Career Discussion course, platform was created for continuous interaction with students particularly for boosting the confidence and improving the including sharing of pedagogic material and answering awareness of challenges in research careers (Figure 4). It questions even after the lectures. Procedures were adopted to would also be great if, taking inspiration from this programme, encourage questions and feedback even during the lecture. similar programmes are initiated in other subjects, as well as This included the presence of additional coordinators in the in other institutions. One possible avenue can be to hold a online classroom apart from the instructor during each lecture. special session/satellite event involving interactive workshops The Moodle platform also provided a seamless connection at various national symposia. to Zoom. Detailed guidelines were prepared, and training All special lectures are available on VV2020 YouTube sessions were held, for instructors, tutors and coordinators. channel [4]. For further information and future updates please Connectivity of every student to the Zoom classroom was visit VV2020 webpage [5]. tested individually before the school.

43 Vol.50(3) News & Events

Figure 3: A group picture of participants, faculty and mentors

Figure 4: Views expressed by some of the participants…

References: 3. Trained scientific women power – how are we losing and 1. https://www.un.org/en/observances/women-and-girls-in why? Anitha Kurup, Maitrayee R., Kantharaju B., Rohini science-day/ Godbole (IAS-NIAS Research Report 2010) 2. Women in Physics: A tale of limits, Physics Today 65, 2, 4. https://www.youtube.com/channel/UCQ_QSNTFQEeX 47 (2012); doi: 10.1063/PT.3.1439 wgMskFV2QtA (VV2020 YouTube channel) 5. http://univ.tifr.res.in/vv2020/vv2020-online.html (VV2020 webpage)

Vol.50(3) 44 Book Review

C. V. Raman and the Press: Science Reporting and Image Building Rajinder Singh from is for the reader to say. It is important to note that the Press Oldenburg, Germany has here refers only to the English language papers. Raman made a career writing on C. featured prominently even in various vernacular newspapers, V. Raman and other Indian in Calcutta and in Madras and Bangalore. Their reporting had scientists who came to a different flavour. This is missing from the book. prominence during the Singh says he got interested in Raman's interaction with the colonial period. He already Press when he read that at the Nobel Award Ceremony -- published three books on `Raman wept because he had to get the award under the British Raman, this is his fourth and flag', and goes on to say he found out that ``Raman indeed the second one on Raman and wept, not due to the British flag, but for other reasons." He the Press, covering the period never bothered to state what those other reasons were, but it is 1933-1948. Singh is clear he missed the point what sitting under the Union Jack particularly adept in digging symbolised, that it reminded Raman he won the award as a up source material for his British subject and not as a citizen of an independent nation. studies and his being in Such mistakes in comprehension occur frequently in the book. Germany, in the heart of Europe, has helped him unearth material from European sources quite easily. Thus his books Singh says Raman's appointment as Director, IISc was and articles contain information generally unknown to the favoured by the Mysore Government because he was from readers in India, and they add a broader perspective to the South India. There is no supporting document to prove this. description of various events. When Martin Foster, the incumbent director, was about to The book, complete in ten chapters, is a chronicle of Raman's retire, the Tatas had informally approached the Royal Society academic life in Bangalore between 1933 and 1948. There are to explore the possibility of having an Indian scientist as the six chapters that describe Raman's turbulent years at the Indian next director and Raman was an automatic choice, there was Institute of Science (hereafter IISc) where he was the Director no other candidate. The State of Mysore had a broad view of 1933 -1937, and had also started the Department of Physics. things and was free of regional biases as we know from the He had to struggle to establish the physics department with fact that they had invited Brojendranath Seal, the great state-of-the-art laboratories, a workshop that could make philosopher from Bengal, to head Mysore University in the precision instruments, a library with physics journals etc. His early 1920s. Singh's insinuation of a regional bias seems quite overbearing personality and somewhat brash methods made baseless. him quite unpopular among the faculty and students of the Singh is right when he says, there is no detailed published other departments. While some excellent science was being work on Jewish scientists emigrating to India. But it is out of produced in the physics department through his guidance and place to mention Erwin SchrÖdinger in the same context as hard work, attention was diverted to the negative aspects of SchrÖdinger was only a sympathiser of the Jews. The earliest his administration. Eventually, he had to resign from the offer to him to come to India was made in 1939, somewhat directorship retaining his position as the head of the later than the peak of Hitlerian purge and the next offer, from department of physics until his retirement in 1948. Allahabad, was in 1940. SchrÖdinger's story has been 1 In the 1940s, Raman landed in fresh controversies with his recounted in great detail elsewhere , but Singh has completely experiments in lattice dynamics and theoretical interpretation ignored it. of the results. He stubbornly refused to accept the correct In 1934, Raman rather hastily founded the Indian Academy of explanation by theorists like Born and Peierls and became Sciences (IASc) in Bangalore ignoring the initiative by the increasingly isolated. Indian Science Congress Association (ISCA) which had The author has covered Raman's journey through these years already started discussing the establishment of such an with the help of newspaper articles and other published academy in Calcutta. This caused dismay and bitterness in the material. Newspaper clippings from the Archives of Raman science circles in the rest of the country and left the science Research Institute that are being used for the first time are an community of India divided. Singh seems to imply that this attractive feature of the book. This is not the first time Raman's action of Raman set in motion the proceedings that eventually story is being told. There are two excellent books, Journey forced Raman to resign from the directorship, IISc. He has into Light by G. Venkataraman and In Pursuit of Excellence elaborated upon the sordid role played by Meghnad Saha and by B. V. Subbarayappa, dealing with the very same events. Syama Prasad Mookerjee in the drama. The chapter describing Whether Singh's retelling changes the perception of the events this has the provocative title `Kolkata strikes back'. But the

1 Kariamanikkam Srinivasa Krishnan, his life and work D C V Mallik S Chatterjee, University Press, 2012, Chapter 14

45 Vol.50(3) Book Review

Council that asked Raman to resign had other members from from the Indian scientists' mission to the West in 1945 and for across the country including representatives of the Tatas and the growing indifference of the scientific community towards the Mysore Government. the great man. When Raman refused to participate in the Empire Scientific Conference in 1946, this further alienated Chapter 4 on the silver jubilee celebrations of ISCA and him, although Singh has justified Raman's refusal on some Raman's deteriorating relationship with that organisation is flimsy grounds. Marginal matters, like NISI (the original something new which was not covered in the books mentioned name of INSA) being recommended as the potential adhering earlier. Raman's unkind remarks on J C Bose, who had passed science body of India in preference over IASc, have been away months before the silver jubilee celebrations and which given undue importance. Even here how Raman's larger-than- had caused considerable adverse reaction among the members life figure delayed matters and the official designation of of the Congress, find no mention at all. That had surely INSA as the adhering body to ICSU had to wait until 1968, influenced some of the decisions of the ISCA officials that have been kept out of the discussion. Raman found unpalatable. The chronology of the events has not been followed in the sequence of the written chapters What the book misses is a discussion of Raman's relation with which has made the reading of the book quite difficult. This is some important physicists of the time who were in IISc with apparent in this chapter when matters related to ISCA's silver Raman. Both Homi Bhabha and Vikram Sarabhai, responsible jubilee celebrations have been coupled with events a good five respectively for the atomic energy programme and the space years later when A V Hill visited India. In 1943, A V Hill was programme, had started their scientific life in India under the invited by the colonial government to visit India to give advice stewardship of Raman. The great mathematician Harish- on the organisation of scientific research in the country. Chandra also started his research career at IISc as a student of Although Hill was a member of the British War Cabinet's Bhabha's during this time. Robert Millikan visited IISc in Scientific Advisory Committee, his visit had nothing to do 1941 and gave a boost to cosmic ray research which later with India's involvement in the war efforts. But Singh mis- flourished under Bhabha at the Tata Institute of Fundamental states the purpose of Hill's visit by saying that he came to Research. None of these have found any mention in the book. involve Indian scientists in war. He has unearthed one vague I have my doubts about the usefulness of such a book in the document to suggest this but it appears this was historical context of the development of science in India. It inconsequential to what followed. Hill's visit and his report does not reveal any aspect of Raman's personality that was not had a tremendous impact on the future course of the already known. Its value must be in the new source material development of science and industrial research in India but the that the author has collected and has meticulously listed in the book is silent on this. Instead, the special meeting of the Royal Notes at the end of each chapter. This will aid future scholars. Society in Delhi, first time outside of England, that Hill presided over, has featured prominently, because Raman was not supportive of it. Singh blames it on Hill for the alleged DCV Mallik neglect that Raman suffered after this with his name missing [email protected]

Vol.50(3) 46 Meet the Physicists

Meet the Physicists!

We profile 4 enthusiastic physicists who work closely with lasers and seem to be enjoying their diverse careers!

Nirmal Viswanathan Teacher, Researcher and Professor, School of Physics, University of Hyderabad Area: Fundamental aspects of light and light-matter interactions, towards understanding spin-orbit interaction of light and emerging topics Years doing physics: 30+ What I like about physics: Allows you to think critically and understand complex aspects of nature Beyond physics: Movies, music, politics… Nirmal

Alka A. Ingale Senior scientist, Raja Ramanna Centre for Advanced Technology, Indore and Professor HBNI, Mumbai Area: Condensed matter physics, primarily using Raman spectroscopy Years doing physics: 36 What I like about physics: Intrigue/mysteries it presents and solutions it provides & how all-encompassing it is! Beyond physics: Reading, travelling, trekking and all creative art forms Alka

Alika Alika Khare Professor, Department of Physics, Indian Institute of Technology, Guwahati Area: Laser Physics, Laser Matter Interaction and Non- Linear Optics Years doing physics: ~35 What I like about physics: The precision and technological developments that optical sciences have been able to bring about! Beyond physics: Photography, travelling and watching young kids play and learn

Subash Pai Subash Researcher turned entrepreneur, CTO, Excel Instruments, Vasai, Maharashtra Area: Design and manufacture of thin film deposition systems and related components, especially pulsed laser deposition systems Years doing physics: 39 What I like about physics: Logical, makes it easy to understand things and above all easy to explain Beyond physics: Sports (Badminton and Cricket), interacting with students, listening to music

47 Vol.50(3)

Backscatter

The charge of the mask brigade

Arnab Bhattacharya Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India E-mail: [email protected]

The COVID-19 pandemic has added one item to our daily pressure drop across the membrane would be huge, making attire – a face mask. Be it a simple handkerchief or dupatta, or breathing through such a mask with sub-micron sized pores a surgical or fancy “N95” mask, covering our mouth and nose extremely difficult. is perhaps the best non-pharmaceutical intervention to keep an Here is where electrostatics comes to the rescue! If the mask airborne infection at bay. While even a simple cloth mask will fibres are electrically charged, so that they can attract and keep out small droplets (usually 3-10 m sized or larger) an ensnare particles, one can have an additional filtration N95 mask, is ideally supposed to capture 95% of particles of 1 mechanism. Typical N95 masks, have a layer of nonwoven 0.3 m size . But what is the difference between N95 and melt-blown polypropylene fibers that are charged, and surgical masks, and simple home-made cloth masks? perform the electrostatic filtration. Polypropylene is an Surprisingly, the simple answer is, it’s all about electrical electret, a dielectric material which can hold a charge or charge! To understand this, let’s get down to the nitty-gritty of possess a net microscopic dipole moment (often the electrical filtering out fine particles. polarization properties are enhanced by various additives). Ideally, a good mask would filter out any unwanted particles This allows the layer to have pores that are much larger, but that we do not want to ingest, while not making breathing still filter out very fine particles using electrostatic interactions more difficult. Most typical filters, are based on a porous to get to the the 95% filtration efficiency levels. Charged fibrous material, and like a sieve, allow particles only smaller fibers can attract both instrisincally charged particles by than approximately the pore size to pass through. Of course in Coulombic forces as well as neutral polar particles (such as the real world, one has to worry about the exact nature of the tiny aqueous droplets) by dielectrophoretic forces that come flow past the fibres. If the flow is laminar, particles would just from the interaction of polarized objects and electric field follow streamlines and go past the obstacles (fibres). gradients. Mechanical capture happens for particles whose inertia is Of course, this presents several practical problems. Unlike large enough to cause a deviation from the streamline and cloth masks which can be easily washed and re-used, or hence impact the typically dense mesh of disinfected with alcohol, such procedures remove the charges fibres of the mask material. For smaller from N95 masks impacting their filtration efficiency. (Beware particles, Brownian diffusion is more of any advertisement for a “washable” N95 masks that doesn’t dominant, and they also encounter the show filtration efficiency data after a wash!). Also, as every mask fibres. Once a particle hits the physicist would know, humidity is detrimental to electro- fibre surface, adhesive (van der statics. Usage for extended periods especially in humid Waals) forces immobilize and trap conditions like the Indian monsoon is likely to degrade a it. For filters based on fibrous mask’s filtration efficiency. Can one somehow put the charge materials and for flow back? Well, there are certainly some groups, including ours2, velocities similar to human that have tried to do so. breathing, the minimum filtration efficiency occurs for However, the biggest challenge is, without doubt, convincing ≈0.3m sized particles. So why the public at large to wear masks, and wear them correctly. not just make the pores smaller? With all its charge, the best N95 mask isn’t going to protect The answer is in the breathability. anyone, if it is just a chin ornament! For filters with very fine pores, the

1 The n95decon.org website has a lot of useful information on 2 E. Hossain et al., https://arxiv.org/abs/2004.13641 “Recharging masks. (Mask cartoon also taken from n95decon.org) and rejuvenation of decontaminated N95 masks”, to appear in Phys. Fluids (2020) DOI: 10.1063/5.0023940

Vol.50(3) 48 Nobel Prizes connected with lasers Ever since its first demonstration, advances in understanding of the science and applications of laser light have been recognized several times with Nobel Prizes in both Physics and Chemistry. Other Nobel prizes have either been key to the development of lasers, or would not have been possible without lasers. PHYSICS 1964 Charles H. Townes, Nicolay G. Basov and Aleksandr M. Prokhorov “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle” 1971 Dennis Gabor “for his invention and development of the holographic method” 1981 Nicolaas Bloembergen and Arthur Leonard Schawlow “for their contribution to the development of laser spectroscopy” 1997 Steven Chu, Claude Cohen-Tannoudji and William D. Phillips “for development of methods to cool and trap atoms with laser light” 2000 Zhores I. Alferov and Herbert Kroemer (part) “for developing semiconductor heterostructures used in high-speed- and opto-electronics” 2001 Eric A. Cornell, Wolfgang Ketterle and Carl E. Wieman “for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates” 2005 Roy J. Glauber “for his contribution to the quantum theory of optical coherence”; John L. Hall and Theodor W. Hänsch “for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique” 2009 Charles Kuen Kao (part) “for groundbreaking achievements concerning the transmission of light in fibers for optical communication” 2012 Serge Haroche and David J. Wineland “for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems” 2017 Rainer Weiss, Barry C. Barish and Kip S. Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves” 2018 Arthur Ashkin “for the optical tweezers and their application to biological systems”; Gérard Mourou and Donna Strickland “for their method of generating high-intensity, ultra-short optical pulses”, both “for groundbreaking inventions in the field of laser physics” CHEMISTRY 1999 Ahmed H. Zewail ”for his studies of the transition states of chemical reactions using femtosecond spectroscopy” 2014 Eric Betzig, Stefan W. Hell and William E. Moerner “for the development of super-resolved fluorescence microscopy” Registered with the Registrar of Newspapers for India under R.N. 20754/71

Published by the Indian Physics Association c/o Dept. of Physics, I.I.T. Bombay, Powai, Mumbai 400076

ONLINE VERSION