LASERS & OPTICS 151 many cases non-destructively. A variety of beam manipulation schemes are avail­ able for the irradiation of samples either directly or after preparation. The unique properties of laser radiation in terms of coherence, intensity and directionality permit remote chemical sensing, where the analytical equipment and the sample are separated by large distances, even of several kilometres. Absorption, and in particular differential absorption, can be utilised in long-path measurements, whereas elastic and inelastic backscatter- ing as well as fluorescence can be used for range-resolved radar-like measure­ ments (LIDAR: light detection and rang­ ing). Laser light can also be efficiently transported in optical fibres to remotely located measurement sites. Various prop­ erties of the fibre itself influence the laser light propagating through the fibre, thus Applications of Laser forming a basis for fibre optic sensors. Applications of laser spectroscopy constitute a vast field, which is difficult to Spectroscopy cover comprehensively in a review. Rather than attempting such a review, examples from a variety of fields are cho­ Costas Fotakis sen for illustrating the power of applied University of Crete, Greece laser spectroscopy. Sune Svanberg Lund Institute of Technology, Sweden Applications in Analytical Chemistry Laser spectroscopy is making a major impact in many traditional fields of ana­ As the new millennium approaches the Above The Italian research vessel Urania, carrying a lytical spectroscopy. For instance, opto- know how in the field of laser-matter Swedish laser radar system, which has sailed under the galvanic spectroscopy on analytical smoky plumes of Sicilian volcanos in Italy measuring the interactions has matured to the stage of sulphur dioxide content flames increases the sensitivity of enabling several exciting real life applica­ absorption and emission flame spec­ tions. Several of these applications rely In recent years, laser sources have troscopy. Extremely sensitive direct on the narrow bandwidth of tuneable become more rugged, easy-to-use and absorption measurements with CW lasers lasers, which allows extreme selectivity in cheaper to allow real world applications can be performed using frequency-mod­ the interaction with free atoms and mole­ of laser spectroscopy. Semiconductor ulation spectroscopy with a t:107 absorp­ cules. In other applications, the high diode lasers have become very reliable tion sensitivity. A high sensitivity in spectral intensity available with pulsed as and easily accessible, and their wave­ absorption can also be achieved in intra­ well as continuous wave (CW) lasers may length range is extended higher and cavity experiments. saturate optical transitions or induce a higher up into the visible region. We can Traces of material can be detected variety of nonlinear effects, as for exam­ mention the emerging blue diode lasers using the REMPI (Resonance Enhanced ple multiphoton ionisation, thus provid­ based on gallium phosphide, and fre­ Multi-Photon Ionisation) or RIMS ing the basis for highly sensitive detec­ quency conversion of the output of reli­ (Resonance Ionisation Mass Spectros­ tion schemes. In fact, single-atom or mol­ able, high-power diode lasers into new copy) techniques, both employing pulsed, ecule detection is widely used, represent­ wavelength regions. New laser materials, tuneable lasers. In the first method, selec­ ing the ultimate sensitivity in analytical such as titanium-doped sapphire, allow tively produced ions of the element are chemistry. In a different trend, ultrafast all-solid-state tuneable systems with con­ detected through the ion current, where­ lasers have allowed detailed temporal siderable power to be constructed. as the second method employs a mass monitoring of chemical reaction dynam­ Diode-laser pumping of solid-state mate­ spectrometer for additional selectivity. ics at femtosecond timescales (femto- rials is making flash-lamp pumping grad­ The breadth of standard analytical- chemistry). In this way, significant insight ually obsolete and enables compact, reli­ chemistry techniques can be further into the basic steps of chemical processes able and energy-efficient laser sources, improved through laser spectroscopy, has been gained. Finally, in the form of especially when combined with current employed for enhanced detection sensi­ holographic techniques the coherence frequency conversion schemes (eg optical tivity and multiplexing. Thus, laser- properties of laser light are the basis of parametric amplifiers). induced fluorescence (LIF) can be ultrasensitive structural diagnostics Laser-based analysis and diagnostics utilised for detecting the separated frac­

applications. can readily be performed in situ and in tions passing the detector position in 1998 July/August News Europhysics 152 LASERS & OPTICS

Fig 1 Left The Italian volcano Stromboli, seen from the research vessel Urania Below The volcanic plume of Mt Etna as recorded by laser radar techniques in September1994 during a ship-borne traverse under the plume, integrated vertical and horizontal profiles of sulphur dioxide are shown.The total flux was 44 tonnes/hour

HPLC (High Performance Liquid Combustion Processes package by a laser pulse (repumping or Chromatography) and Capillary Combustion remains the most important dissociating the molecules) and then Electrophoresis. Further, a whole section means of energy conversion. Laser spec­ identify the package position at a later of the column can be illuminated and the troscopy provides a very powerful, non- time using a second laser pulse. fluorescence along the column can be intrusive measurement capability for the CARS (Coherent Anti-Stokes Raman imaged on a linear array or a CCD detec­ reactive and aggressive media constituted Scattering) is a powerful technique, espe­ tor, to provide simultaneous detection of by burning or exploding combustion cially for temperature measurements in all species undergoing separation. gases. A large number of techniques has strongly luminous media. A coherent, Differences in fluorescence spectra can provided new possibilities to measure the laser-like beam is obtained carrying the also be utilised for further discrimina­ concentration of species including short­ signal, making it largely immune to the tion. Fluorescence labelling can be used, lived radicals, and also the capability of background light. By single-shot CARS detecting simultaneously chromophores determining temperature and flow speed. recordings, PDFs (Probability Density with different fluorescence characteristics The measurements have allowed a realis­ Functions) for temperature can be deter­ and binding to different positions in the tic interplay with the advanced kinetic mined, yielding important information DNA strand. These developments are computer codes that are used for model­ on statistical fluctuations in turbulent very important, for example for the ling combustion. Important issues in combustion. Realistic measurements on Human Genome Project. combustion include NOx and soot forma­ test engines, jet engines and even coal- Laser ablation which occurs upon the tion, engine ignition and engine knock. fired power plants have been performed. interaction of intense laser radiation with LIF has been extensively used, also in Polarisation spectroscopy and DFWM solid targets has provided a number of imaging configurations, to monitor the (Degenerate Four-Wave Mixing) spec­ novel analytical schemes in combination distribution of molecules such as OH, C2, troscopy are further powerful techniques with mass spectrometric or optical detec­ CH and CN. Using a gated and image- in combustion diagnostics. tion. Along these lines, there are several intensified CCD detector the full distrib­ The methods developed for combus­ industrial applications, in which on-line ution can be captured using a single laser tion diagnostics are also applicable for monitoring may occur by laser-induced shot ( = 10-8s). To this end, a sheet of light the monitoring of other reactive media breakdown spectroscopy (LIBS). This tech­ is formed through the burning medium such as in plasma etching or MOCVD nique exploits changes in the emission using cylindrical optics. In a similar way, (Metal Organic Chemical Vapour spectra of laser-produced plasmas depend­ recordings using transitions starting in Deposition) for semiconductor process­ ing on the stage and quality of processing. states with a temperature-dependent ing. In a relevant recent application LIBS has population can obtain temperature dis­ been applied on polymer samples for the tributions. Flow velocities are evaluated Monitoring of the Atmosphere identification and separation of polymeric using Doppler shifts in the recorded sig­ The atmosphere can be monitored by materials for recycling purposes. nals. A further method is to mark a gas laser techniques employing absorption or

Fig 2 Fluorescence imaging of the northern gate of the Lund Cathedral. Images were recorded through pass- bands centred at 438 and 682 nanometers using a scanning laser radar system positioned about 50 meters from the gate. A normal colour photo­ graph of the gate Is

Europhysics News July/August 1998 July/August News Europhysics shown centre LESERS & OPTICS153

Fig 3 Above A restored area on a painting is differentiated with respect to the original and analysed by using quantitative fluorescence multispectral imaging— in visible light no difference is observed. The characterization of the image relies on a narrow hue distribution Fig 4 Right Fluorescence images and tumour diagnostics. Pixels, fulfiling a 'cancer criterion' in fluorescence are shown in false colour superimposed on a normal colour video representation of the area. (Left) Basal cell carcinoma, note the non-cancerous brown spot, which is benign (Right) A tumour on vocal cords laser-induced fluorescence. The LIDAR, Total fluxes from an industrial complex kilometre (truck, helicopter or airplane or laser radar, technique using a pulsed can be evaluated if gas concentration val­ installations). Water LIF signals include a laser as a transmitter and an optical tele­ ues as well as the wind speed at the rele­ sharp OH-stretch Raman signal for water scope as a receiver in a radar-like manner vant height are known. The wind speed that is valuable for referencing, a broad allows a mapping of pollution concentra­ can be determined by video correlation bluish fluorescence light distribution due tions and also meteorological parame­ techniques using visible structures in the to distributed organic matter (DOM), and ters, such as temperature, humidity and moving plume. Lidar techniques are also rather sharp peaks in the near IR region wind velocity. Lidar systems can be oper­ being used for the monitoring of gases of due to algal chlorophyll. The technique is ated from fixed laboratories, trucks, air­ geophysical origin; eg atomic mercury particularly valuable for the monitoring planes or even spacecraft. from Italian geothermal fields and from of oil spills and algal blooms. At atmospheric pressures LIF cannot mercury mines in Italy and Spain have Land vegetation features clear chloro­ be used for species monitoring because been studied. The fluxes of sulphur diox­ phyll signals (the ratio between two red of a strong collisional quenching (com­ ide from the volcanoes Mount Etna, peaks allows an evaluation of the chloro­ bustion diagnostics with LIF is still feasi­ Stromboli and Vulcano have been phyll concentration) as well as blue fluo­ ble because of the possibility of saturat­ assessed in several campaigns with lidar rescence due to a variety of molecules ing the optical transitions, leading to full on board a research vessel making tra­ present in the leaves. Of particular inter­ fluorescence intensity even in the pres­ verses under the volcanic plumes. An est is exploring the possibilities for early ence of collisional transitions). example of results for Mount Etna is detection of forest decline prior to visible Mesospheric monitoring of meteorite- shown in figure 1. signs. By expanding the laser beam, a cer­ derived Li, Na, K and Ca layers using LIF tain area can be illuminated, and by a flu­ lidar works well because of the low pres­ Vegetation Monitoring orescence imaging system the whole sures. Ultraviolet laser pulses transmitted from scene can be captured in properly select­ Tropospheric pollution monitoring is a lidar system can induce fluorescence ed wavelength bands. most frequently performed using DIAL when directed into water or onto vegeta­ (Differential Absorption Lidar). Range- tion. The fluorescence collected by the Cultural Heritage Preservation resolved optical transients due to elastic receiving telescope can be analysed using During recent years lasers have also backscattering from aerosol particles are an optical multichannel analyser found applications in the area of cultur­ recorded for a laser wavelength set on a equipped with an image intensifier, which al heritage. Fluorescence imaging is a characteristic absorption line and just off is gated in synchronization with the powerful method used to detect areas in it for reference, in alternating laser shots. return of the signal. In this way, back­ paintings which have been modified or By dividing the resulting curves, ground due to the ambient light can be retouched. Raman, LIF and LIBS analy­ unknown atmospheric parameters are suppressed. Fluorescence data from the sis of pigments and varnish can also eliminated and the concentration of the earth surface can complement reflectance help reveal the authenticity of a particu­ particular species can be evaluated. data widely collected by multi-spectral lar painting. Intense UV pulses can be Typical ranges for S02, 03, N02, NO and sensors installed in satellites such as used to remove soot and dirt by ablation Hg monitoring are 0.5 to 5 kilometres. LANDSAT or SPOT. Lidar systems are from the surface of icons, paintings etc. The techniques are particularly valuable under development featuring effective Also, stone statues can be cleaned using

for urban and industrial measurements. measurement ranges mostly below one higher pulse energies. Spectroscopic 1998 July/August News Europhysics 154 LASERS & OPTICS imaging and LIBS can be used for the on nation, detection efficiency etc. Fibre sue and electronically detecting only the line monitoring of the cleaning process optic probes, through which both excita­ first emerging photons it is possible to in order to safeguard from potential tion light and induced fluorescence are reject the scattered light and retain an damage. conducted, have been developed and image with good contrast. Many different Recently, the diagnostic techniques utilised for studies of tumours in differ­ techniques working on this general prin­ have been extended for applications also ent organ systems. Multi-colour imaging ciple are now being developed with the on building monuments. For this pur­ devices have also been constructed, mak­ main aim of detection of breast cancer, pose, a scanning LIF lidar system can be ing the presentation of an image the most common malignant tumour dis­ utilised for assessment of modifications processed for cancer detection possible; ease in women. Prototype systems for to the stone surface and the presence of video mixed with the normal white-light optical mammography are being con­ algal or lichen growth. An example is reflectance image obtained through an structed. Similar technology can also be given in figure 2, where data for the endoscope. Tumours detected can be used for oxygenation measurements in northern gate of the Lund Cathedral are treated by photo-dynamic therapy (PDT) the brain, and also possibly for localising shown. Generally, LIF is suitable for the using red laser light, which excites the haematoma following trauma to the non-destructive and in situ analysis of administered sensitiser molecules with skull. It is also useful for spectroscopic pigments and binding media of painted subsequent transfer of ground-state measurements of strongly scattering liq­ artworks. This technique can be comple­ triplet oxygen to the toxic singlet state. A uids such as blood, for which it is of mented by LIBS for obtaining in depth selective necrosis of tumour cells will great interest to monitor the concentra­ profiles of elemental composition. A typi­ result. Of particular interest is the intro­ tion of various molecules, such as urea cal example is shown in figure 3 in which duction of diagnosis and PDT using 6- and glucose. quantitative fluorescence imaging of a aminolevulinic acid (ALA), which is a Spectroscopy in strongly scattering retouched area of a painting called “La natural molecule in the haemoglobin media can also be used in non-medical Bella” (originating from the school of cycle in the human body. Prolonged sen­ applications, such as studies of light Palma di Vecchio and today in a private sitivity of the skin to sun radiation, propagation in green leaves (photosyn­ collection) is delineated and verified by which is a complication for many sensi- thesis) or sheets of paper (quality assess­ LIF and LIBS spectra. In this case, the tisers, is eliminated with ALA. Examples ment through information of fibres etc). extent and composition of the white pig­ of real-time tumour imaging are shown Laser spectroscopy is finding many ments used in the restored area as well as in figure 4. real-world applications. The many power­ in the original painting can be accurately Atherosclerotic plaque can be fibre- ful methods allow advanced sensing, both determined. optically detected in in vivo transluminal locally and remotely. Of great importance monitoring in vessels. Atherosclerotic in the process of taking the technology Laser Spectroscopy in Medicine plaque is characterised by a change in the into everyday applications is the fast The analytical-chemistry applications elastin/collagen balance and can be development of cheap and reliable laser discussed above are of substantial inter­ observed in time integrated or time- sources, fibre-optical components and est in medical research. However, much resolved measurements of LIF. The con­ computers. more direct applications in medical diag­ struction of a guidance device for the nostics have emerged during recent year. safe use of a fibre optic laser coronary Thus, tissue LIF has been extensively angioplasty system on line would be of Further reading studied and applied for early detection of considerable interest. Clinical studies S. Svanberg “Atomic and Molecular malignant tumours and for studies of have demonstrated the potential of this Spectroscopy -Basic Aspects and atherosclerotic plaque. Tissue exhibits a approach. Practical Applications” Springer natural fluorescence when excited by UV Raman spectroscopy is also being Verlag, Heidelberg, 2nd edn., 1992 or violet light. Important natural chro- developed for tissue diagnostics. Much mophores emitting fluorescence are sharper but weak signal features are A.C. Eckbreth “Laser Diagnostics for elastin, collagen, NADH and NAD+. They obtained. In order to suppress competing Flame Temperature and Species” all yield broad, but somewhat different fluorescence the laser irradiation wave­ Abacus Press, Turnbridge Wells, 1987 distributions in the blue-green spectral length is chosen in the near IR region. region. However, exogenously adminis­ Also, direct IR reflectance spectroscopy M. Sigrist (ed.) “Air Monitoring by tered agents such as porphyrins, chlo­ has a diagnostic potential. Spectroscopic Techniques” Wiley, New rines and phtalocyanines, which are York,1994 selectively retained in tumour cells, yield Scattering Spectroscopy sharp and characteristic peaks in the Red light penetrates tissue particularly S. Svanberg New Developments in dark red wavelength region, signaling the well, a fact which is utilised to achieve Laser Medicine Phys. Scr. T72 (1997) presence of cancer. An increase in the red PDT for tissue thickness up to a few mil­ 69 fluorescence is frequently accompanied limetres. The weak penetration of red by a decrease in the blue-green fluores­ light also through thicker tissue layers V. Zafiropulos and C. Fotakis Lasers cence, and thus by a ratio formation an would suggest optical mammography in the Conservation of Painted enhanced tumour demarcation from nor­ without using ionising radiation. Artworks “Laser Cleaning in mal tissue can be achieved. At the same However, the heavy multiple scattering in Conservation: an Introduction” Ed. M. time the monitoring of a dimensionless tissue leads to image blurring reducing Cooper (Butterworth Heinemann, quantity, such as a ratio, makes the data the value strongly. However, by transmit­ Oxford, 1998) chapter 6 pages 79-90

Europhysics News July/August July/August 1998 News Europhysics immune to changes in geometry, illumi­ ting picosecond laser pulses through tis­