DOCSLIB.ORG

Optical Tweezers

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Optical Tweezers Optical Tweezers Experiment OT - sjh,rd University of Florida | Department of Physics PHY4803L | Advanced Physics Laboratory Objective K. Berg-Sorensen and H.Flyvbjerg, Power spectrum analysis for optical tweezers, An optical tweezers apparatus uses a tightly Rev. Sci. Instr. 75 594-612 (2004). focused laser to generate a trapping force that can capture and move small particles under S. Chattopadhyay, R. Moldovan, C. Yeung, a microscope. Because it can precisely and X.L. Wu, Swimming efficiency of bac- non-destructively manipulate objects such as terium Escherichiacoli, Proc. Nat. Acad. individual cells and their internal components, Sci. USA 103 13712-13717 (2006). the optical tweezers is extremely useful in bi- Daniel T. Gillespie The mathematics of ological physics research. In this experiment Brownian motion and Johnson noise, the student will use optical tweezers to trap Amer. J. of Phys. 64 225 (1996). small silica spheres immersed in water. By analyzing the dynamics of particles held in Daniel T. Gillespie Fluctuation and dissipa- the trap, including the frequency spectrum of tion in Brownian motion, Amer. J. of their Brownian motion and their response to Phys. 61 1077 (1993). hydrodynamic drag, the student will charac- terize the physical parameters of the optical S.F. Tolic-Norrelykke et al., Calibration of trap with high precision. The apparatus can optical tweezers with positional detection then be used to measure a microscopic bio- in the back focal plane, Rev. Sci. Instr. logical force, such as the force that propels a 77 103101 (2006). swimming bacterium or the force generated by a transport motor operating inside a plant cell. Introduction The key idea of optical trapping is that a laser References beam brought to a sharp focus generates a restoring force that can pull particles into that D.C. Appleyard et al., Optical trapping for focus. Arthur Ashkin demonstrated the prin- undergraduates, Am. J. Phys. 75 5-14 ciple in 1970 and reported on a working ap- (2007). paratus in 1986. The term optical trapping often refers to laser-based methods for hold- A. Ashkin, Acceleration and Trapping of Par- ing neutral atoms in high vacuum, while the ticles by Radiation Pressure, Phys. Rev. term optical tweezers (or laser tweezers) typ- Lett. 24 156-159 (1970). ically refers to the application studied in this OT - sjh,rd 1 OT - sjh,rd 2 Advanced Physics Laboratory experiment: A microscope is used to bring a low absorption rate and because the particle laser beam to a sharp focus inside an aqueous has good thermal conductance with the sur- sample, such that microscopic, non-absorbing rounding water, effects of heating should be particles such as small beads or individual cells modest. can become trapped at the beam focus. Op- The laser force arises almost entirely from tical tweezers have had a dramatic impact on the elastic scattering of laser photons by the the field of biological physics, as they allow particle whereby the particle alters the direc- experimenters to measure non-destructively tion of the photon momentum without absorb- and with high precision the tiny forces gen- ing any of its energy. The laser force is typ- erated by individual cells and biomolecules. ically decomposed into two components: (1) This includes propulsive forces generated by a scattering force proportional to the laser in- swimming bacteria, elastic forces generated by tensity and (2) a gradient force proportional deformation of biomolecules, and the forces to the gradient of laser intensity. The scatter- generated by processive enzyme motors op- ing force is in the direction of the laser beam, erating within a cell. Experimenting with while the gradient force depends on the spatial an apparatus capable of capturing, transport- dependence of the laser intensity and near the ing, and manipulating individual cells and or- focus provides the Hooke's law force respon- ganelles provides an intriguing introduction to sible for trapping the particle. The origin of the world of biological physics. both forces is similar: the particle elastically A photon of wavelength λ and frequency scatters a photon and alters its momentum. f = c/λ carries an energy E = hf and a Momentum conservation implies that the scat- momentum of magnitude p = h/λ in the di- tered photon imparts an equal and opposite rection of propagation.1 Note that our laser momentum change to the particle. The net power|around 100 mW|focused down to force on the particle is a vector equal and op- a few square microns, imply laser intensi- posite the net rate of change of momentum of ties around 107 W/cm2 at the beam focus. the laser photons. Particles that absorb more than a tiny frac- The theory and practice of laser tweezers are tion of the incident beam will absorb a large highly developed and numerous excellent re- amount of energy relative to their volume views, tutorials, examples, and other resources rather quickly. In fact, light-absorbing par- on the subject are available2. For particles ticles can be quite rapidly vaporized (opti- with diameters d large compared to λ, a ray cuted) by the trapping laser. (Incidentally, optics model gives a good description of the your retina contains many such particles - see physics that generates the force due to the Laser Safety below). While the scatterer and laser. Reflection and refraction of the rays at surrounding fluid always absorbs some energy, the surface of the sphere alter the direction of our infrared laser wavelength (λ = 975 nm) is the rays and thus the momentum they carry. specifically chosen because it is where absorp- The laser force is the cumulative effect from all tion in water and most biological samples is the scattered photons. For smaller particles of lowest. The absorption rate is also near a min- diameter d λ, Rayleigh scattering describes imum for the silica spheres you will study. You the interaction: The particle acts as a point should keep an eye out for evidence of heating dipole, scattering the incident beam in a spa- in your samples, but because of the relatively tially dependent fashion that depends on the 1h = Planck's constant, c = speed of light. 2Google: optical tweezers May 28, 2013 Optical Tweezers OT - sjh,rd 3 location of the particle, leading to a trapping tion. Because the laser beam and focusing op- force on the particle. tics are cylindrically symmetric around the z- The center of the trap will be taken as axis, the trap has the same properties in the (r = 0). For any small displacement (any di- x-direction as in the y-direction. We need only rection) away from the trap center the parti- consider the equations for the x motion of the cle is subject to a Hooke's law restoring force, particle, and a similar set of equations will de- i.e., proportional to and opposite the displace- scribe the motion in the y-direction. However ment. Detailed calculations show that the the trapping force that acts along the z direc- force constants are sensitive to the shape and tion is different than for x and y, as the laser intensity of the laser field, the size and shape intensity in the focal region is clearly not a of the trapped particle, and the optical prop- spherically symmetric pattern. The width of erties of the particle and surrounding fluid. the beam focus in its radial (xy) dimension Consequently the Hooke's law force is diffi- is very narrow. It is limited by wave diffrac- cult to predict. Furthermore, our apparatus tion to roughly one wavelength (λ ∼ 1µm), operates in an intermediate regime of particle whereas this is not the case in z. Hence the sizes where neither the ray optics nor Rayleigh restoring force in z is not necessarily as strong models are truly appropriate. The diameter d as in xy. If the beam is not very tightly fo- of the silica spheres (SiO2) range in size from cused along the z-direction, the trapping force 0.5-5 µm. Thus with the laser wavelength of in the z-direction may be too weak. If the λ = 975 nm, we have d ∼ λ. Fortunately we focal \cone" has too shallow an angle (tech- do not need to calculate or predict the Hooke's nically, a large f-number or small numerical law force constants. We only need to under- aperture), particles may be trapped in the xy stand their physical origin. In this lab you direction but not trapped along z: the laser will learn how to calibrate the trap by measur- beam will tend to pull small particles in to- ing the force constants indirectly on a trapped ward the central optical axis, propel them to- particle in a fluid. With an accurate force cali- ward the focus, and keep pushing them out bration method at hand, one can measure and the other side of the trap. By employing a explore biomolecular forces quantitatively. large numerical aperture, our apparatus pro- vides excellent trapping in all three directions xyz. Physics of the trapped particle We will investigate the motions of the par- The design, operation, and calibration of our ticle in the xy directions only. Consequently, laser tweezers draws on principles of optics, in the discussion that follows, when forces, im- mechanics and statistical physics. We begin pulses, velocities or other vector quantities are with an overview of the physics that is relevant written without vector notation (F instead of to generating and calibrating the trap. We F), they represent the x-component of the cor- then describe the design of the apparatus and responding vector quantity. the procedure for calibration. The ray optics in Figure1 illustrate how Consider the motion and forces in terms of laser beam refraction generates a trapping their components.
Load more

Recommended publications
  • Quantitative Force Measurements with Optical Tweezers: the JPK Nanotracker™
    Quantitative force measurements with optical tweezers: The JPK NanoTracker™ Introduction to optical tweezers manipulate biomolecules and cells, but also to directly and accurately measure the minute forces (on the order of Optical tweezers methodology has developed from proof-of- fractions of picoNewtons) involved. Most often, the principle experiments to an established quantitative biomolecules of interest are not trapped themselves technique in fields ranging from (bio)physics through cell directly, but manipulated through functionalized biology. As the name suggests, optical tweezers are a microspheres. means to manipulate objects with light. With this technique, The correct physical description of optical trapping depends microscopically small objects can be held and manipulated. on the size of the trapped object. One speaks of the ‘ray- At the same time, the forces exerted on the trapped objects optics’ regime when the object’s dimension d is much larger can be accurately measured. The basic physical principle than the wavelength of the trapping light: d>>λ. In this case, underlying optical tweezers is the radiation pressure diffraction effects can be neglected and the trapping forces exerted by light when colliding with matter. For macroscopic of the light can be understood in terms of ray optics. The objects, the radiation pressure exerted by typical light regime where d<<λ is called the Rayleigh regime. In this sources is orders of magnitude too small to have any case, the trapped particles can be treated as point dipoles, measurable effect: we do not feel the light power of the sun as the electromagnetic field is constant on the scale of the pushing us away.
    [Show full text]
  • A Quantum Origin of Life?
    June 26, 2008 11:1 World Scientific Book - 9in x 6in quantum Chapter 1 A Quantum Origin of Life? Paul C. W. Davies The origin of life is one of the great unsolved problems of science. In the nineteenth century, many scientists believed that life was some sort of magic matter. The continued use of the term “organic chemistry” is a hangover from that era. The assumption that there is a chemical recipe for life led to the hope that, if only we knew the details, we could mix up the right stuff in a test tube and make life in the lab. Most research on biogenesis has followed that tradition, by assuming chemistry was a bridge—albeit a long one—from matter to life. Elucidat- ing the chemical pathway has been a tantalizing goal, spurred on by the famous Miller-Urey experiment of 1952, in which amino acids were made by sparking electricity through a mixture of water and common gases [Miller (1953)]. But this concept turned out to be something of a blind alley, and further progress with pre-biotic chemical synthesis has been frustratingly slow. In 1944, Erwin Schr¨odinger published his famous lectures under the ti- tle What is Life? [Schr¨odinger (1944)] and ushered in the age of molecular biology. Sch¨odinger argued that the stable transmission of genetic infor- mation from generation to generation in discrete bits implied a quantum mechanical process, although he was unaware of the role of or the specifics of genetic encoding. The other founders of quantum mechanics, including Niels Bohr, Werner Heisenberg and Eugene Wigner shared Schr¨odinger’s belief that quantum physics was the key to understanding the phenomenon Received May 9, 2007 3 QUANTUM ASPECTS OF LIFE © Imperial College Press http://www.worldscibooks.com/physics/p581.html June 26, 2008 11:1 World Scientific Book - 9in x 6in quantum 4 Quantum Aspects of Life of life.
    [Show full text]
  • Depth-Resolved Measurement of Optical Radiation-Pressure Forces with Optical Coherence Tomography
    Vol. 26, No. 3 | 5 Feb 2018 | OPTICS EXPRESS 2410 Depth-resolved measurement of optical radiation-pressure forces with optical coherence tomography NICHALUK LEARTPRAPUN, RISHYASHRING R. IYER, AND STEVEN G. ADIE* Meinig School of Biomedical Engineering, Cornell University, Ithaca, NY 14853, USA *sga42@cornell.edu Abstract: A weakly focused laser beam can exert sufficient radiation pressure to manipulate microscopic particles over a large depth range. However, depth-resolved continuous measurement of radiation-pressure force profiles over an extended range about the focal plane has not been demonstrated despite decades of research on optical manipulation. Here, we present a method for continuous measurement of axial radiation-pressure forces from a weakly focused beam on polystyrene micro-beads suspended in viscous fluids over a depth range of 400 μm, based on real-time monitoring of particle dynamics using optical coherence tomography (OCT). Measurements of radiation-pressure forces as a function of beam power, wavelength, bead size, and refractive index are consistent with theoretical trends. However, our continuous measurements also reveal localized depth-dependent features in the radiation- pressure force profiles that deviate from theoretical predictions based on an aberration-free Gaussian beam. The combination of long-range radiation pressure and OCT offers a new mode of quantitative optical manipulation and detection with extended spatial coverage. This may find applications in the characterization of optical tractor beams, or volumetric optical manipulation and interrogation of beads in viscoelastic media. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (110.4500) Optical coherence tomography; (350.4855) Optical tweezers or optical manipulation.
    [Show full text]
  • Optical Micromachines for Biological Studies
    micromachines Review Optical Micromachines for Biological Studies Philippa-Kate Andrew 1 , Martin A. K. Williams 2,3 and Ebubekir Avci 1,3,* 1 Department of Mechanical and Electrical Engineering, Massey University, Palmerston North 4410, New Zealand; k.andrew@massey.ac.nz 2 School of Fundamental Sciences, Massey University, Palmerston North 4410, New Zealand; m.williams@massey.ac.nz 3 MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6140, New Zealand * Correspondence: e.avci@massey.ac.nz Received: 21 January 2020; Accepted: 9 February 2020; Published: 13 February 2020 Abstract: Optical tweezers have been used for biological studies since shortly after their inception. However, over the years research has suggested that the intense laser light used to create optical traps may damage the specimens being studied. This review aims to provide a brief overview of optical tweezers and the possible mechanisms for damage, and more importantly examines the role of optical micromachines as tools for biological studies. This review covers the achievements to date in the field of optical micromachines: improvements in the ability to produce micromachines, including multi-body microrobots; and design considerations for both optical microrobots and the optical trapping set-up used for controlling them are all discussed. The review focuses especially on the role of micromachines in biological research, and explores some of the potential that the technology has in this area. Keywords: optical tweezers; multi-component micromanipulators; radiation damage; life sciences; optical microrobots 1. Introduction Improvements in tools for the visualisation of objects at the micro- and nano- scale have given researchers the ability to investigate materials and processes previously out of reach.
    [Show full text]
  • Pulsed Optical Tweezers for Levitation and Manipulation of Stuck Biological
    2005 Conference on Lasers & Electro-Optics (CLEO) CFN3 Pulsed optical tweezers for levitation and manipulation of stuck biological particles Amol Ashok Ambardekar and Yong-qing Li Department ofPhysics, East Carolina University, Greenville, North Carolina 27858 acube3(1!yahoo. covm,livrnail.ecu. ec/ Abstract: We report on optical levitation and manipulation of microscopic particles that are stuck on a glass surface with a pulsed optical tweezers. Both the stuck dielectric beads and biological cells are demonstrated to be levitated. ©2005 Optical Society of America OCIS codes: (170.4520) Optical confinement and manipulation; (140.7010) Trapping Optical tweezers has become a powerful tool for capturing and manipulation of micron-sized particles, typically by using continuous-wave (cw) lasers [1, 2]. It has been routinely applied to manipulate living cells, bacteria, viruses, chromosomes and other organelles [3, 4], To reduce the photodamage to the trapped particles, the average power of the trapping lasers is usually limited to below hundreds of mW and near-infrared (NIR) or infrared lasers were used for trapping [3, 6]. The trapping force generated by the cw optical tweezers is typically in the order of 10-12 N [4, 5]. This weak force is efficient to confine micro-particles suspended in liquids, but not sufficient to levitate the particles that are stuck on the glass surface, where it has to overcome the binding force. Therefore, the stuck particles cannot be manipulated with the optical tweezers that only employs cw lasers. In this paper, we describe a pulsed optical tweezers that employs a pulsed laser for levitation of the stuck particles and a low-power cw laser for successive trapping and manipulation.
    [Show full text]
  • Tip-Enhanced Laser Ablation Sample Transfer for Mass Spectrometry
    Louisiana State University LSU Digital Commons LSU Doctoral Dissertations Graduate School 2016 Tip-Enhanced Laser Ablation Sample Transfer for Mass Spectrometry Chinthaka Aravinda Seneviratne Louisiana State University and Agricultural and Mechanical College, csenev1@lsu.edu Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_dissertations Part of the Chemistry Commons Recommended Citation Seneviratne, Chinthaka Aravinda, "Tip-Enhanced Laser Ablation Sample Transfer for Mass Spectrometry" (2016). LSU Doctoral Dissertations. 2355. https://digitalcommons.lsu.edu/gradschool_dissertations/2355 This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please contactgradetd@lsu.edu. TIP-ENHANCED LASER ABLATION SAMPLE TRANSFER FOR MASS SPECTROMETRY A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Chemistry by Chinthaka Aravinda Seneviratne M.S., Bucknell University, 2011 B.Sc., University of Kelaniya, 2008 May 2016 This dissertation is dedicated with love to my parents, Hector and Dayani Seneviratne my wife Sameera Herath and my little buddy Julian !. ii ACKNOWLEDGEMENTS First and foremost, I give glory to God. Without God’s blessings, none of this would have been possible. It is a great pleasure to offer my unbounding appreciation for all who have supported and guided me over the course my doctoral studies to the completion of this dissertation. I would like to thank my doctoral mentor, Dr.
    [Show full text]
  • Nobel Prize in Physics – 2018
    GENERAL ARTICLE Nobel Prize in Physics – 2018 Debabrata Goswami On Tuesday, 02 October 2018, Arthur Ashkin of the United States, who pioneered a way of using light to manipulate phys- ical objects, shared the first half of the 2018 Nobel Prize in Physics. The second half was divided equally between Gerard´ Mourou of France and Donna Strickland of Canada for their method of generating high-intensity, ultra-short optical pulses. With this announcement, Donna Strickland, who was awarded the Nobel for her work as a PhD student with Gerard´ Mourou, Debabrata Goswami is a became the third woman to have ever won the Physics Nobel Senior Professor at Indian Prize, and the 96-year-old Arthur Ashkin who was awarded Institute of Technology Kanpur, and holds the for his work on optical tweezers and their application to bi- endowed Prof. S Sampath ological systems, became the oldest Nobel Prize winner. Ac- Chair Professorship of cording to Nobel.org, the practical applications leading to the Chemistry. His research work Prize in 2018 are tools made of light that have revolutionised spans across frontiers of interdisciplinary research laser physics – a discipline which in turn is represented by with femtosecond lasers that generations of advancements and not just a single example of have been recognised brilliant work. globally, the latest being the 2018 Galileo Galilei Award of It is easy to take lasers for granted; more so in 2018, as they are the International Commission a near-ubiquitous symbol of technological acumen. Light may be of Optics. As a part of his doctoral thesis at Princeton, a wave, but producing coherent (in-phase), monochromatic (of a Prof.
    [Show full text]
  • Frontiers in Optics 2010/Laser Science XXVI
    Frontiers in Optics 2010/Laser Science XXVI FiO/LS 2010 wrapped up in Rochester after a week of cutting- edge optics and photonics research presentations, powerful networking opportunities, quality educational programming and an exhibit hall featuring leading companies in the field. Headlining the popular Plenary Session and Awards Ceremony were Alain Aspect, speaking on quantum optics; Steven Block, who discussed single molecule biophysics; and award winners Joseph Eberly, Henry Kapteyn and Margaret Murnane. Led by general co-chairs Karl Koch of Corning Inc. and Lukas Novotny of the University of Rochester, FiO/LS 2010 showcased the highest quality optics and photonics research—in many cases merging multiple disciplines, including chemistry, biology, quantum mechanics and materials science, to name a few. This year, highlighted research included using LEDs to treat skin cancer, examining energy trends of communications equipment, quantum encryption over longer distances, and improvements to biological and chemical sensors. Select recorded sessions are now available to all OSA members. Members should log in and go to “Recorded Programs” to view available presentations. FiO 2010 also drew together leading laser scientists for one final celebration of LaserFest – the 50th anniversary of the first laser. In honor of the anniversary, the conference’s Industrial Physics Forum brought together speakers to discuss Applications in Laser Technology in areas like biomedicine, environmental technology and metrology. Other special events included the Arthur Ashkin Symposium, commemorating Ashkin's contributions to the understanding and use of light pressure forces on the 40th anniversary of his seminal paper “Acceleration and trapping of particles by radiation pressure,” and the Symposium on Optical Communications, where speakers reviewed the history and physics of optical fiber communication systems, in honor of 2009 Nobel Prize Winner and “Father of Fiber Optics” Charles Kao.
    [Show full text]
  • Informacja Dla PRAC IPPT
    IPPT Reports on Fundamental Technological Research 2/2016 Krzysztof Zembrzycki, Sylwia Pawłowska Paweł Nakielski, Filippo Pierini DEVELOPMENT OF A HYBRID ATOMIC FORCE MICROSCOPE AND OPTICAL TWEEZERS APPARATUS Institute of Fundamental Technological Research Polish Academy of Sciences Warsaw 2016 IPPT Reports on Fundamental Technological Research ISSN 2299-3657 ISBN 978-83-89687-99-9 Editorial Board/Kolegium Redakcyjne: Wojciech Nasalski (Editor-in-Chief/Redaktor Naczelny), Paweł Dłużewski, Zbigniew Kotulski, Wiera Oliferuk, Jerzy Rojek, Zygmunt Szymański, Yuriy Tasinkevych Reviewer/Recenzent: Jan Masajada Received on 8th April 2016 Copyright © 2016 by IPPT-PAN Institute of Fundamental Technological Research Polish Academy of Sciences Instytut Podstawowych Problemów Techniki Polskiej Akademii Nauk (IPPT-PAN) Pawińskiego 5B, PL 02-106 Warsaw, Poland Printed by/Druk: Drukarnia Braci Grodzickich, Piaseczno, ul. Geodetów 47A Acknowledgements This work was supported by NCN grant no. 2011/03/B/ST8/05481. Research subject carried out with the use of CePT infrastructure financed by the European Union – the European Regional Development Fund within the Operational Program “Innovative Economy” for 2007–2013. The authors gratefully acknowledge NT-MDT for technical support. In addition, we would like to thank IOP Publishing to provide us with the permission to reproduce part of the paper: “Atomic force microscopy combined with optical tweezers (AFM/OT)” [1]. Development of a hybrid Atomic Force Microscope and Optical Tweezers apparatus Krzysztof Zembrzycki, Sylwia Pawłowska Paweł Nakielski, Filippo Pierini Institute of Fundamental Technological Research Polish Academy of Sciences Abstract The role of mechanical properties is essential to understand molecular, biological materials and nanostructures dynamics and interaction processes. Atomic force mi- croscopy (AFM), due to its sensitivity is the most commonly used method of direct force evaluation.
    [Show full text]
  • Integration of Confocal Laser Scanning Microscopy (CLSM) with the JPK Nanotracker™ 2
    Integration of Confocal Laser Scanning Microscopy (CLSM) with the JPK NanoTracker™ 2 This technical note describes the combination of optical that plane is selectively collected by photodetectors. Light manipulation and measurements with JPK’s emitted from other sample layers than the focal plane is NanoTracker™ and simultaneous confocal imaging. As screened using an adjustable pinhole. As a result, only will be discussed below, the unique optical design of the light coming from a small (confocal) volume is analyzed NanoTracker™ 2 allows undisturbed high quality while background signals from other parts of the sample confocal imaging and eliminates unwanted interactions are largely reduced. Besides the ability to image different between the trapping and scanning lasers. planes in the sample, this approach results in a high signal-to-noise ratio and superior spatial resolution Introduction compared to conventional fluorescence microscopy. For Optical manipulation as well as the related measurement more detailed information on confocal imaging of forces and displacements has become a frequently techniques and their biological applications, we used tool in a broad range of disciplines from material recommend the book by James Pawley [6]. sciences to biophysics and biology. Dielectric objects (typically spherical polystyrene or silica beads in the micrometer range) are optically trapped in a tightly focused laser beam and serve as handles for precise manipulations on the nanometer scale down to the single molecule level. With the NanoTracker™ 2, the simultaneous high resolution measurement of piconewton forces and nanometer displacements within the trap becomes available. Many biological applications require the combination of optical manipulation with advanced imaging techniques like differential interference contrast (DIC) and fluorescence microscopy.
    [Show full text]
  • Optical Tweezers with Enhanced Efficiency Based on Laser-Structured Substrates
    Optical tweezers with enhanced efficiency based on laser-structured substrates D. G. Kotsifaki1 , M. Kandyla2, I. Zergioti1, M. Makropoulou1, E. Chatzitheodoridis3, A. A. Serafetinides1,a) 1Physics Department, National Technical University of Athens, Heroon Polytechniou 9, 15780 Athens, Greece 2Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vasileos Constantinou Avenue, 11635 Athens, Greece 3School of Mining and Metallurgical Engineering, National Technical University of Athens, 15780 Athens, Greece a) Electronic mail: aseraf@central.ntua.gr 1 ABSTRACT We present an optical nanotrapping setup that exhibits enhanced efficiency, based on localized plasmonic fields around sharp metallic features. The substrates consist of laser- structured silicon wafers with quasi-ordered microspikes on the surface, coated with a thin silver layer. The resulting optical traps show orders of magnitude enhancement of the trapping force and the effective quality factor. 2 Conventional optical tweezers, initially proposed by Ashkin1, are widely employed for trapping micro- and nanometer-size particles, from micro-colloids and droplets2,3 to living cells4,5, in a microfluidic environment. This technique is based on radiation pressure forces, and stable trapping is achieved when the gradient of the electromagnetic field intensity (gradient force) dominates scattering forces and Brownian motion1. However, in the case of nanometer-size particles, the magnitude of the gradient force decreases. Additionally, damping of the trapped particles decreases rapidly due to a decline in viscous drag6. The above mentioned effects work against stable trapping. Therefore, optical trapping of nanometric particles requires more sophisticated optical techniques7. Alternative methods based on evanescent fields are used for optical nanotrapping, such as scattering by sub- wavelength particles8,9 or confinement of light in a metallic tip10.
    [Show full text]
  • Dual Wavelength Optical Tweezers for Confocal Raman Spectroscopy
    Optics Communications 245 (2005) 465–470 www.elsevier.com/locate/optcom Dual wavelength optical tweezers for confocal Raman spectroscopy C.M. Creely a, G.P. Singh a, D. Petrov a,b,* a ICFO – Institut de Cie`ncies Foto`niques, c/Jordi Girona 29, 08034, Barcelona, Spain b ICREA – Institucio´ Catalana de Recerca i Estudis Avanc¸at, Barcelona, Spain Received 16 September 2004; received in revised form 5 October 2004; accepted 6 October 2004 Abstract We describe the use of dual optical tweezers to manipulate micron-size particles in and out of the focus of a confocal Raman microscope. One of the beams excites the Raman spectrum while the second tweezers improves the sensitivity of the technique and also allows for the manipulation of the environment of the trapped objects. We concentrated on optimising the alignment of both trapping and Raman excitation beams and on the background subtraction method. Even at the low trapping/excitation powers used a single living cell could be trapped and monitored for over 2 h without incurring damage. Ó 2004 Elsevier B.V. All rights reserved. PACS: 87.80.Cc; 87.64.Je; 87.64.Tt Keywords: Optical tweezers; Raman spectroscopy; Microspectroscopy Optical tweezers utilise the force of radiation perform spectroscopy on a single object of micron pressure to trap and manipulate micron and sub- size. A greater signal to noise ratio from the micron sized particles whose refractive index dif- trapped object can be achieved when background fers from that of the environment [1,2]. Since signals from the cover slip and immersion oil are 1984 radiation forces have been used to perform minimised by manipulating the particle inside a spectroscopy on trapped particles, aerosols and sample holder.
    [Show full text]