optical tweezers- 20 years on-david mcgloin

8/12/2019 Optical Tweezers- 20 Years on-David McGloin

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December 2006, published 15, doi: 10.1098/rsta.2006.18913642006Phil. Trans. R. Soc. A

David McGloinOptical tweezers: 20 years on

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Optical tweezers: 20 years on

BY DAVID MCGLOIN*

School of Physics and Astronomy, University of St Andrews North Haugh,St Andrews KY16 9SS, UK

In 1986, Arthur Ashkin and colleagues published a seminal paper in Optics Letters,Observation of a single-beam gradient force optical trap for dielectric particles whichoutlined a technique for trapping micrometre-sized dielectric particles using a focused laserbeam, a technology which is now termed optical tweezers. This paper will provide abackground in optical manipulation technologies and an overview of the applications ofoptical tweezers. It contains some recent work on the optical manipulation of aerosols andconcludes with a critical discussion of where the future might lead this maturing technology.

Keywords: optical tweezers; optical manipulation; colloids; aerosols; light beams

1. Introduction

The idea that light can trap and manipulate particles is what sold me on acareer in research. It is one of those counterintuitive ideas that just seems

wrong at some level, but when it is explained it makes perfect sense. The ideathat light can exert forces on particles so as to push them (rather than trapthem) is not so strange if we consider light as photons which possessmomentum. If light can be reflected from a surface or scatter in some waythen we must allow for the fact that its momentum has been changed and theremust be, from Newtons second law, a force (force is proportional to the rate ofchange of momentum) associated with this change. This is how radiationpressurecan be described. The concept of radiation pressure was considered byJames Clerk Maxwell (1873)as he probed the consequences of his description ofelectromagnetic radiation.

In a medium in which the waves are propagated there is a pressure in the direction normalto the wave, and numerically equal to the energy contained in unit volume.

(Maxwell 1873)

When we consider radiation pressure today, we tend to make use of lasers withtheir associated high intensity, and so it seems remarkable that P. N. Lebedevdemonstrated the existence of radiation pressure using no more than a focusedarc lamp (Lebedev 1901). Moreover, he did this in 1901, pioneering an area thatwould not see real resurgence until the early 1970s. This work would lead to twoNobel prizes (to date) allowing the laser cooling of atoms (e.g.Chu 1998) and thecreation of BoseEinstein condensates in cold atomic gases (e.g.Ketterle 2002).

Phil. Trans. R. Soc. A (2006) 364, 35213537

doi:10.1098/rsta.2006.1891

Published online18 October 2006

One contribution of 23 to a Triennial Issue Mathematics and physics.

*[email protected]

3521 q2006 The Royal Society

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The second type of force that light can exert can again be described byNewtons laws (although this explanation is only strictly valid in the case wherethe wavelength of the light is much smaller than the size of the particle involved)by considering what happens to the light as it traverses a dielectric particle.First, one notices that the light is refracted through the object, and as the lightsdirection is changed so must its momentum: thus a force must be acting on the

particle (figure 1). To understand the direction of the force, we must consider thefact that the experiments that will be discussed in this paper make use of lasers.The typical profile of a laser beam is Gaussian, with the most intense part of thebeam lying in the centre.

Thus, if the refractive index of the particle is greater than that of thesurrounding medium, then the particle is attracted to the centre of the beam; if itis less than that of the surrounding medium, then the particle is repelled from thebeam. Since the force is dependent on the intensity gradient of the beam, this typeof force is called thegradient force(also called the dipole force). The assumption inthis paper is that the relative refractive index (the ratio of the particle to medium

refractive index) is greater than 1 and that we are working in the attractive forceregime. Fromfigure 1we can see that particles should be relatively easy to confinein the transverse direction of the beam, but what about in the direction of beampropagation? Although it may be slightly counterintuitive when considered inlight of radiation pressure we can also observe trapping in this axial direction,whereby the particle is confined very close to the beam focus, provided thegradient force is larger than the radiation pressure force. Thisz-trappingconditionis achieved practically using high numerical aperture optics (the majority ofexperiments make use of oil immersion microscope objectives with NAsO1). Thetechnique developed byAshkinet al. (1986)in which a particle is confined in this

manner, by a single laser beam, is known as optical tweezers and celebrates itstwentieth birthday in 2006. The background, state of the art and future outlook inthe general area of optical manipulation are the subject of this review.

laser(a) (b) laser

trapping

plane

resulting force

Figure 1. (a) The basic optical tweezers principle: take a laser source and focus it through a highnumerical aperture microscope objective. (b) The beam paths through a dielectric sphere. Thethicker line indicates a higher incident beam intensity. The imbalance in intensity between the

inside and the outside of the beam means that the applied force on the bead must act towardsthe higher intensity part of the beam. This illustrates how a particle is confined in the transverse planeof the beam.z-trappingis not shown but works in a similar manner to the transverse trapping.

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2. A brief history of optical forces

After the very early pioneering work, interest in optical forces would largely dieaway until the development of the laser during the 1960s. Arthur Ashkin,working at Bell Labs, pioneered study in this area and produced a stream of

remarkable work that laid the foundations for the field. Indeed, one could arguethat the majority of work in many areas of optical manipulation is really onlyincremental in terms of the work carried out by Ashkin and colleagues. Heinitially focused on the radiation pressure of light. He demonstrated the ability oflight to guide particles (Ashkin 1970), to levitate particles (radiationpressuregravity traps; Ashkin & Dziedzic 1971), to confine particles in dual-beam radiation pressure traps (Ashkin & Dziedzic 1985), the levitation ofairborne droplets (Ashkin & Dziedzic 1975), confinement in vacuum (Ashkin &Dziedzic 1976) and precision trapping via feedback (Ashkin & Dziedzic 1977a).Many of these techniques fell away from what the mainstream optical trapping

community were actively working on but are now seeing a resurgence in interest.My own group, for example, works on optical levitation and guiding and isimplementing dual-beam trapping methods, all primarily to trap airborneparticles (which will be discussed below). Other notable work making use ofradiation included the observation of whispering gallery modes in levitateddroplets byAshkin & Dzeidzic (1977b); such cavity resonance (the droplet actsas a microscopic optical cavity) can be used to experimentally verify Mie theoryand size droplets very accurately.

Up until the demonstration of the single-beam trap, much of the work onoptical forces had been pushing the drive towards laser cooling of atoms (e.g. Chu

et al. 1985) and Ashkins work fitted in as physics of the highest rankheachieved just about everything one could imagine doing with radiation pressureover the course of a decade, but without a definitive focus. Also the availability oflaser sources at the time may have limited work in this area by the widercommunity. In contrast, the optical tweezers technique would open up new areasof study in a short period of time.

The paper Observation of a single-beam gradient force optical trap fordielectric particles (Ashkin et al. 1986) is a classic. Not only does it discuss awholly new technique, but also it outlines exactly how the field would pan outover the next two decades.

They also open a new size regime to optical trapping encompassing macromolecules,

colloids, small aerosols, and possibly biological particles. The results are of relevance to

proposals for the trapping and cooling of atoms by resonance radiation pressure.

(Ashkin et al. 1986)

And this is exactly what people would continue to work on. The paper alsoholds a few surprises. Not only are large Mie particles trapped (10 mm diameter)but also small Rayleigh particles, indeed evidence is presented demonstratingthe trapping of 25 nm diameter silica beads, which still presents a realexperimental challenge today and is of relevance for the developing

nanotechnology field. The paper also outlines the drag and drop techniquefor measuring the forces involved on holding particles, a quick and dirty methodthat is used in many laboratories today.

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3. Techniques

The specific function of optical tweezers is to allow the non-invasivemanipulation of single particles (or in more advanced set-ups many singleparticles simultaneously) and to carry out some kind of study on that particle.Over the past 20 years, the science enabled by optical tweezers has concentratedon the measurement of position and forces to incredible precision, primarily onforce-producing molecules in biology, but also in colloid interactions studies andhydrodynamics. They have also allowed the controlled study of the properties oflight beams and enabled single particle spectroscopy in a controlled manner. Newvariations on the original single-beam trap continue to develop, opening up newstudies and allowing us to underpin our work with a better understanding of thescience behind the interaction of light with matter. So how do these basic

techniques work?One of the most powerful things about optical tweezers systems is their

simplicity. A functional optical tweezers that is used to carry out publicationquality research can be constructed from a laser, a couple of telescopes, a fewmirrors, a microscope objective and some imaging optics with a camera. This hasallowed the proliferation of the technique and its introduction into undergraduateteaching labs worldwide. Indeed, a summer student in my group recentlydeveloped a portable compact optical tweezers system which consisted of a low-power laser diode, a dichroic mirror, an aspheric lens (instead of a microscopeobjective) and a battery powered wireless camera and high brightness LED

(for imaging). The whole system is mounted on a post and is portable in the sensethat it can be lifted in one hand and moved from place to place (and still work).A schematic for a simple optical tweezers set-up is shown infigure 2.

camera

laser

beam steering

mirror

telescope

lenses form

image relay

microscope

objective

sample cell

incoherentillumination

sample

stage

Figure 2. A basic optical tweezers system. The beam is expanded to the desired size by the firsttelescope. The second telescope aids in beam alignment and beam steering. This expanded beampasses through a microscope objective into the sample. Such a system is very simple to design andbuild, and this simplicity is one of the optical tweezers great selling points.

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At present, one of my primary research interests is the use of holographicallygenerated light fields for the manipulation of particles and atoms. This is one of

the newer methods for the trapping and manipulation of multiple particles.These fall into three broad areas: simple beam combination techniques, scanningtechniques and holographic techniques.

ThebeamcombinationtechniquesarebestsummarizedbytheworkofFallman&Axner (1997), in which a laser is split into two separate beams. With carefuloptical design, these can be combined and independently controlled in the focalplane of the microscope objective. Thus, a dual-beam optical tweezers is formed,and this type of system forms the workhorse for much of the force measurementresearch that currently takes place. Scanning techniques generally make use ofacousto-optic deflectors (AODs) which can scan a beam from point to point at

kilohertz rates. This is a very powerful and flexible technique (one of the mostimpressive demonstrations is that of micro-Tetris by Christoph Schmidts group atVrije University (http://www.nat.vu.nl/compl/index-en.html )), which works bytime-sharing the light between trapping sites. So long as the beam returns to thetrapped particle before it diffuses away, then the particle will remain trapped. Thistype of effect can be partially extended into three dimensions by some clever optics,and has been demonstrated by Alfons van Blaaderens group (Vossenet al. 2004).

To achieve true three-dimensional control of multiple trap sites, one must moveto holographic techniques. A hologram is able to control the phase of a light beam,which tells how the beam will propagate. So if one wishes to have a laser beam turn

into a picture of the Royal Society crest a hologram which encodes the phase of thecrest pattern must be generated. Then by reflecting our normal laser beam off thehologram, we can transform that beam into the desired pattern. The holograms canbe generated by the use of well-established algorithms. For complex holograms(designs other than simple arrays of spots), we make use of iterative algorithmsdesigned to solve the inverse problem. The concept is illustrated in figure 3.

Such work was pioneered byFournier et al. (1995), who made use of staticglass holograms to produce beams with multiple trapping sites. Follow-up workto this was done by David Griers group (then in Chicago, now at NYU) whostudied the use of glass-etched holograms for examining the dynamics of colloidal

particles in these extended light fields. This work would form the basis for lateroptical sorting techniques. The next step beyond the fabricated techniques wasto use dynamically alterable devices, spatial light modulators. The pioneers in

(a) (b) (c)

Figure 3. Producing a hologram from an intensity pattern. To recreate an intensity image using ahologram, we first need to compute the desired phase of that object. This is done by taking thedesired image ((a) the Royal Society logo) and feeding it through a computer algorithm, in thiscase an iterative adaptive algorithm. The computer can compute the hologram ( b) and what itpredicts the image will look like when replayed by the illuminating laser beam (c).



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this area were Tizanis group at the University of Stuttgart, who outlined howcomputer controlled holograms (Reicherteret al. 1999;Lieseneret al. 2000) couldbe used to generate trapping patterns that could be iterated in real time andallowed full three-dimensional control over the particles. This work was quicklybuilt upon by the Grier group in the paper Dynamic holographic optical

tweezers (Curtiset al. 2002), and is now regarded by many as the foundation onwhich the growing body of work in this area is based. The 2002 paper outlinedhow iterative algorithms could be used to implement complicated trappingarrays and extended the work of Tiziani from a few to hundreds of trap sites.Although it is not the first paper in the field it did seem to energize thecommunity about this new technique. The advantages of using dynamicholographic optical tweezers are that they offer full control over the spatiallocalization of a trapped particle. This means that each particle can be movedindependently in three dimensions. Further, the use of holograms offers thepotential to correct for optical aberrations in the system (Wulff et al. 2006) as

well as offering a user-friendly experimental experience (if one hologram is wrong,then it is a simple matter to change it to a better one). The disadvantages of thistechnique tend to lie in the speed of the devices, their efficiency and imagefidelity. The issue of speed is one that can easily be seen by trying to recreate thebeam scanning technique of an AOD by a spatial light modulator (SLM). Theparameters for each manufactures SLM are slightly different, but we carried outa time-sharing experiment with a phase modulating Boulder nonlinear systemsdevice (Melvilleet al. 2003), which should have been able to run at above videoframe rates (compared with kilohertz rates for an AOD) and found that inpractice, for trapping experiments, it was limited to around 10 Hz. This wasenough to trap six particles via time sharing but shows that the SLM is notoptimized for rapid dynamic tasks. In the experiment to demonstrate this, wealso showed some of the power of the SLM, in that the particles trapped were alltrapped on different z-planes, spaced approximately 1 mm apart, a trick thatcannot be done with an AOD. Of course, one can just make a hologram to trapthe six particles simultaneously rather than by time sharing. The dynamics of theSLM have not really been an issue in experiments to date and for thoseexperiments that do require speed, such as atom trapping (McGloinet al. 2003),different types of spatial light modulators with much lower efficiency can be used,whereby kilohertz rates can be achieved (Boyer et al. 2004,2005).

To date, much of the work on the SLMs has concentrated on device

characterization and novel colloidal studies, with some work in the biosciences, aswell as on more general beamshaping techniques. To date, two of the main players arethe Grier group at NYU and Miles Padgetts group in Glasgow. The New York grouphas focused on colloidal manipulation, beam shaping and algorithm improvements.The power of SLMs in beam shaping techniques was shown in the study of opticalvortices (optical singularities; Curtis & Grier 2003a,b) in which optical vortex beamscould be created very simply in real time. They could be modulated with ease to alterthe shape, while still retaining the orbital angular momentum associated with suchbeams. The experiments described by Curtis & Grier (2003a) are simple to try usingdynamic techniques, but would require significant fabrication effort to be done

offline. Such vortices have also been used to study hydrodynamic coupling betweencolloidal particles (Ladavac & Grier 2005) and develop microfluidics pumps(Ladavac & Grier 2004). Other colloidal work has made used of dynamically

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evolving light patterns to mimic peristaltic pumps, whereby temporally evolvinglight patterns can be used to transport particles in the absence of flow. Suchtechniques can alsobe usedto achieve and study thermal ratchets (Lee&Grier2005a,2006; Lee et al. 2005) including the observation offlux reversal, whereby particles canbe made to move in the opposite direction to the direction of the optical pattern. My

own group has been working on similar mechanisms using static patterns. We haveexamined how particles move in Bessel beams, which show directed transport ofparticles owing to the intensity imbalance between the outside and the inside of thebeam (Milneet al. 2005;Patersonet al. 2005). This technique can be used to sortparticles, including red and white blood cells in the absence of flow. The sorting isrelatively slow, but may find niche application areas.

Other notable work by the New York group in this area has been thedemonstration of dual wavelength holographic optical tweezers (Lee & Grier2005b), a technique that is likely to be of interest for future optical tweezersspectroscopic tools, the push of optical tweezers as nanotools for the manipulation

of both carbon nanotubes (Plewa et al. 2004) and semiconductor nanowires(Agarwal et al. 2005), and the assembly of quasicrystals (Roichman & Grier 2005).The other main group working on holographic traps is based at the University

of Glasgow, which is the home of the signature experiment in this area. Akin tothe Tetris experiment using AODs, the Smallest strip the willow in the world(Willow) demonstrates the power of the SLM technique (and is beautifully put tomusic to boot). The Glasgow work has focused on complicated beam shaping,and the creation and controlled rotation of three-dimensional crystals (Bingelyteet al. 2003; Jordan et al. 2004;Leach et al. 2004a; Sinclair et al. 2004a), three-dimensional beam propagation algorithms (Sinclair et al. 2004b; Whyte &Courtial 2005) as well as the structure of light beams (Leachet al. 2004b). Theyhave also looked at limiting values in holographic traps and aberration correction(Sinclair et al. 2004c) and are now exploring applications in microfluidics,including using holographic tweezers and video microscopy to map out fluid flowin microchannels and around rotating microobjects (Di Leonardo et al. 2006).

Another recent first for holographic tweezers includes the Raman imaging ofcells (Creelyet al. 2005), in which a cell is manipulated by an array of spots andscanning through the probing Raman beam. For people interested in single particlespectroscopy, optical manipulation seems to offer much: the particle of interest islocalized and static and therefore is easy to probe. There has been increasing workin this area, with Raman spectroscopy being a popular choice among researchers.

Thurn & Kiefer (1984) carried out work on Raman spectroscopy of levitateddroplets, whileBiswaset al. (1989)looked at stimulated Raman scattering (SRS)and this work built onAshkin & Dziedzics (1977b)whispering gallery mode work.Direct Raman spectroscopy on a trapped particle was shown by Ajito & Torimitsu(2001)on droplets in solution and on polystyrene beads.

The use of holographic tweezers in cellular microscopy (Emiliani et al. 2005)has also been recently shown, demonstrating how the technique may be used inbiology to measure forces, generate forces (Emiliani et al. 2004) or to locallyprobe different parts of a cell simultaneously by trapping an array of particlesand moulding them around the cell.

The use of holograms in optical trapping is not limited to the use of spatial lightmodulators. One of the most significant papers in recent years in the field was thedemonstration of particle sorting using an optical lattice (MacDonald et al. 2003).





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Here, a simple-etched hologram was used to create a five-beam interferencepattern which was projected into a sample. As particles flow through the three-dimensional optical structure, they are separated. The separation is due to thedifferent interaction with the light that particles with different polarizabilitieshave. It is also dependent on the connectivity of the lattices sites, with some lightleakage between sites offering the best sorting. The sorting mechanism is due tothe interplay between the Brownian motion of the particles, the optical forces andthe flow-induced forces. By tailoring the relative phases and intensities of theinterfering beams a sophisticated sorting sieve can be created. Such work has alsobeen demonstrated in a slightly simpler set-up using a single line of the SLM-generated traps (Ladavacet al. 2004a). As optical sorting is passive, particles aresorted by their inherent properties and do not necessarily require labelling.Therefore, the goal would be to separate cell types, such as red and white bloodcells, or cancerous and non-cancerous cells merely due to the fact that theyinteract with the light in slightly different ways. Work towards these goals isunderway, but robust and reliable methods of routinely sorting cells, as opposed tonon-biological colloidal particles, are still some way off.

Another developing technique for the manipulation of large numbers ofparticles is evanescent field manipulation (figure 4). This work, pioneered byKawata (Kawata & Sugiura 1992), has seen a resurgence of late (Garces-Chavez

et al. 2005; Quidant et al. 2005) and recent work has demonstrated opticallybound arrays in evanescent fields (Mellor & Bain 2006) and also large areamanipulation using surface plasmon field enhancement (Garces-Chavez et al.2006). While this type of manipulation may have applications in colloidalcrystallization studies, it is not yet evident if it offers any significant advantagesover existing techniques. Further the issue of the control of individual particleswithin the evanescent field has yet to be seriously addressed, but it does showpromise as the areas over which particles can be manipulated are significantlylarger than the microscope-based techniques.

The final technique to discuss is one of the most widely used in optical

manipulation, the ability to detect very small position changes and the ability tosense and apply forces in the piconewton range. These techniques rely on the factthat a simple beam optical tweezers is a harmonic trap and any particle trapped

CCD

CMIL

BS

RL laser

prism

mirror

L2

n2

I(h)

n1

L1

MO

(a)

(b) (c)

Figure 4. (a) Experimental configuration for large area evanescent manipulation. The inset shows thebeam geometry at the interface of the prism and the sample. (b) Field intensity of a five-beaminterference pattern used to trap particles in (c). Reprinted with permission from V. Garces-Chavez.

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in it can be considered a damped harmonic oscillator. This information allows usto calibrate the trap to work out the spring constant and using this informationallows us to sense the position of a trap particle or work out the force beingapplied to the particle. The primary uses of this technique are in the studiesof biological organisms, and in particular, the work on molecular motors

(Neuman & Block 2004). In the past year or so, Steven Blocks group at Stanfordhave developed their already world-leading studies in this area to push theposition sensing capabilities of optical traps to angstrom precision (quiteremarkable when you consider the diffraction limited resolution of the trapitself). They were able to observe the base pair stepping of RNA polymerase,showing that the steps averaged around 3.7 A (Abbondanzieri et al. 2005;Greenleafet al. 2005). Another landmark experiment in this field was also carriedout last year (Rohrbach 2005) in which an optical tweezers was used to measurea controlled 25 fN force on a 533 nm latex sphere. The suggestion is that this isthe smallest switchable force ever measured and illustrates the power of optical

tweezers in this arena. To put this in context, the thermal forces on such aparticle are likely to be in the piconewton regime and the Sun exerts around20 fN on a 75 mm diameter dust particle floating in the atmosphere. These tworesults (position and force sensitivities) push the techniques into new realms thatwill allow us to explore ever more sensitive parameters and is likely to enableoptical tweezers to get more of a handle on the nanoworld.

(a) Optical manipulation derivatives

Recent work by Ming Wus group at Berkeley (Chiou et al. 2005) hasdemonstrated a convergence of two types of manipulation to offer a tantalising

vision of the future of optical manipulation. The concept is essentially an extensionof the trapping technique known as dielectrophoresis (DEP) in which electric fieldsare used instead of optical fields to trap (or repel) particles. Conventional DEPmakes use of patterned electrodes to allow localization of the electric field toenable trapping. As such it is a fixed architecture technique requiring complicatedpatterning to enable more arbitrary functionality. The beauty of the new light-induced dielectrophoresis (LIDEP) is that a large area electrode can be patternedby a light field, by either scanning a pattern on the electrode or a mask (such as ahologram) to project a static pattern, and the optical power level required tocreate an electric trap can be extremely low (microwatts) compared with optical

tweezers. The power of the technique lies in combining the large area effect of DEPwith the precision and reconfigurability of optics. This new technique has mademany in the optical trapping community to sit up and take notetraditionally,the optical and dielectrophoretic communities work in isolation. Work in theOptical Trapping Group in St Andrews has recently produced a proof of conceptLIDEP device with the ultimate goal of producing a large area, high throughputsorting device (S. N. Neale 2005, personal communication).

4. Recent work

At present, one of my research focuses is the manipulation and interrogation ofdroplets. Some of the application of this work is inspired by our collaborators,the group of Jonathan Reid, in the Chemistry Department at Bristol University.





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The focus is on the manipulation of droplets with a view to studying their size,composition and dynamics (gas uptake studies, particle coagulation, etc.), inatmospheric chemistry. Our work concentrates on areas, such as guiding ofdroplets over large distances using radiation pressure, designing new techniquesto trap and probe droplets enabling the manipulation of many dropletssimultaneously to try and create optically driven digital microfluidic chemical

microreactors.We have recently been working on the optical guiding of aerosol droplets(Summers et al. 2006). Typical work on the optical spectroscopy of singledroplets relies on the probing of freely falling droplets. Therefore, single dropletmanipulation techniques have great utility in allowing more accurate and longertime-scale studies. Optical tweezers is one method of achieving single dropletlocalization. However, for some studies, it would be useful to drive a droplet (orother airborne particle) through a number of spectroscopic beams (say, doing afluorescence measurement followed by a Raman measurement) in a controlledfashion; we have been investigating ways to do this optically (figure 5). The

simplest method is to take a Gaussian beam and levitate a particle againstgravity. One can then alter the power in the beam and adjust the equilibriumposition to adjust the height of the particle. Using this method, we can guidedroplets (of water, ethanol and dodecane) over several hundred micrometres.However, a higher guiding distance is desired to give appropriate spatialseparation between our multiple probe beams and so we make use of the non-diffracting properties of the Bessel beam. The Bessel beam has a long, thin corewhich does not spread in the same way as a focused Gaussian beam of similardimension. As such we can use it to controllably guide droplets over much longerdistances, of around 4 mm.

Trapping airborne particles is inherently more difficult than trapping particlesin a suspending medium such as water. This is due to the reduced damping offeredby the medium, and so one cannot simply go and pick up the particle one desires as

Figure 5. Optical levitation of dodecane droplets. We are able to stably trap arrays of droplets

(in this case six) using a Bessel beam. We believe the arrays are optically bound, that is theposition of the droplet above the one below is determined not only by the levitating beam but alsoby the interaction of the light with the lower droplet.

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in conventional fluid sample optical tweezers, rather we must wait until a particlefalls into the trap and then make a decision as to whether it is the one we want.This is one reason why airborne trapping has received far less attention thantrapping in the underdamped regime to date, despite much of Ashkins early workbeing devoted to levitating droplets. The focus for early airborne work up until afew years ago was optical levitation, often combined with Raman spectroscopy(Thurn & Kiefer 1984;Biswaset al. 1989).Omoriet al. (1997)appear to have been

the first group to directly optically trap (as opposed to levitate) an airborneparticle, in this case glass beads. Magome et al. (2003)achieved the same resultwith liquid droplets. Reids group along with Andrew Ward at the RutherfordAppleton Laboratory (RAL) followed up this experiment with work trapping twodroplets in a dual-beam trap (Hopkins et al. 2004) and presented the firstdemonstration of optically controlled coagulation of aerosols. Using the samesystem around the same time (at RAL), Martin King examined how singleseawater and oleic acid droplets reacted with ozone (King et al. 2004). Both ofthese recent papers indicate the potential power of optical trapping techniques toelucidate mechanisms in atmospheric science.

Reids work demonstrated the utility of using cavity-enhanced Ramanscattering in sizing droplets (to within G2 nm, limited by measurementresolution) and also outlined how tweezers may be used to study dropletdynamics by examining what happens during coagulation (figure 6a). We haverecently demonstrated the use of holographic optical tweezers to trap multipleaerosol droplets (Burnham & McGloin 2006; figure 6b,c) with the intention toexplore similar ideas with increasing numbers of particles. We have also shownthat the refresh rate of the SLM does not seem to be a limiting factor to observethe real-time manipulation of droplets and this has allowed us to demonstrate thecontrolled coagulation of water droplets. We have also tentatively demonstrated

the ability of optical tweezers to rotate droplets (via orbital angular momentum)and we believe that this may allow us to create micromixers for airbornechemical reactors and microfluidic devices.

605

(a)

615relative

intensity

/arbitrary

units

625 635

wavelength (nm)

(b) (c)

Figure 6. (a) Droplet coagulation and associated Raman spectra. The sharp spectral peaks are dueto the cavity enhancement provided by the droplets. The uppermost spectra are for the droplet onthe right, the middle spectra are the combined signal for both drops and the lower signal for thecoagulated drop. The combined volume calculated from the spectra is 3.905!10K16 m3, while thecoagulated volume is calculated as 3.902!10K16 m3. (b) Array of six droplets held by holographic

optical tweezers. (c) Array of four droplets in a Y configuration held by holographic tweezers.(Reproduced by permission from the PCCP Owner Societies.)





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I would like to acknowledge the Royal Society for their support as well as the RSC, NERC andEPSRC for funding some of the work mentioned above. The members of the Optical TrappingGroup at St Andrews are also thanked for many stimulating discussions over the past several years.

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Whyte, G. & Courtial, J. C. 2005 Experimental demonstration of holographic three-dimensionallight shaping using a Gerchberg-Saxton algorithm. New J. Phys. 7, 117. (doi:10.1088/1367-2630/7/1/117)

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http://dx.doi.org/doi:10.1088/1367-2630/7/1/117http://dx.doi.org/doi:10.1088/1367-2630/7/1/117http://www.physics.gla.ac.uk/Optics/projects/StripTheWillow/http://dx.doi.org/doi:10.1364/OE.14.004170http://rsta.royalsocietypublishing.org/http://rsta.royalsocietypublishing.org/http://rsta.royalsocietypublishing.org/http://rsta.royalsocietypublishing.org/http://dx.doi.org/doi:10.1364/OE.14.004170http://www.physics.gla.ac.uk/Optics/projects/StripTheWillow/http://dx.doi.org/doi:10.1088/1367-2630/7/1/117http://dx.doi.org/doi:10.1088/1367-2630/7/1/117


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AUTHOR PROFILE

David McGloin

David McGloin is a Royal Society University Research Fellow in the School ofPhysics and Astronomy at the University of St Andrews. He received anMSci(Hons) in laser physics and optoelectronics in 1997 from St Andrewsfollowed by a PhD on electromagnetically induced transparency, also from StAndrews, in 2000. He left academia after his PhD to work for Dstl at FortHalstead on optical imaging techniques before returning to St Andrews as apostdoc. He worked on the laser manipulation of cold atoms and optical tweezers.Awarded a URF in 2003 on the topic of tailored optical potentials for particleand atomic manipulation, he now works on developing manipulation techniques

and applying them in a range of areas from the physical, chemical and biologicalsciences. During 2006, he is spending six months in the Chemistry Department atthe University of Washington in Seattle.


Phil Trans R Soc A (2006)


optical tweezers- 20 years on-david mcgloin

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