IOP PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS

J. Opt. A: Pure Appl. Opt. 10 (2008) 093001 (10pp) doi:10.1088/1464-4258/10/9/093001

REVIEW

Light-induced manipulation withsurface plasmonsM Righini1, C Girard2 and R Quidant1,3

1 ICFO—Institut de Ciencies Fotoniques, Mediterranean Technology Park,08860 Castelldefels (Barcelona), Spain2 NanoSciences Group, CEMES, UPR–CNRS 8011, 29 rue Jeanne Marvig,31055 Toulouse, France3 ICREA—Institucio Catalana de Recerca i Estudis Avancats, 08010 Barcelona, Spain

E-mail: romain.quidant@icfo.es

Received 9 June 2008, accepted for publication 9 July 2008Published 11 August 2008Online at stacks.iop.org/JOptA/10/093001

AbstractWe review recent advances achieved in the field of surface plasmon-based opticalmanipulations. We first discuss enhanced optical forces at the surface of a flat metal film andtheir use for self-organizing a large number of micro-objects. We then show how a suitableengineering of plasmon fields near micro-gold pads enables trapping at a specific location withmuch weaker laser intensity compared to conventional optical tweezers. This part is illustratedby a series of numerical simulations based on the theory of the Green dyadic. Finally, we showthat, beyond their low power requirement, this new generation of integrated optical tweezersoffers new perspectives in optical manipulation including parallel trapping with a single beamand controllable selectivity on the object polarizability.

Keywords: plasmon optics, near-field optics, optical manipulation

(Some figures in this article are in colour only in the electronic version)

M This article features online multimedia enhancements

1. Introduction

The interaction of light with metal micro- and nano-structuressupporting surface plasmon (SP) collective excitation hasrecently received considerable attention which has spreadover different scientific communities, ranging from biologyto integrated optics. In particular, enhanced and confined SPfields have been shown to be efficient for increasing light–matter interactions with small amounts of matter. They havesuccessively been exploited for enhanced spectroscopy [1, 2],sensing of reduced numbers of molecules [3, 4] and to improvethe efficiency of light sources and detectors [5, 6].

We show here how surface plasmon optics can success-fully apply to optical manipulations in coplanar geometry.The latest advances of SP optics are exploited to engineer theoptical near-field landscape at a patterned metal surface and

achieve integrated 2D optical micro-traps. These novel SPtraps have several advantages for integration over conventional3D optical tweezers since they do not need any bulk optics andwork with substantially reduced laser intensities. Furthermore,their coplanar geometry confers onto them unique propertiessuch as their controllable selectivity to the specimen size.

After reviewing the current state of the art of opticalmanipulation based on evanescent fields in section 2, wediscuss in section 3 enhanced optical forces and self-assembling of Mie colloids at a flat metal/dielectric interfacesupporting a surface plasmon. In section 4, we describehow a suitable plasmonic pattern can be designed to trapsingle colloids at determined positions of the surface. Finally,section 5 underlines the specificities of SP traps compared toconventional 3D tweezers and their implication on the trappingselectivity to different particle sizes.

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J. Opt. A: Pure Appl. Opt. 10 (2008) 093001 Review

2. State of the art in evanescent optical trapping

Conventional optical tweezers (OT), as first proposed byAshkin in the 1970s [7] are implemented by tightly focusing,down to the diffraction limit, a laser beam with a highnumerical aperture objective lens [8, 9]. An object locatednearby the laser focus experiences two force contributions.The scattering force (or radiation pressure) is a repulsiveforce pointing along the incident k-vector while the gradientforce, originating from the gradient of the electromagnetic fieldintensity, tends to pull it towards stronger intensity regions(figure 1(A)). The object gets stably trapped when the intensityprofile at the laser focus enables the gradient force to dominateboth scattering forces and Brownian motion. The totalforce magnitude depends on the trap profile and the electricpolarizability of the object, determined by its dimensionsand the dielectric function contrast with the surroundings.In practice, typical laser intensities used for trapping are ofthe order or greater than 109 W m−2. This way OT havebeen used for 3D manipulation of different kinds of systemsimmersed into a fluidic environment from micro-colloids anddroplets [10, 11] to living cells [12–14].

Optical manipulations based on evanescent fields insteadof propagating ones open new perspectives towards theintegration of optical tweezers at the surface of a chip. Indeed,the intrinsic transverse confinement of surface fields enablesmaintaining the manipulated objects close to the surface.

In the simplest configuration, where a plane wave is totallyreflected at an interface, the particle is pulled towards thesurface by the gradient force and pushed by the scatteringforce (figure 1(B)) along the in-plane k-vector [15]. Usingthis concept, Kawata and co-workers first demonstrated opticalguiding of micro- and nano-colloids at the surface of a prismilluminated under total internal reflection [16] and on top ofa channel optical waveguide [17]. Recently, transport alongoptical waveguides has been implemented to achieve opticalsorting of micro-colloids with different sizes [18]. Beyondin-plane guiding, trapping with evanescent fields requiresadditional in-plane field gradients necessary for maintainingthe specimen at a fixed location. While an evanescent trap canbe produced by using a TIR (total internal reflection) objectivelens as reported in [19], several configurations overcoming theuse of high NA objectives have been implemented. Largeperiodic arrays of evanescent traps formed by two interferingcounter-propagating evanescent fields were reported by Cizmaret al [20, 21] (figure 1(C)). Similar results have been achievedby projecting a periodic diffractive element at the samplesurface [22].

By exploiting recent concepts borrowed from nano-optics [23–29], one can use evanescent fields scattered bysubwavelength objects (size � incident wavelength of light)to achieve non-diffraction-limited 3D optical field confinementpotentially able to trap Rayleigh objects. In particular,light confinement at the extremity of a metallic tip [30, 31](figure 1(F)) or at the output of a subwavelength hole in ametallic film [32, 33] (figure 1(E)) was predicted to enableoptical trapping of very small objects down to a few tens ofnanometers. The practical implementation of nano-trapping is

Figure 1. Overview of different trapping configurations.

associated with specific difficulties such as weak forces (dueto small polarizability of particles with small dimensions) andstrong Brownian motions which have until now prevented clearexperimental demonstrations of these predictions.

Beyond conventional evanescent fields, surface plasmons(SPs) supported at metal/dielectric interfaces offer newopportunities in optical manipulation. SPs are resonantoscillations of the quasi-free electrons of a metal at itsinterface with a dielectric that are driven by a p-polarizedelectro-magnetic field. They lead to enhanced surface fieldspropagating along the interface and decaying exponentiallyaway from it [34–36]. Because of their evanescent nature,SPs cannot be directly coupled to propagating light butrequire alternative configurations able to compensate themomentum mismatch with free space photons. In the mostpopular coupling scheme, known as the Kretchmann–Raetherconfiguration, the metal/dielectric interface is illuminatedunder total internal reflection using a glass prism. In thisway, efficient coupling to the SP mode, up to 90%, leads to aconcentration of optical energy at the metal/dielectric interfaceresponsible for a substantial enhancement of the incident field.This enhancement depends on the involved materials at theconsidered incident wavelength [34]. In the specific caseof a gold/water interface, the theory predicts an intensityenhancement of several tens at 633 nm, expected to magnifyboth optical force components compared to conventionalevanescent trapping [37].

In fact, the intrinsic properties of SPs could be of benefitto evanescent optical manipulation in two main manners.

• The enhancements of SP fields are expected to decreasethe trapping intensity requirements.

• The ability of SP fields to be confined down to thesubwavelength regime opens new perspectives to scaleoptical trapping down to the nanometer scale.

3. Enhanced optical forces and self-organization at aflat metal/dielectric interface

In a previous study [38], photonic force microscopy [39, 40]was used to measure quantitatively the enhanced optical forcesexperienced by a micro-object exposed to the SPs at a flatcontinuous metal/water interface. In good agreement with

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Figure 2. Optical configuration where a flat metal film, illuminatedunder total internal reflection by a slightly focused laser beam, isexposed to a solution of dielectric micro-beads.

theoretical predictions [37], experimental data revealed astrong enhancement factor (∼40) of the total force magnituderecorded near the plasmon resonance. In this section, we showhow SP-induced optical forces enable self-organization underlow laser intensity of a large number of free micro-beads at anextended flat metal surface [41].

The experimental set-up used for this experiment issketched in figure 2. A thin gold layer (40 nm) is illuminatedunder total internal reflection by a slightly focused laser beam(∼50 μm diameter in the surface plane) under the plasmonresonant conditions (p-polarization, λ = 1064 nm, θ = 62◦).The surface is exposed to a diluted solution of polystyrene(PS) micro-beads (5 μm) contained into a fluidic chamber ofcontrollable thickness (10 μm). The dynamics of the colloidsis monitored from the top using a long working distanceobjective connected to a camera.

Figure 3 shows the evolution with the incident laser powerof the bead distribution near the illumination region. While,for 20 mW, the beads are randomly dispersed, increasingthe power up to 50 mW makes them concentrate towardsthe center of the beam. Interestingly, at twice as muchpower (100 mW) the beads self-organize to form chainswith specific interparticle distances along and perpendicularto the incident in-plane k-vector. This effect, known as‘optical binding’ [42, 43], results from the interferencebetween the field scattered by each of the particles whichleads to preferential equilibrium positions [41]. Similar self-organization under low laser intensity was demonstrated at thesurface of a vertical dielectric cavity [44, 45].

It must be underlined that different behaviors wereobserved for larger thicknesses of the fluidic chamber. Forinstance, for a 100 μm thickness, strong thermal-inducedhydrodynamics, combining convection and thermophoresis,clearly dominates over optical forces.

4. Stable trapping at finite plasmon micro-structures

We have shown in the previous section how SP fields at ametal/dielectric interface exert enhanced optical forces on anearby object which contribute to their self-organization underlow laser intensity. Using SP forces for trapping a singleobject at a specific location requires an additional in-planefield confinement. As for conventional evanescent fields, thiscan be achieved by focusing [19] or patterning [22, 21] theincident laser light incident on the metal film. We havechosen an alternative strategy where a structured metal filmis illuminated with a homogeneous unfocused laser beam(figure 1(G)) [46, 47].

Figure 3. Evolution with the incident laser power of the distributionof 5 μm PS beads at a flat gold/water interface when illuminatedunder the SP resonance conditions (p-polarization, λ = 1064 nm,θ = 62◦). The ellipse show the approximative illumination area.From [41].

4.1. Numerical simulations

When patterning a metal surface to form finite metal pads, theboundaries confine the SP to the metal region and consequentlyreduce the spatial extension of enhanced SP fields. Theresulting contrast between the enhanced field at the metalinterface and the field at the bare substrate surface leads toan in-plane electromagnetic intensity gradient which can beengineered to form a stable potential well able to trap particleslocated in its vicinity.

To illustrate this concept, we start by computing theelectric near-field intensity distribution nearby a gold patternformed by several gold pads lying on a transparent surfaceand immersed in water. Let us consider the geometry shownin figure 4(A). In this configuration, a plane wave illuminatesthe system through the glass substrate under total internalreflection. We define E0(r, ω0) and S(r, r′, ω0) as the incidentelectric field and the field-susceptibility of the system in theabsence of supported structure (this second-rank tensor is alsocalled the Green dyadic function in the literature [27, 48]),respectively. One can show that the new electric field state(in the presence of the metal structures) is the solution of theLippmann–Schwinger integral equation:

E(r, ω0) = E0(r, ω0)

+ ε(ω0) − n2sur

4π

∫τ

S(r, r′, ω0) · E(r′, ω0) dr′, (1)

where ε(ω0) represents the permittivity of the metal, nsur theoptical index of the surroundings, and the integral runs overthe volume τ occupied by the metallic structure.

In maps (B) and (C) of figure 4, two different incidentpolarizations are considered: p-polarization, where theincident electric field is contained in the incident plane, ands-polarization, where it is parallel to the glass/water interface.Each of the gold pads behaves as a dipolar polarizable objectfeaturing different near-field optical responses depending onthe polarization of the driving incident field. Under p-polarization, the dominant field component is perpendicular tothe sample surface, leading to a field intensity maximum ontop of each of the pads. Conversely, under s-polarization, thepads are polarized in the surface plane, leading to two maximalocated just above their lateral edges. Finally, we remark thatthe disordered arrangement of the pads tends to downgrade theimage–object relation even in the near-field zone.

A small object, located at position R, and exposed toa plasmonic near-field optical landscape, such as the oneof figure 4, experiences an optical potential susceptible to

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Figure 4. Example of simulations performed near disorderedsquare-shaped gold pads embedded in water and supported by aplane transparent surface. The system is illuminated in total internalreflection: (A) top view of the model. The numerical method isdescribed in [27]. (B)–(C) Maps of the normalized electric near-fieldintensity in p- and s-polarization, computed in a plane located at35 nm from the top of gold pads (100 nm wide and 40 nm high). Thesurface wave is p-polarized in (B) where the scale varies from 0.65 to5.2, and s-polarized in (C) where the scale increases from 0.1 to 5.3(image size: 730 nm × 550 nm; incident wavelength 710 nm; andincident angle 70◦).

influencing its dynamics. Figure 5 sketches the differentforce components experienced by the object. On the onehand, the asymmetrical illumination leads to a homogeneous

Figure 5. Sketch of the force components experienced by an objectnear a finite plasmonic structure.

scattering force Fscat pointing along the incident in-plane k-vector which will tend to guide the object along the surface.On the other hand, the field gradient near the gold structurescreates attractive gradient forces Fgrad, both in the plane andperpendicular to it, pulling the object towards the field intensitymaxima [49]:

Fgrad = −∇RUgrad(R), (2)

where the potential Ugrad(R) is a function of the optical indicesnbead and nsur of the particle and surroundings, respectively, andthe electric near-field intensity |E(r, ω0)|2 is generated by thelocalized plasmons inside the bead according to

Ugrad(R) = −n2bead − n2

sur

16π

∫v

dv |E(r, ω0)|2. (3)

Let us note that the well amplitude associated with thispotential energy can be tuned at will by varying the incidentwavelength λ0 = 2πc/ω0. In the vicinity of a plasmonresonance, significant enhancement effects produced by theincreasing of the integral

∫v

dv |E(r, ω0)|2 can be achieved.To illustrate the concept of optical potential engineering

near plasmonic structures, we consider the case of gold disksand study the evolution of force and energy maps with respectto physical and optical parameters, including light polarization,size pads and arrangement effects. Under total internalreflection, the field intensity distribution around each padfeatures a maximum centered at the forward edge. Such a near-field concentration leads to a potential well for a polystyrene(PS) sphere (diameter 80 nm) located at 10 nm from the topof the metal pads (figure 6). In practice, stable trapping inthis well is conditioned by a proper balance between restoring

Figure 6. Optical binding energy maps computed by scanning a PS bead (80 nm in diameter) above a set of five gold disks (300 nm indiameter) embedded in water and supported by a plane glass surface. The sample is illuminated in total internal reflection underp-polarization. (A) Disordered arrangement (image size: 2250 nm × 1500 nm); the scale varies from −1.2 (dark) to −0.24 meV (bright).(B) Aligned arrangement (image size: 1800 nm × 1200 nm); the scale varies from −1.2 (dark) to −0.32 meV (bright). The incidentwavelength and angle are 710 nm and 70◦, respectively.

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J. Opt. A: Pure Appl. Opt. 10 (2008) 093001 Review

Figure 7. Vertical force maps (in fN) computed by scanning a polystyrene bead (40 nm in diameter) above single gold disks of differentdiameters D embedded in water and supported by a plane transparent surface. The sample is illuminated in total internal reflection (imagesize: 1200 nm × 1200 nm). The incident wavelength and angle are 710 nm and 70◦, respectively. The laser intensity is fixed at 109 W m−2.(A) s-polarization and D = 300 nm; (B) p-polarization and D = 300 nm; (C) s-polarization and D = 500 nm; (D) p-polarization andD = 500 nm; (E) s-polarization and D = 700 nm; (F) p-polarization and D = 700 nm.

gradient forces (associated to the depth of the potential well) onthe one hand and the Brownian energy kBT (where kB and Tare the Boltzmann constant and the temperature, respectively)and the in-plane scattering force on the other hand. Oursimulations for this geometry show that the restoring gradientforces dominate for a p-polarized incident surface wave andincident angles greater than 68◦. In addition, as illustrated infigure 6, the well shape is highly sensitive to arrangement ofthe metal pads. For example, linear pad arrays produce thinenergy channels that could be applied to the transport of a tinyamount of matter in coplanar geometry.

Figure 7 shows the distribution of the total normal forceexperienced by the bead as a function of the metal paddiameter and the incident polarization. This force component,FgradZ , oriented along the axis perpendicular to the sample,characterizes the attraction of the bead by the plasmonicstructure. For a given position R = (X, Y, Z = cst) ofthe bead, it is computed by taking the first derivative ofequation (3) and adding the normal component of the radiativeforce (see [50]). The regions where FgradZ takes significantvalues (in blue in figure 7) are rounded up by the circlesthat delimit the metallic disks. For all disk sizes, the trap

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Figure 8. (A)–(C) Chronological frame sequence recorded for an incidence angle θ = 66◦ and p-polarization showing the trapping of a4.88 μm PS bead at a 4.8 μm gold pad. Frame (D) was recorded after switching the incident polarization from p to s (see video 1 atstacks.iop.org/JOptA/10/093001).

area appears to be significantly more confined in p-polarizedthan in s-polarized modes. Also, the force distribution getsmore complex for increasing pad diameters due to retardationeffects. Nevertheless, under p-polarization, all force maps aredominated by a sharp maximum at the forwards edge. We alsoobserve, on the map (F) of figure 7, an enhancement by a factor50 of the binding force FgradZ produced by the metal disk (withrespect to its value away from the metal).

Making a simple analogy with 3D OT, similarly to the highNA objective lens in a conventional OT set-up, the metal disksomehow acts as a micro-lens that is able to provide from anextended illumination the in-plane field confinement necessaryfor trapping. This way, parallel trapping should also occur inan array of disks using the same homogeneous illumination asillustrated in figure 6.

4.2. Experimental results

Based on previous numerical predictions, experiments werecarried out using the set-up of figure 2. Instead of a flatgold film, different patterns of micro-sized gold disks werefabricated using e-beam lithography combined with lift-off.In order to minimize the contribution of thermal-inducedhydrodynamics, in addition to working with thin chamberthicknesses (<20 μm), we restricted ourselves to a low densityof gold micro-disks.

Figure 8 summarizes, in a sequence of successive frames,a first experiment performed with an isolated gold disk (4.8 μmdiameter and 40 nm height) located at the center of the screenand exposed to a solution of 4.88 μm PS beads. The samplesurface is illuminated with a 710 nm laser beam near theplasmon resonance angle (θ = 66◦). The illumination area

and the incident power are 0.1 mm2 and 250 mW, respectively,corresponding to an intensity I ≈ 2.5 × 106 W m−2. Ourmeasurements first show how the beads, away from the goldpad, are guided along a straight line from bottom to top. Thisis the scattering force (or radiation pressure) which pushesthe beads at the glass surface along the incident in-plane k-vector. This movement is actually exploited to bring the beadstowards the trapping area without using any fluidic flow. Oneof the guided beads passing upon the gold pad gets trapped andremains immobilized to a small portion of the disk as long asthe illumination is maintained [47].

At this stage, one can get deeper insight into SP trapproperties by analyzing the time-dependence position ofthe trapped bead and exploiting concepts of photonic forcemicroscopy [38]. In order to maximize the accuracy of themeasurement, we increase the total recording time by reducingas much as possible the scattering force magnitude. Thisleads us to work with an incident angle θ = 68◦, where thecontribution of the scattering force becomes weak while therestoring SP forces are maximum. Minimizing the scatteringforce also enables us to trap enough time under s-polarizationand to assess the trap features when not coupling to the SPmode.

The tracking diagram of a 3.55 μm PS sphere trapped by a4.8 μm gold disk under p-polarization (9 min acquisition time)is shown in figure 9(A). Already from these data it is interestingto see how the particle remains confined to a forward regionof the pad as suggested by the numerical calculations offigure 6. This confinement is specific to p-polarization wherethe incident laser couples to the SP mode of the pad. In s-polarization the bead’s trajectory extends over nearly the wholepad area.

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J. Opt. A: Pure Appl. Opt. 10 (2008) 093001 Review

Figure 9. Tracking diagram of a 3.55 μm PS bead trapped at a 4.8 μm gold disk under both (A) p-polarization and (B) s-polarization, andcorresponding profile of the optical potential along the X and Y axes.

From the histogram of the bead positions, one canassess the shape of the potential function U(l) (l = X, Y )it experiences. The Boltzmann distribution describes theprobability density r(l) = exp(−U(l)/kBT )/Z , where Zis the partition function normalizing the probability densityfunction, kB the Boltzmann constant and T the absolutetemperature. This method permits us to check the assumptionof a harmonic potential and when it is possible to proceed to thecorrelation [51] and the power spectral density analysis [52] ofthe position time-series for the evaluation of the trap stiffnessas illustrated in figure 9. For the weak incident laser intensityused in our experiment, the potential depth in p-polarization isfound to be greater than 4 kBT . The resulting trap stiffnessis about 30 fN μm−1 along X and 17 fN μm−1 along Y .These values are one order of magnitude greater than unders-polarization [53].

To end this section, we now investigate experimentally theability to achieve simultaneous parallel trapping in a pattern ofseveral gold disks located within a homogeneous illuminationarea. Figure 10 shows parallel trapping of seven particlesachieved in a hexagonal array of disks. Conceptually, themethod can be extended to any type of pattern while thenumber of traps is limited by the size of the incident beamable to provide the minimum incident laser intensity necessaryfor trapping. However, it should be underlined that beyonda certain density of gold (i.e. for shorter distances betweenneighboring pads), agglomeration of beads occurs, disturbingtheir proper organization according to the gold pattern.

5. Tunability and selectivity of SP traps

After describing the concept of SP tweezers and theirexperimental implementation, we discuss in this last sectiontheir main physical specificities over conventional 3D tweezersand show how the latter can be exploited for engineeringthe trap and confer upon it a certain selectivity to the objectpolarizability.

Similarly to 3D tweezers, the ability to stably trapa nearby object is controlled by a right balance between(repulsive) scattering forces and (attractive) gradient forces.In conventional 3D tweezers, the scattering force points alongthe incident k-vector while gradient forces act both along the

Figure 10. Parallel trapping over a hexagonal array of goldmicro-traps.

optical axis and perpendicular to it. Due to the 2D layoutof SP tweezers, both contributions are in this case containedwithin the surface plane (see figure 5). The major differencethough of SP tweezers over conventional 3D tweezers arisesfrom the dependence of both scattering and gradient forces onthe illumination parameters and in particular on the incidentangle. On the one hand, the scattering force magnitude riseswhen decreasing the incident angle by improving the spatialoverlap between the tail of the evanescent field and the objectvolume (see figure 11(A)). On the other hand, the strengthof the gradient force follows the resonant behavior of the SPmode and reaches a maximum at the plasmon angle θSP = 68◦(see figure 11(B)). In fact the amount of energy coupled tothe SP can also be accurately tuned by controlling the incidentpolarization, thus affecting the weight of the incident verticalfield component. Consequently, the trapping properties of SPtweezers can be tuned by changing the balance between thetwo force components.

In order to evaluate the combined effect of the incidentangle θ on the trapping properties of a single pad, we plot infigure 11(C) the trapping probability for a 4.88 μm bead asa function of θ . In the experiment, a probability of one hasbeen attributed for trapping times longer than 120 s. Applyingthis definition, three different regimes have been identified

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J. Opt. A: Pure Appl. Opt. 10 (2008) 093001 Review

0

0.0

0.2

0.4

0.6

0.8

1.0

64 65 66 67 68 69

64 65 66 67 68 69

64

0.4

0.6

0.8

65 66 67 68 69

1

2

3

Figure 11. (A) Evolution with the incident angle of the guiding velocity of 3.55 and 4.88 μm PS beads at a glass/water interface.(B) Coupling efficiency to the SP at a flat 50 nm thick gold film. (C) Evolution with the incident angle of the trapping probability of a 4.88 μmPS bead near a 4.8 μm gold pad under both p- and s-polarization. (D) Illustration of the trapping mechanism for an incidence angle θ = 66◦.

64 65 66 67 68 69

0.0

0.3

0.6

0.9

Figure 12. (A) Evolution with the incident angle of the trapping probability for both 3.55 and 4.88 μm PS beads under p-polarization.(B) Illustration of the selective mechanism (see video 2 at stacks.iop.org/JOptA/10/093001).

within the considered range of angles 64◦ < θ < 69◦ underp-polarization. While for θ < 65◦ the gradient restoringforces are weak since we stand out from the SP resonanceand the scattering force is maximum, no trapping is achieved.Conversely, for θ > 67◦, the local intensity at the pad surfaceis maximum while the scattering force is minimum, leadingto an efficient trapping of the bead. The scattering force isactually so weak in this case that significant trapping is alsoobserved under s-polarization without involving SPs. Beyondthis similarity in the trapping probability, the effect on thetrapping potential introduced by the polarization state has beendiscussed previously (figure 9). Between these two limitcases, there is a transient regime where the ratio between bothforce contributions varies sharply with θ . Within this range,we have identified incidence conditions for which the beadis systematically stably trapped under p-polarization while itquickly escapes under s-polarization. In this last case, the

uncertainty in the time to release the bead arises from itsBrownian dynamics.

Beyond the tunability of SP tweezers with the illuminationparameters, one can additionally exploit the intrinsic depen-dence of both force components on the bead polarizability tocontrol the trapping selectivity to the bead size observed in ourprior works [47]. In order to illustrate this concept, we plot infigure 12 the trapping probability as a function of the incidentangle (under p-polarization) for both 3.55 and 4.88 μm beads.Although for θ < 65◦ and θ > 68◦ both bead sizes behavesimilarly, between 66◦ and 67◦ the smaller particle is efficientlytrapped while the bigger one is quickly released. We interpretthis effect by remarking that the scattering contribution, twotimes stronger for the 4.88 μm bead (see figure 11), makes thebalance between scattering and restoring forces opposite forthe two bead sizes. This feature, specific to SP tweezers, of-fers in practice the possibility of controlling their selectivity to

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J. Opt. A: Pure Appl. Opt. 10 (2008) 093001 Review

determine objects out of a mix by simply adjusting the incidentangle.

6. Conclusion

Optical manipulation based on surface plasmons is a noveland promising application of plasmonics which opens newopportunities for the elaboration of future integrated opto-fluidic devices. By exploiting the enhanced optical forces athomogeneous and patterned metal surfaces, self-organizationand parallel trapping is achieved under weak laser intensitycompared to conventional optical tweezers. Interestingly, thedifferent force contributions involved in SP tweezers can betuned by adjusting the illumination parameters, permittingdynamic control of their trapping ability and selectivity on theobject polarizability.

While the method is well controlled for the manipulationof micrometer-sized objects, the future challenge is inextending SP-based manipulation to objects much smaller thanthe incident wavelength. Among the possible strategies, itwas recently suggested to exploit the electromagnetic couplingbetween metal nanoparticles [46, 54].

Acknowledgments

This work was supported by the Spanish Ministry of Sciencesthrough grant TEC2007-60186 and Consolider Nanolight.esand the fundacion CELLEX. The authors thank GiovanniVolpe and Dmitri Petrov for their contribution to thiswork. In addition, we have benefited from the computingfacilities provided by the massively parallel center CALMIPof Toulouse.

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