Multi force optical tweezers to generate gradients of forces

Multi force optical tweezers to generategradients of forces

Valentina Emiliani, Daniele Sanvitto1, Morad Zahid,Fabien Gerbal2, and Maite Coppey-Moisan

Complexes Macromolculaires en Cellules Vivantes, Institut Jacques Monod, UMR 7592,CNRS, Universites P6/P7, 75251 Paris cedex 05, France

1Present address: Department of Physics, University of Sheffield, Sheffield S3 7RH

2Present adress: Laboratoire de Biorheologie et d’Hydrodynamique Physico-Chimique,Universites P6/P7, France

[email protected]

Abstract: We present a multi trap optical tweezes system that enables togenerate two-dimensional dynamical configurations of focal spot where thetrapping force of each element of the pattern can be individually changed.Force gradients in the pN range can be generated on a micrometer scale.

© 2004 Optical Society of America

OCIS codes: (230.1040) Acousto-Optical devices; (170.4520) Optical confinement and ma-nipulation; (020.7010) Trapping.

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(C) 2004 OSA 23 August 2004 / Vol. 12, No. 17 / OPTICS EXPRESS 3906#4400 - $15.00 US Received 18 May 2004; revised 4 August 2004; accepted 5 August 2004

mailto:[email protected]

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1. Introduction

In 1986 optical trapping of dielectric particles by means of a focused laser beam was demon-strated by Ashkinet al. [1]. Since then, the optical tweezers technique gained a continuous in-creasing interest especially in view of its application in biological research [2]. This techniqueallows non invasive manipulation of biological samples like viruses, bacteria, and living cells[3, 4, 5]. Precise measurement of elasticity, forces, torsion and surface tension are also possiblewith a sub-pN accuracy [6, 7, 8]. In cellular biology, it enables to investigate cell reaction toexternal cues via the controlled application of localized mechanical and chemical stimuli oncell cortex [9, 10, 11]. A basic optical tweezers set up requires a strongly focused laser beam,a coarse positioning system to center the sample and an imaging system to monitor the experi-ment [12]. However, the increasing use of this technique in biological research has required theconception of more and more tricky experimental apparatus. Improvements are required bothin the imaging part and in the “tweezing” technique. For imaging, sensitive fluorescence tech-niques combined with optical manipulation is desired in most applications. For manipulation,

Xe lampObjective

Sample stage

Las

er

AOD

CCD

Mg lamp

Dichroic

Dichroic

Attenuator

Fig. 1.Schematic of the multi trap optical tweezers set up.


Fig. 2.Polystyrene latex beads, 2µm in diameter, trapped by a circle of laser spot(a); deformed in anellipse (b); deformed by leaving one part of the circle fixed (c); same beads trapped in a squared array(d); deformed along the x direction (e); traps can be moved independently (f).

it is advantageous in some cases to produce multiple optical traps.One possibility to realize multi trap systems consists in rapidly scanning asingle laser beam

using fast beam deflectors. In this way, different traps can be generated by time sharing thelaser beam between several positions. A multi-spot pattern is then obtained and its geometrycontrolled via the deflector system. The use of fast beam deflectors is of crucial importance asthe time the trap is ’off’, servicing another position, has to be shorter than the time the particleneeds to diffuse away from its trapping position. Fast beam deflection is very well achievedby the use of acousto-optic deflectors (AOD)[14, 13, 15]. In this case the rise time to producedifferent deflection angles is of the order ofµs, thus allowing the synthesis of multi trapsof high stability. An alternative way to generate multi traps makes use of diffractive opticalelements (DOEs) [16, 17]. In this case, multi traps aresimultaneously generated by focusingthe laser beam on a suitable patterned optical element. As a result, the laser is diffracted ina desired arrangement of multi spots. The advantage in respect to the AOD based system isthe possibility to generate traps distributed in 3D volumes [18]. Moreover, when the DOEs areimplemented on a spatial light modulator (SLM), the optical traps can be independently movedby changing the relative phase of the diffracted pattern [19, 20, 21].

One interesting extension of the multi trap optical tweezers system is to tune separately thetrapping force of each spot.

This opportunity can enable promising applications, for example several objects can be ma-nipulated at the same time with a force independently adjusted on the single object. Cell re-actions, as adhesion reinforcement, cytoskeleton organization or gene expression, to externalgradient of forces can be monitored.

Tuning of the trap stiffness has been already proposed in single trap optical tweezers set upto generate an efficient system of feedback control [22]. In that case modulation of the trapstrength has been achieved by an acousto optical modulator.

In this paper, we propose to create a multi trap pattern where the trapping strength of each


Fig. 3. Intensity distribution of the spots reflected on the coverslip and imaged onto a CCD camera,for a 4x4 array and a circular distribution.

spot can be tuned in such a way that 2D gradient of force can be generated. In this case, wepropose to achieve force-tuning by controlling the number of time the laser beam passes onthe same spot. This is achieved by the use of a dual axis AOD system. Examples of possiblespot distributions and the corresponding gradient of force one can realize with this method arepresented.

Figure 1 shows the multi trap optical tweezers system. The trapping source is a 15W Nd:Yaglaser (Spectra Physics, J40) with an emission wavelength of 1064 nm. After a first beam ex-pansion the laser is sent into two orthogonally mounted AODs (AA opto-electronique), theoutgoing steered beam is further expanded in order to match the entrance of the objective. Avariable intensity attenuator obtained with a combination of aλ /2 plate and a linear polarizerallows us to vary the laser intensity. After the second beam expander the laser beam is sent intoan inverted microscope (Zeiss AxioVert 135) and finally, by a dichroic mirror, to the focusingobjective (Zeiss Neofluor 100x oil, NA=1.3). The dichroic is positioned above the fluorescencefilter block. In this way excitation light from a Mercury lamp mounted on the rear port of themicroscope can be focused to the sample with the same objective used for trapping. Fluores-cence from the sample is sent to a high sensistive CCD camera (cool Snap HQ,Roper) placedat the left port of the microscope. This configuration allows us to perform at the same timeoptical trapping and fluorescence imaging. Finally, the transmitted light from an arc lamp issent to a second CCD camera to image the bead positions. The AODs work with a center drivefrequency of 76 MHz, a bandwith of 15 MHz and an access time of 1µsec.

Examples of dynamic geometries of beads that is possible to realize with a typical AOD basedsystem are shown in Fig. 2(a)-(f). As described in the introduction, by varying the successivepattern of the laser beam, the shape of the array can be varied in a symmetric way or along apreferential axis (Fig. 2 (a)- (e)). Each trap can be moved separately from the rest of the pattern(Fig. 2 (f)).

In the distributions of beads shown in Fig. 2, the trapping force is constant over the entirepattern. By further modifying the sequence of laser beam pattern we can select the number oftime the laser passes on each spot in one cycle. In this way, we control the force at each trapindependently from the others.

Examples of this, are shown in Fig. 3(a)-(b) for two geometries. Here in order to highlightthe intensity distribution, we image on the CCD camera the laser spots (without beads) reflected


Fig. 4. (a) Intensity profile along the arrow of Fig. 3, the row number is indicated in thefigure. (b) Force calibration corresponding to the different rows of the array.

on the glass of a coverslip. The two figures show the intensity distribution in a 4x4 array and ina circular distribution of spots.

Figure 4(a) shows the intensity profile along the white arrow in Fig. 3(a). In this case wegenerate a 4x4 pattern of laser spots with an intensity profile which can be described byI(n) =I1×n1.5, n being the row index andI1 the laser intensity along the first row of the array. In thiscase the laser intensity at the entrance of the AOD was 3 W, corresponding to about 1 W atthe entrance of the objective (considering the losses due to the passive optics and the AODs).Considering the further losses due to the objective (about 50%) we estimateI1 = 5 mW.

The forces corresponding to different rows of the array have been derived by measuring theescape velocity of the beads as the arrays are translated horizontally at increasing speed. Foreach row we derived the escape velocity,ve(n). From the values ofve(n), we derived the laserforcesF(n) by using the Stokes’s law,F(n) = 6πηRve(n), (η being the viscosity of the water,R the bead radius). The results are reported in Fig. 4(b). In this case, a trapping force rangingfrom 0.4 pN in the first row to 3 pN in the last one has been obtained. The 15W laser powerallow for this geometry to have a gradientI(n) = I1×nα , with a value forα up to 3.

In conclusion we have presented a multi force optical tweezers system where pattern ofmulti spots with a force independently regulated at each spot can be generated. This charac-teristic opens interesting prospectives as the possibility to investigate cell reaction to externalcues via the controlled application of mechanical gradient on cell cortex or the simultaneousmanipulation of several objects with forces calibrated independently on each object.

We thank the staff of theplatform ’Imaging of Dynamical Processes in Cellular and Devel-opmental Biology’ of the IJM for helpful advices in the realization of the system. This workwas supported by the CNRS (DRAB) by the ARC (Association pour la Reserche sur Cancer)and by the GEFLUC grant. D. Sanvitto was supported by a Marie Curie fellowship, V. Emilianifrom CNRS and ARC.


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Multi force optical tweezers to generate gradients of forces