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Real time interactive optical micromanipulation of a mixture of high and low index particles Peter John Rodrigo Vincent Ricardo Daria and Jesper Gl ckstad Optics and Plasma Research Department Ris National Laboratory DK 4000 Roskilde Denmark jesper gluckstad risoe dk http www risoe dk ofd competence ppo htm Abstract We demonstrate real time interactive optical micromanipulation of a colloidal mixture consisting of particles with both lower nL n0 refractive indices than that of the suspending medium n0 Spherical high and low index particles are trapped in the transverse plane by an array of confining optical potentials created by trapping beams with top hat and annular cross sectional intensity profiles respectively The applied method offers extensive reconfigurability in the spatial distribution and individual geometry of the optical traps We experimentally demonstrate this unique feature by simultaneously trapping and independently manipulating various sizes of spherical soda lime micro shells nL 1 2 and polystyrene micro beads nH 1 57 suspended in water n0 1 33 2004 Optical Society of America OCIS codes 140 7010 Trapping 170 4520 Optical confinement and manipulation and 230 6120 Spatial Light Modulators References and links 1 A Ashkin Optical trapping and manipulation of neutral particles using lasers Proc Natl Acad Sci USA 94 4853 4860 1997 2 K Svoboda and S M Block Biological applications of optical forces Annu Rev Biophys Biomol Struct 23 247 285 1994 3 D G Grier A revolution in optical manipulation Nature 424 810 816 2003 4 M P MacDonald G C Spalding and K Dholakia Microfluidic sorting in an optical lattice Nature 426 421 424 2003 5 J Gl ckstad Microfluidics Sorting particles with light Nature Materials 3 9 10 2004 6 A Ashkin Acceleration and trapping of particles by radiation pressure Phys Rev Lett 24 156 159 1970 7 A Ashkin J M Dziedzic J E Bjorkholm and S Chu Observation of a single beam gradient force optical trap for dielectric particles Opt Lett 11 288 290 1986 8 K Sasaki M Koshioka H Misawa N Kitamura and H Masuhara Optical trapping of a metal particle and a water droplet by a scanning laser beam Appl Phys Lett 60 807 809 1992 9 K T Gahagan and G A Swartzlander Trapping of low index microparticles in an optical vortex J Opt Soc Am B 15 524 533 1998 10 K T Gahagan and G A Swartzlander Simultaneous trapping of low index and high index microparticles observed with an optical vortex trap J Opt Soc Am B 16 533 1999 11 M P MacDonald L Paterson W Sibbett K Dholakia P Bryant Trapping and manipulation of low index particles in a two dimensional interferometric optical trap Opt Lett 26 863 865 2001 12 R L Eriksen V R Daria and J Gl ckstad Fully dynamic multiple beam optical tweezers Opt Express 10 597 602 2002 http www opticsexpress org abstract cfm URI OPEX 10 14 597 13 P J Rodrigo R L Eriksen V R Daria and J Gl ckstad Interactive light driven and parallel manipulation of inhomogeneous particles Opt Express 10 1550 1556 2002 http www opticsexpress org abstract cfm URI OPEX 10 26 1550 14 V Daria P J Rodrigo and J Gl ckstad Dynamic array of dark optical traps Appl Phys Lett 84 323 325 2004 15 J Gl ckstad and P C Mogensen Optimal phase contrast in common path interferometry Appl Opt 40 268 282 2001 16 S Maruo K Ikuta and H Korogi Submicron manipulation tools driven by light in a liquid Appl Phys Lett 82 133 135 2003 3781 15 00 USReceived 4 February 2004 revised 29 March 2004 accepted 29 March 2004 C 2004 OSA5 April 2004 Vol 12 No 7 OPTICS EXPRESS 1417 1 Introduction Light carries both linear and angular momenta Momentum transfer that accompanies light matter interaction has provided us means to trap and manipulate particles in the mesoscopic scale Significant developments in the past decades have resulted in a variety of applications of conventional optical trapping in the biological and the physical fields and the emergence of a next generation of optical micromanipulation schemes 1 5 In 1970 Ashkin demonstrated that a transparent dielectric micro sphere suspended in water is radially drawn towards the optical axis of a Gaussian laser beam where the intensity is strongest 6 He observed this behavior with latex spheres having relative refractive index m greater than unity m n n0 where n and n0 are the refractive indices of the particle and the suspending medium respectively Upon radial attraction towards the region of stronger intensity the high index particle accelerates in the direction of the Poynting vector due to an axial scattering force On the other hand Ashkin noted that for an air bubble m 1 in water the sign of the radial force due to the intensity gradient is reversed hence the low index particle is repelled away from the beam axis Ashkin and co workers later showed that by tightly focusing a Gaussian beam to a high index particle an axial force due to an intensity gradient is also produced strong enough to counteract the scattering force resulting in a stable 3D confinement of the particle 7 However a stationary tightly focused Gaussian beam does not provide a confining potential for low index particles Optical trapping of a low index microscopic particle requires a beam with an annular intensity profile A straightforward approach is to apply high speed deflectable mirrors that enable time multiplexing of a desired beam pattern at the trapping plane Scanning the beam in a circular locus creates a ring of light that confines a low index particle in its dark central spot 8 A low index particle can also be trapped in an optical vortex produced from a focused TEM01 beam 9 An optical vortex has been used to trap a low index sphere and a high index sphere at the same time in two neighboring positions along the beam axis 10 Low index particles were also trapped between bright interference fringes produced at the focal plane of an objective lens where two coherent plane waves converge 11 However dynamic and parallel manipulation of a larger array of high and low index particles has not been achieved with the above techniques Here we demonstrate real time user interactive manipulation of a mixture of high and low index particles by reading out 2D phase patterns encoded onto an input beam by a programmable spatial light modulator SLM using the generalized phase contrast GPC approach to produce tailored light distributions that result in optical confinement of the mixed particles in the transverse plane For spherical particles trapping beams with radial symmetry are utilized High index micro spheres were efficiently trapped and manipulated using trapping beams with top hat transverse profiles at the trapping plane 12 13 On the other hand low index particles are trapped using beams with annular transverse profiles 14 We demonstrate that unlike other methods the GPC approach readily provides both the ability to create independently controllable optical traps for high and low index particles and the flexibility to render in real time arbitrary dynamics for these two types of particles simultaneously This exceptional functionality may facilitate particle encapsulation in air bubbles or in water in oil emulsions applied in petroleum food and drug processing 2 Experiment Trapping and manipulation of colloidal particles is achieved using the experimental setup shown in Fig 1 The system makes use of a continuous wave CW Titanium Sapphire Ti S laser wavelength tunable Spectra Physics 3900s pumped with a CW frequency doubled Neodymium Yttrium Vanadate Nd YVO4 laser 532 nm Spectra Physics Millenia V The Ti S laser utilizes built in birefringent quartz filter plates to select the operating wavelength within the near infrared NIR spectrum from 700 to 850 nm In our experiments the operation wavelength is set to 830 nm With a maximum pump power of 5 0 W from the Nd YVO4 the Ti S laser provides a maximum power of 1 5 W The laser is expanded and 3781 15 00 USReceived 4 February 2004 revised 29 March 2004 accepted 29 March 2004 C 2004 OSA5 April 2004 Vol 12 No 7 OPTICS EXPRESS 1418 collimated before incidence on a reflection type phase only SLM The SLM employing parallel aligned nematic liquid crystals Hamamatsu Photonics is optically addressed by a VGA resolution 480 x480 pixels liquid crystal projector element that is controlled from the video output of a computer Fig 1 Experimental setup for simultaneous optical manipulation of high and low index particles at the trapping plane The expanded beam 830 nm incident at the spatial light modulator SLM comes from a CW Ti Sapphire Ti S laser pumped by a visible CW Nd YVO4 laser Under computer control arbitrary 2D phase patterns are encoded onto the reflective SLM A high contrast intensity mapping of the phase pattern is formed at the image plane IP and is captured by a CCD camera via partial reflection from a pellicle The intensity distribution is optically relayed to the trapping plane Standard brightfield detection is used to observe the trapped particles PCF phase contrast filter Ir iris diaphragm L1 L2 and L3 lenses MO microscope objective DM dichroic mirror TL tube lens We use the SLM to imprint a programmable 2D binary phase pattern 0 or phase delays to the wavefront of the 830 nm laser beam The phase modulated wavefront is directed into a 4 f filtering system composed of lenses L1 and L2 and a phase contrast filter PCF located at the Fourier plane The PCF is constructed by depositing a 30 m diameter circular transparent photoresist Shipley Microposit S1818 structure on an optical flat Centered at the Fourier plane the PCF introduces a phase shift between low and high spatial frequency components of the phase encoded beam The diameters of the SLM iris Ir and the on axis PCF are adjusted to optimize the throughput and contrast of the output intensity distribution 15 A high contrast intensity distribution which is geometrically identical to the phase pattern at the SLM is generated at the image plane IP To monitor the output intensity distribution a pellicle is inserted in the path and directs a small fraction 3 of the light towards a CCD camera The intensity pattern at the IP is scaled and relayed by lens L3 and the microscope objective MO to a conjugate plane trapping plane The fluorescence port of the inverted microscope Leica DM IRB is used to direct the near infrared laser light to the back focal plane of the MO via a dichroic mirror The same MO and a built in microscope tube lens allow brightfield images to be captured by a second CCD camera The quality of the intensity patterns synthesized at the image plane via the GPC approach is depicted in Fig 2 where variably sized beams with top hat and annular transverse profiles are generated at different positions at the transverse x y plane The condition for achieving 3781 15 00 USReceived 4 February 2004 revised 29 March 2004 accepted 29 March 2004 C 2004 OSA5 April 2004 Vol 12 No 7 OPTICS EXPRESS 1419 optimal intensity contrast is described in the previous analysis of the GPC method 15 Optimum phase to intensity conversion requires that the ratio of the SLM area encoded with phase shift to that with 0 phase remains less than or equal to 0 25 for the operating diameters of the SLM iris and the PCF When the condition is satisfied the maximum intensity of the trapping pattern is approximately four times the average intensity of the SLM input beam Fig 2 a Measured high contrast intensity pattern at the output plane IP Corresponding surface intensity plots for the representative b top hat in yellow square and c annular or doughnut in green square trapping beams A trapping beam with a top hat transverse intensity profile provides a radially symmetric potential well for a high index particle as shown in Fig 3 a When a top hat beam is positioned in the vicinity of a high index particle the particle gets attracted to the beam axis We have observed previously that a beam with diameter slightly larger than that of the particle provides better transverse confinement especially when the trapped particle is moved along the horizontal plane 12 In contrast a top hat beam acts as a potential barrier for a low index particle Unstable at the beam center the low index particle gets repelled to either side of the optical potential as shown in Fig 3 a This is evident in the experiment we performed with spherical shells made of soda lime glass material Polysciences with de ionized water as host medium These air filled hollow glass spheres have shell thickness of 1 m and outer diameters in the range of 2 20 m The hollow glass spheres with outer diameters greater than 5 m effectively behave as low index particles in water n0 1 33 Similar hollow glass spheres where found to have average density of 0 2 g mL and effective refractive index nL 1 2 9 A 6 m hollow sphere in the presence of a top hat beam is shown in Fig 4 The sequence of images shows the displacement of the low index particle as a result of its repulsion from the region of stronger light intensity A low index particle finds a minimum potential at the center of the beam with an annular transverse intensity profile as shown in Fig 3 b However unlike the spontaneous attraction of a high index particle towards the center of a top hat beam a low index particle is not readily drawn to the dark central spot of the annular beam From the outer region to the dark center of the annular beam the low index particle needs to overcome the potential barrier associated with the bright ring of light 3781 15 00 USReceived 4 February 2004 revised 29 March 2004 accepted 29 March 2004 C 2004 OSA5 April 2004 Vol 12 No 7 OPTICS EXPRESS 1420 Fig 3 Diagram of the optical potential a for a high index solid curve and a low index dashed curve particle due to a beam with top hat transverse intensity profile and b for a low index particle due to a beam with annular transverse intensity profile Fig 4 AVI 1 656 MB Deflection of a soda lime hollow glass sphere from a computer mouse controlled trapping beam with top hat intensity profile An arrow in each frame indicates the location of the beam at that instant Scale bar 10 m Next we demonstrate a scheme where we take advantage of the repulsive forces induced by intensity gradients to low index particles The sample we prepared contained a mixture of polystyrene micro spheres index nH 1 57 Bangs Laboratories and the low index hollow spheres in de ionized water in 30 m thick glass cell The sample is mounted on the microscope stage Due to density mismatch the polystyrene spheres 1 05 g mL settle to the bottom surface of glass cell while the air filled hollow glass spheres 0 2 g mL float to the top portion Axial adjustment of the MO allows us to view the two types of particles To bring more particles into a particular region we generate and scan a vertical line beam pattern resulting in the simultaneous deflection of low index particles in the scan direction as shown in Fig 5 Raking of the low index particles is made either by non mechanical scanning of the linear beam pattern using the graphical user interface or by horizontal displacement of the microscope stage This simple procedure allows us to drag a number of low index particles into the operating region where polystyrene spheres are found directly below 3781 15 00 USReceived 4 February 2004 revised 29 March 2004 accepted 29 March 2004 C 2004 OSA5 April 2004 Vol 12 No 7 OPTICS EXPRESS 1421 Fig 5 AVI 1 126 MB Raking of low index particles to a region of interest achieved by scanning a bright linear intensity pattern in the x y plane The arrow frame 1 indicates the scanning direction Scale bar 10 m The ability to interactively generate and change phase patterns at the SLM in real time allows each doughnut trap to be independently switched on and off and be transversely displaced such that it correctly coincides with the position of the corresponding particle In Fig 6 we demonstrate the steps for trapping low index particles with doughnut optical traps In the first frame a doughnut trap is positioned next to a particle which is located almost outside the field of view From its initial position the trap is then positioned directly in the location of the particle and moved slightly to the center of the observation region In the third frame a new trap is added by the click of the computer mouse and brought to one of the untrapped particles The same procedure is done in the succeeding frames until all four particles are trapped as shown in the 15th frame Once all particles are trapped they are brought into a diamond formation 20th frame and then into a linear arrangement 25th frame The sizes of the particles vary within 6 10 m and the corresponding doughnut traps are configured with appropriate diameters and thickness by a click and draw computer mouse sequence The high index polystyrene spheres are lifted off the bottom surface of the sample cell by corresponding optical traps with top hat profiles As the high index particles accelerate upward they appear in focus with the low index particles pre positioned at the upper surface of the sample cell As high index particles are brought to the upper glass surface by top hat beams doughnut optical traps are also created for low index particles Figure 7 shows a mixture of high and low index particles simultaneously trapped by top hat and annular trapping beams respectively From an irregular spatial distribution the particles are individually displaced and sorted according to their index contrast with the suspending medium This process illustrates the versatility of the GPC method in generating trapping patterns with arbitrary symmetric or asymmetric spatial configurations in real time 3781 15 00 USReceived 4 February 2004 revised 29 March 2004 accepted 29 March 2004 C 2004 OSA5 April 2004 Vol 12 No 7 OPTICS EXPRESS 1422 Fig 6 AVI 2 512 MB User interactive procedure for trapping different sizes of hollow glass spheres using doughnut optical traps Fig 7 AVI 1 113 MB Image sequences of trapping and user interactive sorting of an i