Xuechang Ren a *, Canhui Wang, Yanshuang Li, Shaoxin Shen, Shou Liu

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1 Available online at Physics Procedia 22 (2011) International Conference on Physics Science and Technology (ICPST 2011) Optical Tweezers Array System Based on 2D Photonic Crystals Xuechang Ren a *, Canhui Wang, Yanshuang Li, Shaoxin Shen, Shou Liu Xiamen University, Physics Department, Xiamen, , China Abstract A simple optical interference method for creating multiple optical tweezers from a single laser beam, using two dimentional photonic crystals (PhCs) as a diffractive beam splitter, was described. To obtained clear periodic traps, all diffracted beams sould be used and the intensity of each splitted beam should be same. So the period and the surface features of PhCs was adjusted in the present study As a demonstration of this technique, using 2D PhCs with 700 nanometer period, hexagonal lattice patterns with one micrometer period have been implemented. The image of periodic intensity gradient of light fabricated by this method is presented Published by Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of Garry Lee. Keywords: optical tweezers; multiple optical traps; a diffractive beam splitter; photonic crystals; interference * Corresponding author. Tel.: ; fax: address: xuechangren@xmu.edu.cn Published by Elsevier B.V. Selection and/or peer-review under responsibility of Garry Lee. Open access under CC BY-NC-ND license. doi: /j.phpro

2 494 Xuechang Ren et al. / Physics Procedia 22 (2011) Introduction Since Ashkin et al. introduced optical traps in 1986[1], single beam optical tweezers have become indispensable tools for a wide range of interdisciplinary research, such as the exploration of small interaction forces [2], the manipulation of single cells [3], and the assembly of microstructures on the micro- and nanoscales [4]. Optical tweezers work on the principle of creating an intensity gradient that is exerted by tightly focused laser beams to trap and move objects. When a number of particles have to be trapped simultaneously, some techniques to create multiple optical traps have been reported [5]. For instance, dynamic holographic optical tweezers were used to transfer particles along arbitrary paths [6], and an approach based on computer-generated holograms (CGH) was demonstrated [7]. A photonic crystal (PhC) is an artificial structure whose refractive index is periodically modulated. A PhC can alter the propagation property of light with a wavelength of roughly twice that of its period [8]. Due to its promising applications, PhCs have been intensively investigated. [9] In this paper we demonstrate a simple and effective means to create multiple optical tweezers in hexagonal lattice patterns from a single laser beam using 2D PhCs. These tweezers arrays and their variants should have immediate applications for probing phenomena in biological systems and complex fluids, and in organizing soft matter into mesoscopically textured composite materials. The image of periodic intensity gradient of light fabricated by this method is presented. 2. Methods and data In 2001, T. Kondo demonstrated a simple optical interference method to fabricate microperiodic structures [9]. The optical setup is schematically shown in Fig. 1. Femtosecond laser pulse was split by a diffractive beam splitter (DBS) and overlapped with two lenses. Temporal overlap of the split femtosecond pulses was automatically achieved by this optical setup. One-, two-, and three-dimensional periodic microstructures with micrometer-order periods were fabricated using this method. Fig.1. Optical setup. DBS: diffractive beam splitter, L1 and L2: lenses, and AA: aperture array. The inset shows beam configuration around the sample. So according to the techniques presented by T. Kondo [9] and Eric R. Dufresne [10], 2D PhCs is used as a diffractive beam splitter in our paper. In their methods, not all diffracted beams can be used and the intensity of each splitted beam was not same. If the period and surface features of PhCs should be

$Xuechang Ren et al. / Physics Procedia 22 (2011) 493 497 495 optimized, these two problems could be solved. For convenience, 1D PhC (i.e., grating) is taken into account, as shown in Fig. 2. Fig. 2. Schematic diagram of the diffraction of 1D PhC Fig.$ $m, the diffraction order;, the wavelength of incident light.$

3 Xuechang Ren et al. / Physics Procedia 22 (2011) optimized, these two problems could be solved. For convenience, 1D PhC (i.e., grating) is taken into account, as shown in Fig. 2. Fig. 2. Schematic diagram of the diffraction of 1D PhC Fig. 3. Schematic diagram of a practical diffractively generated optical tweezer array The grating equation is: d(sin sin ) m (1) d, the period of the grating;, the incident angle;, the diffraction angle; m, the diffraction order;, the wavelength of incident light. For the normal incidence, m d sin (2) Here in order to make good use of the laser power, only zero- and first-order should be diffracted, so the following conditions should be met o 1 o d 2, and 30 sin d 90. (3) The schematic diagram in Fig. 3 shows one implementation of diffractively generated optical tweezer array. The optical tweezer array is powered by a 140 mw argon ion laser operating at nm. The laser

$n sin 1 sin 0.65. (4) Here n is the refractive index of media; is one-half aperture angle of the objective, and meantime is equal to the diffraction angle.$ Inserting equation (4) into equation (2), the period of the grating d is deduced to be 704nm. Fig. 4. Optical setup for PhCs fabrication Micrographs showing optical tweezers Fig. 5.

Inserting equation (4) into equation (2), the period of the grating d is deduced to be 704nm. Fig. 4. Optical setup for PhCs fabrication Micrographs showing optical tweezers Fig. 5.

4 496 Xuechang Ren et al. / Physics Procedia 22 (2011) beam passes through photonics crystals vertically, and then the microscope s objective lens (40, N.A. 0.65). So we have N. A. n sin 1 sin (4) Here n is the refractive index of media; is one-half aperture angle of the objective, and meantime is equal to the diffraction angle. Inserting equation (4) into equation (2), the period of the grating d is deduced to be 704nm. Fig. 4. Optical setup for PhCs fabrication Micrographs showing optical tweezers Fig. 5. The array generated in the experiment The PhCs were fabricated according to the technique already described by Xiangsu Zhang [11], illustrated in Fig. 4. In PhC fabrication, the material used was BP212 positive photoresist with 8 m thickness, and the light source used was the same as above. The exposure and development time were 40s and 8s, respectively. Since on the basis of these parameters, the estimated beam intensity ratio of each diffracted beam of the PhCs is about 1, the visibility of interference pattern would be better.

5 Xuechang Ren et al. / Physics Procedia 22 (2011) Results and discussion For the sake of observation, the microscope s objective lens (10, N.A. 0.25) was arranged as the second lens L2. Therefore the period of the tweezer array was about 1.2 m. The micrograph in Fig. 5 was taken from the array generated in the experiment with a CCD camera under the magnification of a microscope (400 ). The bar in the micrograph is 1 m diameter fiber. Calculated results indicate that the same diffracted beam intensity provides higher contrast of interference pattern, so that materials with high-diffraction efficiency are favorable to be used for making PhCs. And then the argon ion laser can be replaced by fiber optic laser with lower power for low cost operation. Acknowledgements This work is supported by the Fundamental Research Funds for the Central Universities # References [1] Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu. Observation of a single-beam gradient force optical trap for dielectric particles. Opt. Lett. 1986; [2] S. B. Smith, Y. Cui, and C. Bustamante. Overstretching B-DNA: The Elastic Response of Individual Double-Stranded and Single-Stranded DNA Molecules. Science 1996; [3] Ashkin, J. M. Dziedzic, and T. Yamane. Optical trapping and manipulation of single cells using infrared laser beams. Nature 1987; [4] K. Svoboda and S. M. Block. Biological applications of optical forces. Annu. Rev. Biophys. Biomol. Struct. 1994; [5] J. Kuo and K. Chen. Multiple Optical Traps with a Single-Beam Optical Tweezer Utilizing Surface Micromachined Planar Curved Grating. Japanese Journal of Applied Physics 2010; [6] J. E. Curtis, B. A. Koss, and D. G. Grier. Dynamic holographic optical tweezers. Opt. Commun. 2002; [7] E. R. Dufresne, G. C. Spalding, M. T. Dearing, S. A. Sheets, and D. G. Grier. Computer-generated holographic optical tweezer arrays. Rev. Sci. Instrum. 2001; [8] E. Yablonovitch. Photonic band-gap structures. J. Opt. Soc. Am. B 1993; [9] T. Kondo, S. Matsuo, S. Juodkazis, and H. Misawa. Femtosecond laser interference technique with diffractive beam splitter for fabrication of three-dimensional photonic crystals. Appl. Phys. Lett. 2001; 79(6) [10] E. R. Dufresne and D. G. Grier. Optical tweezer arrays and optical substrates created with diffractive optics. Review of science instruments ; [11] X. Zhang,, S. Liu and Y. Liu. Fabrication of large-area 3D photonic crystals using a holographic optical element. Optics & Lasers in engineering 2006; 44,

Multiple optical traps from a single laser beam using a mechanical element

Multiple optical traps from a single laser beam using a mechanical element J.A. Dharmadhikari, A.K. Dharmadhikari, and D. Mathur * Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400