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Theoretical and Experimental Studies on Holographic Optical Tweezers and Related Techniques

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Tutor: YaoBaoLi
School: Xi'an Optics and Fine Mechanics
Course: Optics
Keywords: Optical trapping,Holographic optical tweezers (HOT),Spatial lightmodulator (SLM)
CLC: O439
Type: PhD thesis
Year:  2012
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Since its invention by Ashkin in1986, optical tweezers has been evolved to be animportant technique of optical micro-manipulation after tens of years, which is beingextensively used in the studies of various microscopic fields ranged from an atom tohundreds of micrometers. Now optical tweezers is developing in a variety ofdirections, one of which is holographic optical tweezers (HOT). Rather than a fixedsingle optical trap, HOT is able to generate3D multiple optical traps with dynamicaland individual control. Besides, beams unlike traditional Gaussian ones can beemployed to generate optical traps with novel characteristics in HOT system. Theseadvantages of HOT provide enriched accesses to optical micro-manipulation, andconsequently HOT is known as a revolution of the optical tweezers and becomes a hotspot of research in this field. Focused on HOT and related techniques, this thesiscovers the following research works.1. Trapping forces of focused beams on dielectric particles are calculated based onthe electromagnetic scattering model. Effects of various parameters, such asnumerical aperture (NA) of objective, polarization state of trapping beams andrefractive indexes of the particles, on the trapping forces are revealed, whichserve as guidance for constructing and operating the optical tweezers system.2. The principle of liquid crystal spatial light modulator (LCSLM), a key element ofHOT system, is studied theoretically and experimentally. The amplitude/phasemodulation principle of commonly used LCSLM is analyzed. A Jones matrix oftwist nematic liquid crystal display (TN-LCD), usually working for intensitymodulation, is established, based on which the possibility for a TN-LCD torealize phase-only modulation is predicted precisely and verified experimentally.These results supply significant basis of modulating spatial light field byTN-LCD.3. Algorithms for calculating phase holograms to reconstruct desired light intensitydistributions are studied extensively. Based on the classical Gerchberg-Saxton (GS) algorithm, a modified algorithm is proposed to calculate computer generatedhologram (CGH) for three dimensional (3D) targets. Compared with the classical3D GS algorithm, the proposed algorithm can greatly reduce the computation cost,and the reconstructed field is higher in quality.4.2D and3D structures of optical traps and optical vortexes are generated anddynamically controlled by the calculated CGHs. Images locating at severaldistinct axial planes and a simple stereo target are reconstructed, which areattempts to digital holographic3D display/project.5. A HOT system is constructed based on thorough analysis of the influence of eachdevice to the system. Several kinds of samples are manipulated by this HOTsystem. Various operations, such as3D stable trapping, multi-plane trapping,dynamic trapping and optical rotation, are realized.
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