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Position Detection for Optical Tweezers Using Moiré Technique

Khorshad, Ali Akbar | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51680 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Seyed-Reihani, Nader; Tavassoly, Mohammad Taghi
  7. Abstract:
  8. Optical Tweezers (OT) are non-invasive micro-manipulation tools based on a tightly focused Gaussian laser beam for applying and measuring pico-Newton range optical forces. They are also utilized for trapping and moving particles and biological specimens. The force exerted by OT is mainly determined by detecting the position of the trapped object inside the harmonic potential well of OT. Conventionally,a Quadrant Photo-Diode (QPD) positioned at the Back Focal Plane (BFP) of the condenser lens provides the position of the bead with MHz temporal and nanometer spatial resolution. As a disadvantage of this detection scheme, it is impossible to obtain simultaneously the maximal lateral and axial detection sensitivity with a fix detection numerical aperture. In this thesis, we show, by theory (Rayleigh scattering approximation) and experiment, that moiré technique could measure nanometer displacements of the trapped bead more sensitive than that of provided by the QPD method. To this end, we locate a Moiré Deflectometer (MD) or Talbot Interferometer (TI) (it consists of two linear amplitude grating with the 2th grating positioned at a Talbot distance) at the BFP of the condenser lens. After impinging the unscattered and scattered beams on the double gratings, rotational and parallel moiré fringes will be appeared. It is shown that the intensity fluctuation of the whole moiré pattern or opposite fringe pair (one bright fringe in the right-hand side of the pattern and its counterpart in the left-hand side) is linearly dependent on the displacement of the trapped bead, such that the fringe pairs provide different detection sensitivity. The simulation results show that, first, the outermost fringe pair provides the maximal detection sensitivity, second, there is an Optimal Talbot Distance (OTD) such that by positioning the 2th grating at the OTD, the MD detection system with lower spatial periods provides larger detection sensitivity respect to the QPD method. Finally, the experiments done by 2:17m and 1:26m polystyrene beads show that the MD results are in good agreement with the QPD findings and theoretical predictions.Furthermore, by selecting the proper fringe pair the lateral and axial detection sensitivities for 2:17m bead could be improved %100 and %279 respect to the QPD method, respectively
  9. Keywords:
  10. Optical Tweezer ; Photodetector ; Quadrant Photodiode ; Moire Deflectometer ; Talbot Interferometer ; Optical Trapping ; Diffraction

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