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Optimization and Accurate Measurement of the Force Acting on the Spherical Nano/micro Particles Trapped by Optical Tweezers: Theoretical and Experimental Study

Gorjizadeh Alinezhad, Hossein | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 55168 (04)
  4. University: Sharif University of Technolog
  5. Department: Physics
  6. Advisor(s): Seyed Reihani, Nader; Mahdavi, Mohammad
  7. Abstract:
  8. In this thesis, the optical force acting on the spherical polystyrene particle trapped by optical tweezers is calculated. The calculation is based on the generalized Lorenz-Mie theory. The effect of various parameters on the optical force is also investigated. It is shown that, tuning the different parameters, one can significantly enhance the optical force acting on the trapped particle. Besides, one can achieve a stable 3D trap of a sub-micron particle somewhere far deep inside the sample chamber. Our results indicate that: (1) By obstructing the central region of the incident Gaussian laser beam at the back focal plane of the objective lens, the trap stiffness, the maximum linear force, and the maximum applicable force in the axial direction can be considerable increased. (2) For optimal trapping of a particle with a certain size, an optimal numerical aperture and an appropriate laser power should be utilized. In this condition, the maximum force acting on the particle in the lateral direction would also be considerable enhanced. (3) By tuning the correction collar of the dry objective lens, the spherical aberration induced by the refractive index mismatch between the immersion and sample media, can be compensated. This would produce a stable 3D trap for a sub-micron particle somewhere far deep inside the sample chamber. Our theoretical predictions are in good agreement with our experimental results.In this thesis, we also introduce a nonlinear Kramers-Moyal based calibration method for optical tweezers. This allows for accurate measurement of the trapping stiffness of optical tweezers and also the diffusion coefficient of the trapped particle. Our results show that the measured positional time series of the trapped bead contain some nonlinear features. More specifically the diffusion constant of the particle depends almost quadratically on its distance from the center of the trap. This is in contrast with the essential assumption made in the commonly used power spectrum calibration method. We show that this nonlinearity vanishes only for zero laser power. Therefore, the commonly used power spectrum method would only be valid for very low laser powers. We also show that utilizing power spectrum calibration method for regularly used powers result in an over estimation of the trap stiffness, as well as underestimation of the voltage-to-position conversion factor.
  9. Keywords:
  10. Optical Tweezer ; Optical Trapping ; Numerical Aperture of Objective ; Kramers-Moyal Coefficient ; Optical Trap Stiffness ; Optimal Trapping in Depth ; Laser Power

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