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Theoretical Calculation of Optimal Trapping Conditions for Particles with Nanometer to Micrometers Sizes Using Generalized Lorenz-Mie Theory

Gorjizadeh, Hossein | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46669 (04)
  4. University: Sharif University Technology
  5. Department: Physics
  6. Advisor(s): Seyed Reihani, Nader
  7. Abstract:
  8. Optical tweezers consist of a tightly focused laser beam. Particle with refractive index grater than that of the surrounding medium are eligible for optical trapping. This device is used for measuring forces from pico-newton to nano-newton, and displacements from nanometer to micrometer. In biopolymers, for example, it is used for studying the elasticity of DNA molecules, mechanical properties of cell membrane; and also in Microfluidics, it can is used for mixing two liquids flowing in a micro-channel by rotating a trapped particle. In general, physics of optical tweezers (or interaction of the trapped particle with the tightly focused laser beam) is treated in three different regimes:1) Geometrical Optics; which is dealt with the condition when the size of the particle is substantially greater than the wavelength of the laser.2) Rayleigh Optics; when the size of the particle is very small compared to the wavelength of the laser.3) Mie Optics; when wavelength of the laser is comparable to size of the particle. In this thesis, the optimal trapping conditions are determined in mentioned regimes. The optimal condition for a given laser power is defined as the maximal force exerted on the trapped particle. This is done by tuning the numerical aperture of the objective lens. Varying the numerical aperture of the objective changes the spot size, and hence, the interacting volume fraction of the particle. Yet, in this thesis variation of the optimal numerical aperture as a function of the object’s size is determined. In other words, for a particle with a given size there is an optimal interacting volume, which not only leads to a maximal restoring force, but also reduces the probable irradiation damages by widening the focal spot. This could be very important when optical trapping of biological cells are on demand
  9. Keywords:
  10. Optical Tweezer ; Geometrical Optics ; Rayleigh Regime ; Mie Regime ; Numerical Aperture of Objective ; Maximal Trapping Force

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