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Green’s Function Formulation for Studying Optomechanics of Subwavelength Nanoparticles

Abbassi, Mohammad Ali | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54936 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Mehrany, Khashayar
  7. Abstract:
  8. In this thesis, we study optomechanics of subwavelength nanoparticles based on the Green's function formulation. First, we investigate the optical force exerted upon Rayleigh particles in the free space using the dipole approximation method. Then, we present a new method based on the Taylor expansion of the polarization field to calculate the optical forces beyond the Rayleigh regime. Subsequently, we study the optical force exerted upon Rayleigh particles in non-free spaces, and model the backaction effect using the scattering Green's function. We show that the backaction effect can modify the polarizability of the particle and thereby can affect the gradient force, radiation pressure, and spin curl force. In addition, the back-scattered field can directly exert a force upon the particle that is proportional to the first order derivatives of the scattering Green's function. Then, we apply the proposed formulation to study the backaction effect in optical resonators and waveguides, in detail. We show that the backaction can help us to increase the ratio of potential depth to the local intensity at the trap, which can be advantageous to realize non-destructive trapping of small nanoparticles. We also show that the polarizability of the nanoparticle can resonate in the evanescent regime of guided structure that provides strong forces applied to the nanoparticle. It should be noted that the backaction's strength can be enhanced by increasing the Purcell factors in optical resonators and decreasing the group velocities in the waveguides. We also study the backaction effect in planar layered media, and show that the polarizability of a nanoparticle can resonate near a left-handed substrate. Finally, we propose a new formulation for studying dynamics of the particle's motion based on the Green's function. This formulation enables us to study the cooling process when the particle interacts with a continuum of electromagnetic modes
  9. Keywords:
  10. Multipole Expansion ; Optical Cooling ; Optical Trapping ; Electromagnetic Force ; Nanoparticles ; Optomechanics ; Green Function

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