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Optical trapping using a photonic crystal cavity : design and sample fabrication Mirsadeghi, Seyed Hamed

Abstract

A photonic crystal (PC) structure for trapping a 50nm radius dielectric particle at a precise location on a silicon surface in an organic solvent environment has been designed and all of its key components have been fabricated. The high gradient of electric field intensity in a PC cavity mode, with wavelength ~ 1.5 microns, exerts a radiation force toward the center of the cavity. The Finite Difference Time Domain (FDTD) modeling method was used to design a symmetric (input/output) structure that consists of two grating couplers, two parabolic tapered waveguides, two single mode ridge waveguides, two photonic crystal waveguides and a single three-missing-hole (L3) PC cavity. The radiation force on the dielectric sphere was exactly calculated using FDTD simulations to evaluate the Maxwell Stress Tensor (MST) in the presence of the particle to be trapped. This result was compared to that obtained using the simpler dipole approximation, and good agreement between them was found. The fabrication of the structure was done by electron beam lithography and chlorine plasma etching. The Q factors for some of the fabricated samples were measured from the cavity enhanced photoluminescence emission of PbSe quantum dots deposited on the sample surface. A vertical Q factor of 3600 (in vacuum environment) was measured for an isolated cavity, which corresponds to a Qv of 3800 ( in solvent environment) in the FDTD simulations. Also, the Q, of the overall structure (cavity and the waveguides) was measure to be 1050 in vacuum, which from simulations is equivalent to a Q of 1800 in a solvent. These Q values and the resonant frequencies of the modes are in close, but not perfect agreement with the simulation results.

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