Date of Award

8-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Electrical Engineering

Advisor

Dr. Lin Zhu

Committee Member

Dr. Pingshan Wang

Committee Member

Dr. Liang Dong

Committee Member

Dr. John Ballato

Abstract

Nowadays, optical forces are widely used to control or measure the position of micrometer- to nanometer-sized particles, such as living cells, DNA and bacteria. Two major categories of optical forces are discussed here: gradient or dipole force and scattering force. The gradient force is originated from the fact that a polarizable micro-particle placed in a laterally varying optical field will experience different forces in the positively and negatively charged sides. In this thesis, we investigate the optical gradient force in 2D hybrid and plasmonic waveguides. Compared to the conventional dielectric waveguides, we show that the optical force can be enhanced by at least 1 order of magnitude in the hybrid and plasmonic waveguides. The scattering force is an axial force, which is the result of the momentum transfer from the radiating field to the dielectric medium. To investigate the radiation force, we mainly study two types of micro-resonators: wheel resonators and small disk resonators. To investigate the optical gradient force, we calculate the optical forces in two dimensional hybrid waveguides in which a dielectric waveguide is coupled with a metal substrate and plasmonic waveguides in which two identical metal waveguides are coupled. We compare coupled plasmonic waveguides with different geometries, including rectangular, circular, and triangular cross sections. In comparison with their corresponding dielectric structures, optical forces in both hybrid and plasmonic waveguides are greatly enhanced due to stronger evanescent waves and larger field gradients. To investigate the radiation force, we study three types of optomechanical oscillators: silicon nitride disk resonator, silicon nitride wheel resonator, and small silicon disk resonator. The experimental results show that the optomechanical coupling coefficiency increases from 0.8GHz/nm to 110GHz/nm, the effective mass decreases from 2ng to 6pg, the oscillation frequency increases from 10MHz to 1GHz, the device size shrinks from 60&\mu m to only 4&\mu m and the oscillation threshold improves from 500&\mu W to 8&\mu W. We also systematically investigate GHz optomechanical microdisk oscillators on the platform of SOI. We have tested small silicon disks with different radii. For the disks with the radii of 1.5&\mu m, 2&\mu m, 2.5&\mu m and 3&\mu m, fundamental breathing modes are detected. The mechanical Q is sensitively dependent on the undercut, since it plays an important role in the mechanical energy dissipation. The silicon disk with the radius of 2&\mu m and the undercut ratio of 90% oscillates at 1.27GHz under the dropped power below 10&\mu W. We also demonstrate the phenomena of the instability and RF mixing above optomechanical oscillation threshold. Then we study single, two side-coupled, and serially-coupled triple wheel resonator system. The single wheel resonator and two side-coupled wheel resonators have the same dimension with inner radius of 24.3&\mu m and outer radius of 34.3\mu m. In the single resonator, we observe that both the fundamental radial breathing mode and the flapping mode couple to a high Q optical mode and generate frequency mixing through the nonlinear optical transfer function. The harmonic generation of the flapping mode produces a comb-like frequency mixing spectrum. Instead of using the external pump modulation, we show that the regenerative oscillation of an internal mechanical mode can be used as a modulation source for optomechanical RF mixing. In the side-coupled resonators, the optomechanical transduction is related to the energy distribution in the two resonators, which is strongly dependent on the input detuning. Compared to a single resonator, the coupled resonators can still provide very sensitive optomechanical transduction even if the optical and mechanical quality factors of one resonator are degraded. The serially-coupled triple resonators have the dimension with inner radius of 20&\mu m and outer radius of 25&\mu m. A freestanding beam is placed in the vicinity of the middle resonator. In this coupled system, we demonstrate that the mechanical mode of the freestanding beam can be selectively coupled to different resonance supermodes through the near field interaction. All the devices are fabricated on the 300nm thick silicon nitride material with the silicon oxide as the sacrificial layer, and they are fully suspended with three thin spokes connecting to a center support. They have effective mass around 1ng and optomechancial coefficient in the level of 1GHz/nm.

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