Theses Master's

Guided Resonances in Photonic Crystal Slabs for Cavity Optomechanics

Bui, Catvu Huu

In this thesis, the phenomenon of Fano-type guided resonances found in photonic crystal slab structures is investigated numerically and experimentally as a means to enhance the reflectivity of an ultrathin silicon nitride micromechanical membrane while preserving its thickness, low eigenmode mechanical mass and high mechanical Q factor. This enhancement finds its use in the growing field of cavity optomechanics, particularly in the membrane-in-the-middle configuration, in which a 50-nm-thick silicon nitride membrane with a high mechanical Q is coupled to the optical field inside a high-finesse Fabry-Perot cavity excited by a 1064 nm laser. It is predicted that ground state cooling and the observations of quantum behaviors in such system could be made feasible should a more demanding set of requirements, among which is an improved reflectivity of the membrane, be satisfied. Addressing this requirement for high reflectivity, this thesis proposes and demonstrates the incorporation of photonic crystals, in the form of small-area square-lattice hole arrays, into the freestanding micromechanical membranes. At normal incidence plane wave illumination, Fano-type guided resonances in such photonic crystal membrane are excited and provide a mechanism via which strong reflectivity peaks could be tuned to target wavelengths. To accomplish this, transmission spectral line shapes, field distributions inside the membrane and photonic band diagrams are studied using finite-difference
time-domain and plane wave expansion methods. Then, using electron beam lithography, photonic crystal structures designed for a transmission dip near 1064 nm are patterned into freestanding silicon nitride membranes. The fabricated membranes are tested optically for normal-incidence transmission spectra using a broadband source. Guided resonances and improved reflectivity near 1064 nm are verified against simulations. Preservation of the high Q factors of mechanical modes of patterned membranes is confirmed experimentally via mechanical ringdown measurements, verifying that the patterned membranes retain important characteristics in the context of optomechanics, namely their small thicknesses, low eigenmode mechanical masses and high Q factors. By realizing in the same device a good reflector and a high-Q mechanical resonator, these results should lead to improved performances of optomechanical systems of the membrane-in-the-middle type and present a path towards motional ground state cooling of such membrane as well as observations of related quantum effects.


More About This Work

Academic Units
Mechanical Engineering
Thesis Advisors
Wong, Chee Wei
M.S., Columbia University
Published Here
January 31, 2013