Integrated Acousto-optic Devices based on Brillouin Optomechanics

Thesis (Ph.D.)--University of Washington, 2020Brillouin scattering is the inelastic light-sound interaction. It has given birth to two popular research topics: stimulated Brillouin scattering and acousto-optics. With recent advances in nanotechnology, research on both stimulated Brillouin scattering and acousto-optics has focused on scaling down the traditional bulky material to nanophotonic chips for functional devices. The integrated platforms have not only the advantage of a small footprint but also the flexibility of design and fabrication.On-chip stimulated Brillouin scattering has been demonstrated in varieties of platforms for integrated photonics. The key factor of stimulated Brillouin scattering is the Brillouin gain, which is determined by the intensity of the coherent acoustic waves. Because the acoustic wave frequency is about five orders of magnitude lower than that of light, the energy efficiency of acoustic generation through intense pump light is relatively low. On the contrary, in acousto-optics, acoustic waves are generated piezoelectrically with microwave at the same frequency, which is much efficient. However, previous work in acousto-optics focused on either acoustic modulation of cavities or small deflection of light. The interaction of light and sound with the wavelength in the same scale remains unexplored. My first project focused on exploring new phenomena and inventing novel devices by combining the advantages of stimulated Brillouin scattering and the acousto-optic modulation. With successfully integrating sub-optical wavelength acoustic transducers onto aluminum nitride photonic integrated circuits, we experimentally observed the backscattering of photons with piezoelectrically excited phonons for the first time. We systematically studied the electromechanical Brillouin scattering system. Potential applications as single-sideband modulators and microwave photonic links have also been demonstrated. In the second project, a subwavelength suspended phononic waveguide was realized. The coupling of an acoustic wave was achieved through a parabolic acoustic coupler. Different acoustic couplers with interdigital transducers with different apertures are fabricated for comparison and optimization. Transmission of the Lamb wave at 3.4 GHz was detected even in the presence of high waveguide loss. Furthermore, the phenomenon of mode conversion from membrane acoustic waves to subwavelength guided modes was first visualized through microwave impedance microscopy. This project provides guidance for the future design of co-confining light and sound in subwavelength structures. Besides these two projects, I will also discuss the fundamental of surface acoustic waves and the technical details in surface acoustic wave device design in Chapter 2. Future perspectives for integrated acousto-optic devices are included in the last chapter