Fabrication Of A High-Q Photonic Molecule Based On A Novel Coupling Approach And Its Application For Cavity Optomechanics

For decades, optical microcavities have drawn more and more research interest and played a much important role both in basic study and application area, due to their small mode volume and high optical quality factor. They have been exploited in cavity optomechanics, cavity quantum electrodynamics, nonlinear optics, low threshold microlasers, sensors and other fields. Among these, cavity optomechanics has been an active research field which focuses on the interaction between electromagnetic radiation and mechanical motions in optical cavities. In this research activity, silica whispering gallery mode (WGM) microcavities have become a powerful platform partly due to their high quality optical and mechanical properties. As examples, silica microcavities have been largely exploited in the studies of mechanical oscillation, optomechanically induced transparency and optomechanical cooling with single microcavity.In this thesis, our work on multimode optomechanics with two coupled high optical quality factor silica microcavities for low-threshold phonon laser is described. Also, preliminary results on high-frequency phonon laser are present.In the first part, we demonstrate a compound structure of two microtoroids through a new coupling method for phonon laser. This structure is also called as photonic molecule, for the reason that it can produce two supermodes with different frequency between the microcavities, which is similar to two electronic levels with different energy in molecule. One microtoroid located at the corner of chip is turned upside down to couple with another one with very thin silicon pillar which has a high mechanical quality factor (pillar diameter less than one micron and the mechanical quality factor in vacuum is 9000). Two microtoroid cavities with intrinsic optical Q-factors of 9.7×107 and 9.3×107 and scattering-induced mode splittings of 11.2 MHz,7.6 MHz, are used to form a photonic molecule with phonon laser threshold of 1.3 μW, which is 5 times lower than the previous work.In the second part, we try to fabricate small-diameter silica WGM microcavities and use the same coupled method in the first part to obtain a high frequency phonon laser. We have already fabricated a 12.3 μm-diameter microsphere cavity with a 294 MHz radial breathing mode frequency and observed the mechanical oscillation with single microsphere cavity successfully. High frequency phonon laser using this photonic molecule structure will be conducted soon.