PhD defense: Sho Tamaki

Optomechanical Crystals with Gallium Phosphide

Faithful quantum state transfer between telecom photons and microwave frequency mechanical oscillations necessitate a fast conversion rate and low thermal noise. Gallium Phosphide (GaP) is a promising material thanks to its large electronic bandgap of 2.26 eV, which suppresses two-photon absorption, and to its high refractive index n = 3.05 at the telecom C-band, leading to a high-Q optical mode.

 In this thesis, we propose 2 designs of optomechanical crystals (OMCs) made of GaP. The first design is two-dimensional OMC which enables sufficiently high mechanical frequency (1-10 GHz). This places our device in the resolved-sideband regime, a prerequisite for many quantum protocols. It can also support higher thermal conductance than 1D structures, mitigating the parasitic laser absorption heating. The other design is a Su–Schrieffer–Heeger (SSH) based topological nanobeam. The topologically protected optical and mechanical modes are expected to be robust against geometrical impurity and to have consistent eigenfrequencies. This will make those modes indistinguishable and thus suitable for quantum communication and computations.

 We fabricate and characterise the 2D OMC made of GaP. We realise a high optical Q-factor of 7.9×10^4, corresponding to a linewidth κ/2π = 2.5 GHz at the telecom frequency 195.6 THz. This optical mode couples to several mechanical modes, whose frequencies all exceed the optical linewidth. The most strongly coupled mode oscillates at 7.7 GHz, more than 3 times the optical linewidth, while achieving a substantial vacuum optomechanical coupling rate g0/2π = 450 kHz. This makes the platform a promising candidate for a long-lived, deterministic quantum memory for telecom photons at low temperatures.

Zoom link: https://ucph-ku.zoom.us/j/69617265696?pwd=oeJNxPOYDRGSWDdLrJbPhe5NFkpbDe.1