A two-dimensional gallium phosphide optomechanical crystal in the resolved-sideband regime

TL;DR

This paper reports the fabrication and characterization of a gallium phosphide two-dimensional optomechanical crystal that achieves high optical quality, strong mechanical coupling, and operates in the resolved-sideband regime, suitable for quantum memory applications.

Contribution

The work demonstrates a 2D GaP optomechanical crystal with high optical Q-factor, GHz mechanical modes, and strong optomechanical coupling, advancing quantum information processing capabilities.

Findings

Achieved optical Q-factor of 7.9×10^4 at telecom frequency

Realized mechanical modes exceeding the optical linewidth, with the strongest at 7.7 GHz

Obtained a vacuum optomechanical coupling rate of 450 kHz

Abstract

Faithful quantum state transfer between telecom photons and microwave frequency mechanical oscillations necessitate a fast conversion rate and low thermal noise. Two-dimensional (2D) optomechanical crystals (OMCs) are favorable candidates that satisfy those requirements. 2D OMCs enable sufficiently high mechanical frequency (1$\sim$10 GHz) to make the resolved-sideband regime achievable, a prerequisite for many quantum protocols. It also supports higher thermal conductance than 1D structures, mitigating the parasitic laser absorption heating. Furthermore, gallium phosphide (GaP) is a promising material choice thanks to its large electronic bandgap of 2.26 eV, which suppresses two-photon absorption, and high refractive index $n$ = 3.05 at the telecom C-band, leading to a high-$Q$ optical mode. Here, we fabricate and characterize a 2D OMC made of GaP. We realize a high optical $Q$-factor of $7.9\times 10^{4}$, corresponding to a linewidth $\kappa/2\pi$ = 2.5 GHz at the telecom frequency 195.6 THz. This optical mode couples to several mechanical modes, whose frequencies all exceed the cavity 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 $g_{\mathrm{0}}/2\pi$ = 450 kHz. This makes the platform a promising candidate for a long-lived, deterministic quantum memory for telecom photons at low temperatures.