Phase Noise of an In_0.5Ga_0.5P Optomechanical Crystal Oscillating at 3 GHz

We report on the first GaInP bichromatic 1D optomechanical crystal. Self-sustained oscillations are observed with extremely low linewidth, low threshold and a phase noise limit of -113 dBc/Hz at 1 MHz offset.

The co-localization of light and mechanical oscillation in a tiny optomechanical crystal results in a large optomechanical coupling at GHz frequencies. However, linear and nonlinear absorption and related thermal effects hinder the study of fundamental light-matter interactions [1]. Here we introduce a new material for optomechanical crystals : Gallium Indium Phosphide (GaInP). Owing to its large electronic band gap, two-photon absorption is suppressed in the practical Telecom spectral domain. The design of the one dimensional cavity (Fig.1a) is based on the concept of the bichromatic lattice[2]. It consists of a line of holes with fixed period a and, on the sides, half-ellipses, with period a`=0.98a. This design is defined by only 4 parameters and confines optical modes as well as mechanical modes, as can be seen in Fig.1b.

The theoretical radiation-limited Q-factor is 6 x 10⁶. The experimental optical quality factor is limited by fabrication disorder and ranges from Q=10⁴ to a maximum of 1.3 x 10⁵, depending on the coupling to the input waveguide. The mechanical resonance ranges between 2.9 and 3.0 GHz, in agreement with calculations, with quality factor Q_m≈2300, limited by internal losses. The calculated and measured vacuum optomechanical coupling coefficient is g₀≈ 2π x 300 kHz, in close agreement with theory. While nonlinear absorption is effectively suppressed, the residual absorption is responsible for a thermooptic bistability (Fig.1c), which reveals a linear dependence on the power. This allows the estimate of the absorption rate Γ_abs ≈ 60 MHz, namely 1\% of the total optical losses, which are dominated by elastic scattering.
The self-sustained oscillations regime is achieved at a threshold power of 40 µW and the Lorentzian part of the mechanical linewidth is narrowed down to 70Hz (Fig.1d).
The phase noise of the free running oscillator was also measured using the heterodyne detection. The frequency fluctuation (1/f² noise) is 37 dB above the reference, and the pure phase noise limit is equal to -113 dBc/Hz at 1MHz offset, which is close to the values found in previous oscillators in the MHz range[3].
Further research is aimed at reducing the phase noise of the OM oscillator.

[1] R. Norte et al, Physical Review Letters 116 (2016)

[2] F. Alpeggiani et al, Applied Physics Letters 107, 261110 (2015)

[3] X. Luan, et al., Scientific Reports 4 (2014)

e-mail address : ines.ghorbel@thalesgroup.com