High Frequency GaAs Nano-Optomechanical Disk Resonator

Optomechanical coupling between a mechanical oscillator and light trapped in a cavity increases when the coupling takes place in a reduced volume. Here we demonstrate a GaAs semiconductor optomechanical disk system where both optical and mechanical energy can be confined in a subwavelength scale interaction volume. We observe a giant optomechanical coupling rate up to $100\mathrm{GHz}/\mathrm{nm}$ involving picogram mass mechanical modes with a frequency between 100 MHz and 1 GHz. The mechanical modes are singled-out measuring their dispersion as a function of disk geometry. Their Brownian motion is optically resolved with a sensitivity of ${10}^{-17}\mathrm{m}/\sqrt{\mathrm{Hz}}$ at room temperature and pressure, approaching the quantum limit imprecision.

Figure 1

Optomechanical study of a GaAs disk. (a) Scanning electron microscope (SEM) view of a GaAs disk ($4.5\mu \mathrm{m}$ diameter and 200 nm thickness) suspended on an AlGaAs pedestal. (b) Schematics of the near-field optomechanical spectroscopy experiment. FPC stands for fiber polarization control, PBS for polarization beam splitter.

Figure 2

Selected vibrational spectra of GaAs nano-optomechanical disks. (a) Optically measured motional noise spectrum of a disk of radius $5.6\mu \mathrm{m}$ and thickness 200 nm. (b) Calibrated displacement noise resonance of a disk of radius $4.5\mu \mathrm{m}$, showing a sensitivity of $2\times {10}^{-17}\mathrm{m}/\sqrt{\mathrm{Hz}}$. (c),(d) High-frequency mechanical resonances of a disk of radius $3.6\mu \mathrm{m}$ in the 500 MHz–1 GHz band.

Figure 3

Dispersion lines of GaAs disk mechanical modes measured vs disk radius. Thickness is 200 nm. Data points were acquired on different disks of the same sample. Transparent “guide to the eyes” bands correspond to numerical simulations with an error bar represented by their thickness.