Spectra of mechanical cavity modes in distributed Bragg reflector based vertical GaAs resonators

Distributed Bragg reflector based semiconductor resonators constitute paradigmatic systems where cavity optomechanical and optoelectronic phenomena can be simultaneously active in the same device. High GHz range mechanical frequencies and ultrastrong optomechanical couplings are additional attractive features for applications. We report here a detailed spectroscopic study of the fundamental optomechanical resonances of such a device. The existent challenge to study vibrational frequencies that are above the bandwidth of current electronics is solved using a purposely made tandem Fabry-Perot-triple spectrometer. A full theoretical description of the Raman process including electronic, vibrational, and optical confinement is presented to describe the experiments. These results open the path for the demonstration of polariton optomechanical phenomena in these devices.

Figure 1

Color map corresponding to the in-plane dispersion of the optical cavity modes. The double-resonant Raman scattering process is accomplished by sending light at a specific angle ${\theta }_0$ and wavelength ${\lambda }_l$, and collecting the ${\lambda }_s$ scattered light at $\theta =0$. The inset is a scheme of the experimental configuration.

Figure 2

Raman spectra measured on different spots on the sample, so as to tune the optical cavity mode. Spectra were taken for double-resonant conditions with incidence angle $\theta \approx 4^{\circ }$ (left) and $\theta \approx {47}^{\circ }$ (right), tuned to enhance the spectra corresponding to optomechanical vibrations and optical phonons of the structure, respectively. Vertical dashed lines and spectra with thicker curves are shown to highlight the peaks that can be identified as due to phonons.

Figure 3

Raman spectra taken with incidence angle $\theta \approx 3.4^{\circ }$ (double-resonant condition tuned around $1.9$ cm ${}^{-1}$) with the triple-additive spectrometer (top) and the Fabry-Perot-spectrometer tandem (middle). Solid darker curves correspond to the experiment, dotted curves to the model (see text for details). The peaks marked with single and double asterisks are assigned to laser light. In particular the strong peaks close to 25 and 50 GHz signal the successive tuning of the Fabry-Perot modes with the laser energy, their difference corresponding to the FSR. The bottom panel shows the calculated acoustic reflectivity of a microcavity surrounded by GaAs (instead of air in one side), and the acoustic dispersion of an infinite DBR. Vertical lines highlight relevant spectral features at the Brillouin zone edge and center.

Figure 4

Calculated optical and acoustic modes of the optomechanical resonator. From top to bottom: electric intensity associated with the optical cavity mode, normalized to an incidence of amplitude 1, and strain associated with relevant modes at $\approx 19$, 38, and 56 GHz, appropriately normalized assuming the system in thermal equilibrium. Note the confined (extended) character of the modes at 19 and 56 (38) GHz.