Micropillar Resonators for Optomechanics in the Extremely High 19–95-GHz Frequency Range

Strong confinement, in all dimensions, and high mechanical frequencies are highly desirable for quantum optomechanical applications. We show that GaAs/AlAs micropillar cavities fully confine not only photons but also extremely high frequency (19–95 GHz) acoustic phonons. A strong increase of the optomechanical coupling upon reducing the pillar size is observed, together with record room-temperature $Q$-frequency products of $10^{14}$. These mechanical resonators can integrate quantum emitters or polariton condensates, opening exciting perspectives at the interface with nonlinear and quantum optics.

Figure 1

(a) SEM images of an array of circular and square pillars with lateral sizes ranging from 50 to $1\mu \mathrm{m}$. The inset presents a zoom on a $5\mu \mathrm{m}$ square pillar. (b) $k$-space image of the optical cavity modes for a square pillars of $8\mu \mathrm{m}$ lateral size. The shaded ellipse represents the profile (energy broadening and angular dispersion) of the pump and probe laser pulses. (c) Spatial distribution of $|dV/V|$ associated with a confined mechanical mode around 19 GHz, calculated using finite element methods. (d) Scheme of the ultrafast resonant laser microspectroscopy setup. (e) Scheme of the process leading to mechanical signals in reflectance difference time resolved spectroscopy.

Figure 2

Top: As-measured differential reflectance time trace for a $5\mu \mathrm{m}$ pillar, varying the delay $t$ between the pump and probe pulses. The inset is a schematic representation of the laser pulse energy distribution, and the cavity mode (dip in the reflected probe) at different times after pump excitation. The time before(after) the arrival of the pump is indicated with the initial red(blue) star and dip. Excited carrier relaxation leads to the recovery of the cavity mode, which around 2000 ps passes through the central energy of the laser (vertical dashed line, green star, and dip). At longer times the equilibrium situation is recovered (final red star and dip). Middle: filtered time trace corresponding to frequencies between 5 and 100 GHz. Bottom: windowed Fourier transform (WFT) of the filtered trace, obtained with 1000 ps windows.

Figure 3

(a) Vibrational spectra of a $5\mu \mathrm{m}$ pillar obtained through the Fourier transform of the oscillating part extracted from the reflectance difference signal. The fundamental cavity mode at approximately 19 GHz and two overtones at 58 and 95 GHz are highlighted. The inset compares the 19 GHz mode of pillars of 3, 4, and $5\mu \mathrm{m}$ size. (b) Lifetime of the 19 and 58 GHz cavity confined mechanical vibration as a function of pillar size $L$. Error bars reflect the measured amplitude uncertainty. The colored region marks the resolution limit of our pump and probe technique. (c) Mechanical signal amplitude ($\mathrm{\Delta }R/R_0$) corresponding to the three confined acoustic modes. Values are normalized to the amplitude observed for the $50\mu \mathrm{m}$ pillar, and have been vertically shifted for clarity. Dashed curves are fits with a $1/L$ dependence (see text for details).