Polariton-driven phonon laser

Abstract

Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser. Hybrid quantum systems combining cavity quantum electrodynamics and optomechanics constitute a novel platform with potential for operation at the extremely high frequency range (30-300 GHz). We report on laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity. The studied system comprises GaAs/AlAs quantum wells coupled to cavity-confined optical and vibrational modes. The non-resonant continuous wave laser excitation of a polariton BEC in an individual trap of a trap array, induces coherent mechanical self-oscillation, leading to the formation of spectral sidebands displaced by harmonics of the fundamental 20 GHz mode vibration frequency. This phonon "lasing" enhances the phonon occupation five orders of magnitude above the thermal value when tunable neighbor traps are red-shifted with respect to the pumped trap BEC emission at even harmonics of the vibration mode. These experiments, supported by a theoretical model, constitute the first demonstration of coherent cavity optomechanical phenomena with exciton polaritons, paving the way for new hybrid designs for quantum technologies, phonon lasers, and phonon-photon bidirectional translators.

Fig. 1. Bose–Einstein condensate optomechanical induced amplification.

a, b Photoluminescence (PL) color maps of exciton–polaritons confined in a 40-μm-wide stripe. The two spatial images were recorded for nonresonant excitation powers well below (a, P_Pump = 10⁻⁴ P_Th) and above (b, P_Pump = 2.3 P_Th) the condensation threshold power P_Th. c Color map of the spectral PL dependence on the probe laser energy. d The integrated PL intensity. e The emission amplitude of the fundamental BEC state as a function of the probe laser energy. The arrows highlight the peaks corresponding to the OMIA processes. Their energies as a function of the peak order are displayed in f.

Fig. 2. Bose–Einstein condensation in a polariton trap array.

a Photoluminescence color map of exciton–polaritons confined in a square array of 1.6-μm-wide square traps as a function of the nonresonant excitation power (given in terms of the threshold power P_Th ~ 19 mW). For clarity each spectrum has been normalized to its maximum amplitude, and a logarithmic amplitude scale is used. The 3-μm-wide excitation spot was focused on a trap site: emission from both the pumped trap and from its redshifted neighbor traps can be clearly observed. The lower panels show the spatially resolved PL images for excitation below (b) and above (c) the BEC threshold. The emission from the neighboring traps can be clearly identified at x ~ ±4.8 μm in b. The inset in c shows the well-resolved vibrational sidebands.

Fig. 3. Regenerative mechanical self-oscillation induced by a BEC.

a PL spectra for varying cw nonresonant excitation powers, for a 1.6 μm polariton square trap array. All spectra have been normalized to the maximum intensity. The numbered arrows highlight: (1) low-energy vibrational sidebands spaced by ${\nu }_{\mathrm{m}}^{\mathrm{0}}=20$ GHz ~ 83 μeV; (2) clearly resolved sidebands at both sides of the fundamental and first excited polariton BEC states; and (3) reappearance of the sidebands at a high excitation power. The left and right insets show details of the fundamental BEC and first excited polariton state, respectively. b PL spectra shifted in energy and shown relative to the fundamental BEC state, indicating PL contributions from neighbor traps. The spectra with pronounced sidebands (corresponding to the regions 1–3 in a) are highlighted with thicker traces. For these spectra, the energy of a neighbor trap (circular labels) is redshifted by an even multiple of the confined mechanical mode ${\nu }_{\mathrm{m}}^{\mathrm{0}}~20$ GHz.

Fig. 4. BEC optomechanics: self-oscillation and mechanical induced BEC sidebands.

PL spectra of the fundamental (a) and first excited (b) polariton BEC mode with mechanically induced sidebands (red connected symbols). The solid thin black line is a fit to Eq. (1) yielding χ ~ 0.65 (see text for details). The asterisk indicates a peak due to PL from a neighbor trap, which was added ad hoc in the model.