We present an ultrahigh resolution Raman study of the lifetime of 1 THz acoustic phonons confined in nanocavities. We demonstrate that the cavity Q factor can be controlled by design. Anharmonicity contributes only marginally to limit the cavity phonon lifetime, even at room temperature, while thickness fluctuations in the scale of 1/10 of a unit cell are the main limitation for the performance of THz phonon cavities.

We have realized GaAs/AlAs quantum-cascade lasers operating at 4.75 THz exhibiting more than three times higher wall plug efficiencies than GaAs/Al0.25Ga0.75As lasers with an almost identical design. At the same time, the threshold current density at 10 K is reduced from about 350 A/cm2 for the GaAs/Al0.25Ga0.75As laser to about 120 A/cm2 for the GaAs/AlAs laser. Substituting AlAs for Al0.25Ga0.75As barriers leads to a larger energy separation between the subbands reducing the probability for leakage currents through parasitic states and for reabsorption of the laser light. The higher barriers allow for a shift of the quasi-continuum of states to much higher energies. The use of a binary barrier material may also reduce detrimental effects due to the expected composition fluctuations in ternary alloys.

Resonant photoelastic coupling in semiconductor nanostructures opens new perspectives for strongly enhanced light-sound interaction in optomechanical resonators. One potential problem, however, is the reduction of the cavity Q-factor induced by dissipation when the resonance is approached. We show in this letter that cavity-polariton mediation in the light-matter process overcomes this limitation allowing for a strongly enhanced photon-phonon coupling without significant lifetime reduction in the strongly-coupled regime. Huge optomechanical coupling factors in the PetaHz/nm range are envisaged, three orders of magnitude larger than the backaction produced by the mechanical displacement of the cavity mirrors.Optomechanical resonators, that is, cavities that confine light and mechanical vibrations in the same space, strongly coupling the electromagnetic and elastic deformation fields, have emerged as novel paradigms for new fundamental ideas and applications.1-11 Optomechanical non-linearities, laser cooling, and phonon lasing 12,13 have been demonstrated. In addition, optomechanical devices have been cooled down to the quantum ground state of mechanical motion, signaling a new era of quantum phononics with implications for quantum information processing, sensitive measurements and fundamental research.14-17 Very recently hybrid systems combining cavity quantum electrodynamics (CQED) and cavity optomechanics have been theoretically proposed as a means to evidence unconventional dissipative couplings and cooling at the single-polariton level.18,19 Here we experimentally demonstrate an additional relevant characteristic of cavity polariton optomechanics, i.e., the possibility to access a hugely enhanced optomechanical coupling of dispersive photoelastic resonant nature, without significant dissipation-induced cavity Q-factor quenching.Two issues that have been identified as relevant for the development of cavity optomechanics are, on one side, the push for ever higher frequencies [20][21][22] and, on the other side, the search for new stronger optomechanical coupling mechanisms.22-25 While micromechanical devices typically oscillate in the KHz-MHz range, GHz-THz frequencies have been attained using nano-size toroids 21 and distributed Bragg reflector (DBR)-based microcavities. 22Radiation pressure is usually identified at the origin of optomechanical coupling. Direct transfer of impulse from the photon field to the resonator mirrors induces vibrations on the latter, which in turn results in a backaction on the electromagnetic field due to the resonator optical detuning induced by the mechanical displacement of the mirrors. We have recently reported that GaAs DBR- * email:afains@cab.cnea.gov.ar based microcavities constitute optimized optomechanical resonators operating in the GHz-THz range, with the potential of adding an additional photoelastic term to the above described purely "mechanical" mechanism.22 The two complementary sides of the coin in this case are electrostriction (for the generation of phonons by ligh...