Phonon lasers, known as “sasers,” leverage stimulated emission of sound and have traditionally been an academic curiosity, unlike photon lasers. However, their importance is growing due to their potential for on-chip information processing at ultra-high frequencies and in the quantum realm within integrated photonic and optomechanical devices.
Inspired by unipolar and quantum cascade lasers (QCLs), a quantum cascade phonon laser (QCPL) has been proposed and implemented. This QCPL operates by optically inducing a condensate of exciton-photon quasiparticles, called polaritons, in a microstructured semiconductor device. These polaritons descend a ladder of carefully engineered energy levels. A key distinction from QCLs is that the QCPL uses bosonic polaritons, enabling double stimulation. This polariton cascade is accompanied by the efficient stimulated emission of phonons at frequencies of approximately 20, 60, and 100 GHz, which are designed for strong on-chip interaction with the polaritons.
The engineering of polaritonic energy levels is achieved by microstructuring the spacer layer of an (Al,Ga)As microcavity into a “wedged stripe” with varying lateral thickness. This configuration creates an effective potential, resulting in polariton states that are nearly equidistant in energy, with separations matching the fundamental confined vibrational mode (~20 GHz) and its overtones. Coulomb interactions and dissipation contribute to the asynchronous locking of energy levels to optomechanical resonance when phonon self-oscillation is induced.
Experimental evidence for this multimode phonon lasing in the 10-100 GHz range includes: Inter-level locking (orbital and pseudo-spin) at confined phonon cavity frequencies, such as ~20 GHz, ~60 GHz, and ~100 GHz. The emergence of equidistant sidebands in the polariton emission spectrum, indicating mechanical modulation and period-doubling behavior.
This concept paves the way for the design of high-frequency integrated optomechanical devices, including non-reciprocal photon transport and multi-wavelength Brillouin lasers. The significant vibrational amplitudes achievable suggest great potential for nonlinear phononics. While the current GaAs-based system requires low-temperature operation (T < 100 K) due to small exciton binding energies, alternative molecular systems could potentially operate at room temperature. The principle of bosonic cascade phonon lasing also has implications for ultrafast temporal modulation of photonic lattices, the design of synthetic magnetic fields, non-reciprocal transport, and dynamical gauge field simulators.
