Stimulated Non-Collinear Intersubband Polariton-Polariton Scattering in a Dispersive Multi-THz Microcavity

Non-collinear pump-probe spectroscopy with phase-stable multi-THz pulses reveals the ultrafast scattering dynamics of intersubband polaritons in a dispersive microcavity. For resonant excitation of the lower polariton at a finite in-plane momentum, $k_{||}$, a narrowband maximum emerges in the probe reflectivity at the polaritonic ground state at $k_{||}=0$. A quantum mechanical model reproduces the experimental key results quantitatively and confirms the underlying microscopic process as stimulated polariton-polariton scattering. These results are an important milestone on the way to bosonic lasing in custom tailored polaritonic systems in the multi-THz and THz spectral regions.