Intersubband polariton -- LO phonon interaction in mid-infrared non-dispersive cavities: experimental demonstration of spontaneous scattering and perspectives towards polariton lasing

TL;DR

The paper investigates spontaneous and stimulated interactions between intersubband polaritons and longitudinal optical phonons in non-dispersive mid-infrared cavities. It combines an experimental demonstration of LO-phonon–assisted polariton scattering with a theoretical pump–probe framework based on generalized Optical Bloch Equations and Fröhlich coupling to predict conditions for gain. The results show spontaneous scattering with emission locked to the LO-phonon energy and, importantly, identify pump–probe parameters that could realize stimulated scattering and optical gain, suggesting a route toward inversionless lasing and polariton condensation in the MIR/THz regime. This work provides foundational insight and a practical pathway for achieving gain in ISB polariton systems, with potential impact on MIR/THz photonics and quantum technologies.

Abstract

We report experimental evidence of the interaction between intersubband polaritons and longitudinal optical phonons in non-dispersive mid-infrared cavities, under resonant optical injection. The light emission originating from spontaneous polariton-phonon scattering is observed at a frequency corresponding to an energy shift of one phonon below the pump frequency. Given the extremely low spontaneous scattering rate, we employ a custom-developed quantum mechanical model to numerically demonstrate the feasibility to stimulate such process using a pump-probe scheme. Based on this analysis, we identify a set of experimental conditions under which optical gain may be realized in a mid-infrared intersubband polaritonic system.

Figures (6)

• Figure 1: (a) Simulated energy band profile and wavefunctions using a commercial Poisson-Schrodinger solver. (b) Transmittance of the sample shaped in a multipass prism configuration. The measurement is done at room temperature within an FTIR.
• Figure 2: (a) Reflectance measurement of the microcavity device with a stripe size of 1.2$\mu$m (the one closest to the anti-crossing point), along with the transmittance of the bare ISB transition (in transparency). The dash line marks the central frequency of the latter. In the inset, the optical image shows 6 microcavity arrays, each of them with a different metal stripe size. (b) Reconstructed experimental reflectance as function of the strip size. The arrow marks the position of the anti-crossing point within the polaritonic dispersion.
• Figure 3: (a) Experimental optical set-up with the tunable QCL and goniometric arms that allow the different experimental pump positions.(b) Upper panel, experimental reflectance of the microcavity with $\Lambda$ = 0.95 $\mu$m lateral size, recorded at 78K. The light blue dot corresponds to the pump state while the dark blue one is the expected final state. Lower panel, FFT of the recorded interferogram of the sample under resonant injection of the QCL at a frequency of 34.2 THz. The main peak is the remaining stray light from the pump. The lower peak corresponds to the photons emitted by the small fraction of polaritons scattered via LO-phonons. The blue shaded area is the QCL tuning range.
• Figure 4: (a) Numerically simulated transmittivity spectrum as a function of the incidence angle in the $xz$ plane. The different experimental pump positions are indicated. (b) Left panel, emission spectra when the pump frequency is changed while the angle is kept constant at 41$^{\circ}$. Right panel, emission spectra when the pump angle is changed while frequency is kept constant at 33.9 THz. The polaritonic spontaneous emission is locked 8.75 THz below the pump.
• Figure 5: (a) Numerically calculated reflectance of the sample at the conditions of no pump (blue) and of maximum observed gain (orange). (b) The absolute value of the pump field (dashed dark line), the probe field sent to probe the system (blue), the probe field after probing the system (orange), along with the phonon field (grey). The inset shows a temporal zoom of the fields in logarithmic scale.