Unveiling intersubband polaritonic coupling within quantum-cascade metasurfaces

We investigate an amplifying quantum-cascade (QC) metasurface, which serves as a crucial component in the QC vertical-external-cavity surface-emitting-laser (VECSEL), under various current densities by employing reflection-mode terahertz time domain spectroscopy. Our findings reveal a near-perfect absorption and an atypical reflection phase characteristic observed at zero bias, which diminishes as the device undergoes larger bias to induce gain. This phenomenon is attributed to the critical coupling between a lossy intersubband transition within the active material and the metasurface resonance. These outcomes offer valuable insights into the interaction dynamics between the intersubband QC-gain material and the metasurface, thereby refining design principles for QC-VECSELs in both biased and unbiased operating regimes.