Observation of intersubband polaritonic coupling in a quantum-cascade metasurface

An amplifying quantum-cascade (QC) metasurface - the key component of the QC vertical-external-cavity surface-emitting-laser (VECSEL) - is studied as a function of current density using reflection-mode terahertz time domain spectroscopy. Nearly perfect absorption and an anomalous reflection phase characteristic is measured at zero bias, which disappears as the device is biased to provide gain. This is attributed to critical coupling between a lossy intersubband transition within the active material and the metasurface resonance. These results provide insight into the interaction between the intersubband QC-gain material and the metasurface and modify the design rules for QC-VECSELs for both biased and unbiased regions.