Midinfrared Ultrastrong Light–Matter Coupling for THz Thermal Emission

In condensed matter, THz optical resonances (1–5 THz) are very sensitive to thermally activated phenomena that can either equate the populations between the levels or irremediably broaden the transition to a point where it disappears. It is therefore very difficult to exploit THz electronic transitions for thermal emission. To bypass this problem, we have used a transition in the mid-infrared (10–50 THz), ultrastrongly coupled to a resonant mode of a highly subwavelength microcavity. The coupling strength in our system reaches 90% of the energy of the matter resonance and becomes so high that the lower-polariton state lies in the THz region. This mixed light–matter resonance is therefore issued from an optical transition that is much less sensitive to thermal effects, as we have experimentally demonstrated. Our system has been optimized by tailoring the cavity thickness and engineering the matter resonance, which arises from a set of heavily doped quantum wells, in order to increase the THz emissivity. By injecting a lateral current in the quantum wells that raises the electronic temperature, we have observed THz emission up to room temperature.