Terahertz spectroscopy of semiconductor microcavity lasers: Photon lasers

Semiconductor microcavities can exhibit various macroscopic quantum phenomena, including Bose-Einstein condensation of polaritons, Bardeen-Cooper-Schrieffer (BCS) states of polaritons, and photon lasing (lasing with negligible Coulombic exciton effects). An important aspect of possible experimental identification of these states is a gap in the excitation spectrum (the BCS gap in the case of a polaritonic BCS state). Similar to the polaritonic BCS gap, a light-induced gap can exist in photon lasers. Although polaritonic BCS states have been observed on the basis of spectroscopy in the vicinity of the laser frequency, the direct observation of polaritonic BCS gaps using light spectrally centered at or around the emission frequency has not been achieved. It has been conjectured that low-frequency (terahertz) spectroscopy should be able to identify such gaps. In this first of two studies, a theory aimed at identifying features of light-induced gaps in the linear terahertz spectroscopy of photon lasers is developed and numerically evaluated. It is shown that spectral features in the intraband conductivity, and therefore in the system's transmissivity and absorptivity, can be related to the light-induced gap. For sufficiently small Drude damping this includes spectral regions of THz gain. A future study will generalize the present formalism to include Coulomb effects.