Infrared absorption and reflection spectra of high-density electron–hole systems

We analyze infrared absorption and reflection spectra of high-density electron–hole (e–h) systems. The BCS-like energy gap (BEG) is found in the calculated spectra. We incorporate the large quantum fluctuation with the quasi-static Eliashberg equation for e–h systems which allows us to calculate the renormalized band energy, the BEG, and the Coulomb renormalization factor. We find that the collective fluctuation significantly modifies the spectra, and that the strong visible-light excitation distinctly stabilizes the e–h BCS state.