Far-infrared emission by resonant-polaron effects in narrow-band-gapHg1−xCdxTe

We report on the first experimental observation of far-infrared emission within the reststrahlen band of a semiconductor. We have performed Fourier-transform emission experiments on narrow-band-gap ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Cd}}_{\mathit{x}}$Te (x≊0.17). Under ${\mathrm{Ar}}^{+}$-laser excitation, we find an emission spectrum at the longitudinal-optical-phonon energies of the material (137.5 and 157.5 ${\mathrm{cm}}^{\mathrm{-}1}$). The appearance of the optical emission signal is caused by interband polaron effects arising from the resonant matching between longitudinal-optical phonons and electron-hole excitations, as suggested in recent theoretical calculations.