Room temperature superradiance due to coherent coupling between light and extended single quantum state

Abstract We have established the newly-developed growth method of high-quality CuCl thin film based on molecular-beam epitaxy. In qualified films, where harmonized coupling between an excitonic polarization wave and a resonant electromagnetic wave over a range of multiple wavelengths is achieved, we have observed an exceptionally high speed nonlinear response of excitons at room temperature, wherein the excitons decay radiatively before its coherence is destroyed by dephasing. A component with extremely large radiative width can be observed in the nonlinear optical spectrum only when the other components with smaller radiative widths disappear by the room-temperature dephasing. The radiative decay time of the excitonic state with the largest radiative width reaches the order of 100 fs, which is much faster than the dephasing process. The mechanism demonstrated by our results contradicts the conventional physical description of light–matter interaction based on the long-wavelength approximation. The shapes of the measured degenerate four-wave mixing spectrum and the radiative decay profile closely reflect those of the calculated induced-polarization spectrum and the radiative decay profile obtained by real-time analysis, respectively.