Periodic superradiance in an Er:YSO crystal

We observed periodic optical pulses from an Er:YSO crystal during irradiating with a continuous-wave excitation laser. We refer to this phenomenon as ``periodic superradiance.'' This periodicity can be understood qualitatively by a simple model, in which a cyclic process of a continuous supply of population inversion and a sudden burst of superradiance is repeated. The excitation power dependences of peak interval and the pulse area can be interpreted with our simple model. In addition, the linewidth of superradiance is much narrower than an inhomogeneous broadening in a crystal. This result suggests that only ${\mathrm{Er}}^{3+}$ ions in a specific environment are involved in superradiance.