Second-harmonic free induction decay in a two-level spin system

Two-photon free-induction decay (FID) was experimentally investigated in a two-level electron-spin system by revealing the second-harmonic wave emitted by the system. Oscillatory FID signals were detected for large-area pulses of the input power, analogously to recent observations by Kunitomo et al. for single-quantum resonances. Moreover, the measured decay time of the FID signal following a steady-state preparation was found to be larger than the theoretical value by a few orders of magnitude.