Vibrational two-photon emission from coherently excited solid parahydrogen

We report observation of two-photon emission from coherently excited vibrational state of solid parahydrogen, known as one of quantum solids. The coherent state between the ground and the excited states is prepared by stimulated Raman scattering using two visible laser pulses. The two-photon emission is triggered by another mid-infrared laser pulse. The observed two-photon emission persists even when the trigger pulse is injected long after the excitation. The observed phenomenon is due to a long decoherence time of the vibrational states of solid parahydrogen, which is attributed to special nature as quantum solid. Dependence of the decoherence on target temperature and residual orthohydrogen concentration are studied along with its time evolution. It is found that the emission intensity increases even after the excitation pulses pass through the target completely. The coherence development is highly suppressed at high target temperature and high residual orthohydrogen concentration. Effects of target annealing and laser-induced damage on the target are also observed.