Observation of coherent two-photon emission from the first vibrationally excited state of hydrogen molecules

In this paper, we describe an experiment which was conducted to explore the macro-coherent amplification mechanism using a two-photon emission process from the first vibrationally excited state of the para-hydrogen molecule. Large coherence in the initial state was prepared by the adiabatic Raman method, and the lowest Stokes sideband was used as a trigger field. We observed coherent two-photon emission consistent with the expectation of the Maxwell–Bloch equation derived for the process, whose rate is larger by many orders of magnitude than that of spontaneous emission.