Coherent two-photon emission from hydrogen molecules excited by counter-propagating laser pulses

We observed two-photon emission signals from the first vibrationally excited state of parahydrogen gas excited coherently by counter-propagating laser pulses. A single narrow-linewidth laser source was used to excite the parahydrogen molecules and induce the two-photon emission process. We measured how the signal energy depended on the detuning, target gas pressure, and input pulse energies. These results are qualitatively consistent with those obtained by numerical simulations based on the Maxwell-Bloch equations with one spatial dimension and one temporal dimension. This study of two-photon emission with counter-propagating injection is an important step toward neutrino mass spectroscopy.