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 para-hydrogen molecule. Large coherence in the initial state was prepared by the adiabatic Raman population transfer method, and the lowest Stokes sideband was used as a trigger field. We observed the coherent two-photon emission consistent with the expectation of the paired super-radiance master equation. 
Yuki Miyamoto, Hideaki Hara, Susumu Kuma, Takahiko Masuda, Masayuki   Katsuragawa, Itsuo Nakano, Chiaki Ohae, Noboru Sasao, ...
physics.atom-ph physics.optics