Parametric downconversion via vibronic transition.

This Letter presents the first, to the best of our knowledge, demonstration of noncritically birefringent-phase-matched parametric downconversion, which is associated with stimulated emission via vibronic transition in a laser gain medium. The so-called self-difference frequency generation is realized along the a-axis of a Cr:CdSe single crystal pumped by a Tm:YAG laser pulse at 2.013 µm, directly producing an infrared spectrum centered at 9 µm with the maximized effective nonlinearity. The light source, which benefits from the broad vibronic spectroscopic properties together with the wide transparency range of the host material, is expected to generate noncritically phase-matched, mid-infrared spectra beyond 20 µm along with birefringence engineering in the solid solution Cr:CdSxSe1-x.