Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections

In this paper we discuss the quantum efficiency of the metastable
⁴
F_{3/2}
state in Nd:YAG. We show that the vibronic interaction disrupts the equality between absorption and emission cross section ("reciprocity") upon which spectroscopic quantum efficiency measurements are based. Even if the electron-lattice coupling is weak, a phonon whose energy matches the energy difference between two electronic states can have a significant effect. Our spectroscopic measurements clearly show nonreciprocity and establish the importance of vibronics in Nd:YAG. The data are consistent with a high quantum efficiency.