Experimental distinctions between population inversion gain and hyper-Raman gain in xenon

Abstract

Three-level systems in which the middle level has opposite parity from the remaining levels can develop optical gain on the excited-state transition when a two-photon resonant field couples the ground state with the uppermost state. Both population gain, which leads to amplified spontaneous emission, and hyper-Raman gain, which leads to stimulated hyper-Raman scattering, occur.1 Evidence for both types of process appears in two-photon-excited bidirectional emission in atomic xenon.2 This paper presents and compares threshold scaling characteristics, temporal behavior, directional energy distribution patterns, and tuning behavior as evidence for the gain mechanism in xenon. Near threshold, clear evidence for population inversion gain exists. Above threshold, axial emission energy exceeds off-axial (spontaneous emission) energy by a factor which increases with excitation energy. This excess of energy indicates the presence of hyper-Raman and/or four-wave mixing gain.