Observation of the continuous transformation of a four level laser into a two level system

A longstanding and fundamental question of laser and maser physics concerns the energy separation ( Delta [Greek Lunate Epsilon]) required between the pumped and lasing states of a three (or four) level system. We observe a four level laser gradually transform into a two level (nonlasing) system by photoassociating Cs-Ar thermal atomic pairs and monitoring subsequent stimulated emission on the 6p2P3/2 arrow right 6s2S1/2 (D2) transition of Cs at 852.1 nm. Laser excitation spectra demonstrate that the value of Delta [Greek Lunate Epsilon] below which the system rapidly assumes the form of a two level structure and lasing ceases is similar to 0.7kT which corresponds to a Boltzmann factor (exp[- Delta [Greek Lunate Epsilon]/kT]) of 0.5.