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 (Δ ε) 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 6p ²P _3/2 → 6s ²S _1/2 (D ₂) transition of Cs at 852.1 nm. Laser excitation spectra demonstrate that the value of Δ ε below which the system rapidly assumes the form of a two level structure and lasing ceases is ∼ 0.7 kT which corresponds to a Boltzmann factor (exp[- Δ ε / kT]) of 0.5.