Material providing optical gain via population inversion — Nd:YAG crystal, He-Ne gas, semiconductor heterostructure, dye solution. Characterised by gain cross-section σ, upper-level lifetime τ, saturation intensity I_sat = hν/(στ).
Material providing optical gain via population inversion — Nd:YAG crystal, He-Ne gas, semiconductor heterostructure, dye solution. Characterised by gain cross-section σ, upper-level lifetime τ, saturation intensity I_sat = hν/(στ).