Luminescence of layered compounds excited by high-intensity two-photon pumping

High-intensity excitation luminescence measurements in three layered semiconductors (GaSe, GaS, and Pb${\mathrm{I}}_2$) are reported. The excitation is achieved by two-photon pumping. Some new results are obtained: direct-gap emission in GaS and anti-Stokes "photon replica" in Pb${\mathrm{I}}_2$. Spontaneous spectra of GaSe and Pb${\mathrm{I}}_2$ show emission due to cooperative processes recombination, also giving stimulated emission. The optical gain of Pb${\mathrm{I}}_2$ has been measured.