Two-photon absorption and stimulated emission in poly-crystalline Zinc Selenide with femtosecond laser excitation

Abstract

The optical nonlinearity in polycrystalline zinc selenide (ZnSe), excited with 775 nm, 1 kHz femtosecond laser pulses was investigated via the nonlinear transmission with material thickness and the Z scan technique. The measured two photon absorption coefficient <em>β</em> was intensity dependent, inferring that reverse saturated absorption (RSA) is also relevant during high intensity excitation in ZnSe. At low peak intensity <em>I</em> &lt; 5 GW cm^–2, we find <em>β</em> = 3.5 cm GW^–1 at 775 nm. The spectral properties of the broad blueish two-photon induced fluorescence (460 nm-500 nm) was studied, displaying self-absorption near the band edge while the upper state lifetime was measured to be τ_e ~ 3.3 ns. Stimulated emission was observed when pumping a 0.5 mm thick polycrystalline ZnSe sample within an optical cavity, confirmed by significant line narrowing from Δ<em>λ</em> = 11 nm (cavity blocked) to Δ<em>λ</em> = 2.8 nm at peak wavelength <em>λ</em>_p = 475 nm while the upper state lifetime also decreased. These results suggest that with more optimum pumping conditions and crystal cooling, polycrystalline ZnSe might reach lasing threshold via two-photon pumping at <em>λ</em> = 775 nm.