Anisotropy of two-photon absorption in gallium selenide at 1064 nm

The Bridgman method is used to grow single crystals of ε-GaSe. The two-photon absorption (TPA) coefficient β was measured for especially un-doped crystals at room temperature for the ordinary o-ray (β_⊥) and extraordinary e-ray (β_∥) using single-wavelength excitation by a Nd:YVO₄ laser at 1.064 μm with a pulse duration of 10 ps and a repetition rate of 81 MHz. No large anisotropy was observed for the TPA coefficients between the two geometries. The TPA coefficients were found to be β_⊥ = 1.07 × 10⁻⁹ cm/W and β_∥ = 1.88 × 10⁻⁹ cm/W. It was found that doping slightly increases the TPA coefficient. The value of the TPA for crystals doped with 0.5 at.% of Tl is β_⊥ = 7.56 × 10⁻⁹ cm/W.