GaSe layered semiconductor possesses outstanding properties for applications as a non-linear material. A few investigated phenomena can be mentioned: (i) high efficient second harmonic light generation in the mid-IR, (ii) high stimulated photoluminescence after above band gap excitation at low temperatures, (iii) pronounced anti-Stokes photoluminescence bands excited with focused cw-HeNe laser beam below the band gap at room temperature. Existing data of two-photon absorption (TPA) measured far away from band edge cannot, however, explain these effects. Weak anisotropy and only slight enhancement of the TPA coefficient with doping was reported so far. In this paper we present for the first time the experimental results, which demonstrate the existence of the TPA in the narrow range near the band edge.
The ε-GaSe samples with sufficiently mirror smooth lateral surfaces were prepared by the original technique. Excitation was provided with variable wavelength of 2.5 ns laser pulses around the band edge. The changes of induced free-carrier absorption (FCA) have been measured using time- and spatially-resolved technique with orthogonal pump-probe geometry and with variable probe light polarization in respect to c-axis. Strong dependence of the cross-section for light absorption on the probe beam polarization has been derived. Anisotropy of this quantity reaches 50 and peaks when polarization is perpendicular to c-axis for probe wavelength 1300 and 861 nm.
We detected quadratic FCA increasing with pump light intensity above indirect exciton position in a narrow spectral range of about kT width. Fine structures of absorption due to assisted phonons were resolved on the long wavelength side at 75 K. The phenomenon is observed over medium injections and persists in temperature range up to 300 K. This phenomenon is explained by TPA. Its coefficient exceeds 10-7 cm/W in the peak. Since linear absorption is weak below the direct exciton, the TPA is resolved in up to 500 µm depth of the sample. The discovered phenomenon shows that TPA in GaSe gives an explanation for previously observed non-linear phenomena listed above. A discussion about mechanism of the TPA is presented.
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