Gap States Contribution in the Non-Linear Optical Response of Glassy Semiconductors

Abstract

Due to lattice disorder, glassy semiconductors have energy levels in their bandgaps (gap states) that can be populated by sub-gap illumination or due to recombination of photo-excited carriers. The non-linear optical (NLO) response induced by high-intensity femtosecond (fs) laser pulses develops together with the gap states excitation. These effects in non-crystalline semiconductors have not yet been well studied. In this work, we use two realizations of the pump-probe method [1,2] to study the NLO response of chalcogenide glassy semiconductors of the systems As 40 S x Se 1-x and As 40 Se x Te 1-x with fs resolution in time upon illumination by fs laser pulses with the peak wavelengths λ p of 0.79 and 1.57 μm. By the partial replacement of S by Se in the As 40 S x Se 1-x system and of Se by Te in the As 40 Se x Te 1-x system, the bandgap energy Eg was varied so that the ratio R= hv/E g of the photon energy hv to the bandgap energy Eg was tailored in the range between 0.3 and 0.9. Magnitudes of the NLO coefficients of refraction n 2 and absorption β 2 have been obtained from measurements at each λ p with thin glass samples (thickness ∼1 mm) of each composition. Comparison of the data with the dispersion function G 2 (R) derived in the theory of NLO response of direct-gap crystalline semiconductors [3] (Fig.1a) allowed us to reveal that in the range 0.5 2 for a glass composition decreases with R in a similar way but it is always positive-valued. By solution of kinetic equations for densities of the photo-excited charge carriers, the dielectric constant e was evaluated by using the Drude-Lorentz model. We found that dynamics of the NLO response develop differently with or without a single-photon excitation of gap states [4,5]. In the range 0.3 2 increases with R (Fig.la). Here in the samples with R > 0.38, the two-photon excitation of gap states is possible; this is confirmed by Fig. 1b where the time-resolved NLO response of the samples is compared with the typical trace (inset) obtained in [2] for two-beam coupling signal from fused silica at the wavelength of 1.22 μm (R<0.3).