Nonlinear and saturable absorption characteristics of amorphous InSe thin films

Abstract

We prepared very thin amorphous InSe films and investigated the thickness dependence of the nonlinear absorption by pump-probe and open aperture Z-scan techniques. While thinner films (20 and 52 nm) exhibit saturable absorption, thicker films (70 and 104 nm) exhibit nonlinear absorption for 4 ns, 65 ps, and 44 fs pulse durations. This behavior is attributed to increasing localized defect states in the energy band gap as the film thickness increases. We developed a theoretical model incorporating one photon, two photon, and free carrier absorptions and their saturations to derive the transmission in the open aperture Z-scan experiment. The theory of open aperture Gaussian beam Z-scan based on the Adomian decomposition method was used to fit the experimental curves. Nonlinear absorption coefficients along with saturation intensity thresholds were extracted from fitting the experimental results for all pulse durations. The lowest saturation threshold was found about 3×10−3 GW/cm2 for 20 nm film thickness with nanosecond pulse duration and increased about four orders of magnitude for 104 nm film thickness.

The effect of thickness and doping on the nonlinear absorption behaviour of IIIA-VIA group amorphous semiconductor thin films

“…Nonlinear absorption in semiconductors with thickness values in the range of microns is extensively studied in the literature [9,10]. Recently in our studies, we show that decreasing thickness as well as dopant ratio results in decreasing I sat and increasing thickness and dopant ratio results in increasing β eff value for InSe and GaSe amorphous thin films [11,12].…”

“…The I sat and β eff values were extracted according to the Ref. [11] which we proposed a model incorporating OPA, TPA, and FCA and their saturation to derive the transmission in the OA Z-scan experiments (equation (1)). …”

The effect of thickness and/or doping on the nonlinear and saturable absorption behaviors in amorphous GaSe thin films

“…These applications include Q-switching, mode-locking, upconversion lasing, and optical limiting. 11 Thinner films ͑20 and 52 nm͒ exhibited SA behaviors while thicker films ͑70 and 104 nm͒ exhibited NA behaviors for 4 ns, 65 ps, and 44 fs pulse durations. TPA process and its saturation have been widely studied in semiconducting crystals.…”

“…NA can be classified into two types: ͑i͒ transmittance increases with increasing optical intensity; this case corresponds to well known saturable absorption ͑SA͒; ͑ii͒ transmittance reduces with increasing optical intensity; this type includes two-photon absorption ͑TPA͒, multiphoton absorption, and reverse SA. 13 In our recent work, 11 we extended this theory by considering OPA saturation, TPA, and free carrier absorption ͑FCA͒ saturation. 1-4 It is well known that substitutional doping is a defining property of semiconductors.…”

Structural and optical properties of gallium sulfide thin film

“…These values are less than the band gap values of bulk gallium sulfide crystal. This behavior is thought to be due to the blurring of the band gap by the defect states [25,26]. Steepness of the exponential absorption tail is determined from the absorption spectrum as:…”