Nonlinear optical and optical limiting response of PLD nc-Si thin films

Abstract

The effect of deposition temperature on the structural as well as linear and nonlinear optical properties of nanocrystalline silicon (nc-Si) thin films fabricated by pulsed-laser deposition (PLD) are reported in this study. The films were deposited at substrate temperatures (T_s) ranging from room temperature (RT) to 700 °C. The X-ray diffraction spectra of the films displayed the characteristic peaks of Si(111), (220) and (311) confirming their polycrystalline nature. Raman maps confirmed that the films were composed of nc-Si domains embedded in the a-Si matrix. The UV-Vis-NIR transmission spectra of the Si films were used to estimate the absorption coefficient (α), the refractive index and the thickness of the films. The band gap energy was found to vary non-monotonically from 1.35 to 1.56 eV as a function of T_s. The nonlinear absorption coefficient (β) and nonlinear refraction coefficient (n₂) of all the films were estimated using a modified Z-scan technique under cw He–Ne laser irradiation. The open aperture Z-scan of the thin films indicated strong reverse saturation absorption and the value of β for the nc-Si films was observed to be ∼10 cm W⁻¹. The closed aperture Z-scan curves confirmed the presence of self-focusing properties corresponding to positive nonlinear refraction. The value of n₂ for the Si films was observed to be ∼10⁻⁴ cm² W⁻¹. A large third-order nonlinear optical susceptibility of the order of 10⁻¹ esu was observed in these PLD nc-Si films, which is 10⁹ times higher as compared to that of bulk Si. The nc-Si thin films also exhibited optical limiting properties with optical limiting thresholds increasing with increasing values of β.