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

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 ( n 2 ) 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 n 2 for the Si films was observed to be ∼10 −4 cm 2 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 10 9 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 . PLD nc-Si films exhibited deposition temperature-dependent large RSA and positive NLR ( χ (3) ∼ 10 −1 esu) and optical limiting behavior.