Meanwhile, the localized defect states are also proposed to SIS3 research buy affect the nonlinear optical properties of nc-Si films. Ito et al. found that the nonlinear refractive index did not decrease monotonously with the size of nc-Si, and they believed that www.selleckchem.com/products/XL184.html both the quantized electronic states and defect states contributed to the large nonlinear refractive index [12]. In our present work, we systematically studied the nonlinear optical properties of Si/SiO2

multilayers during the transition process from amorphous phase to nanocrystalline Si state. We found tunable nonlinear optical behaviors, reverse saturation absorption in the amorphous-phase dominant samples, and saturation absorption in the nanocrystalline-phase dominant ones, under femtosecond laser excitation. The nonlinear refraction was also simultaneously changed. We proposed that the interface states of nc-Si play the important role in the changing of nonlinear optical behaviors. Methods The nc-Si/SiO2 multilayer samples with 9.5 periods studied here were obtained by thermally annealing amorphous

Si/SiO2 stacked structure prepared in conventional plasma-enhanced chemical vapor deposition (PECVD) system. During the deposition process, the substrate temperature and radio frequency power were kept at 250°C and 50 W, Selleckchem PR 171 respectively. The details of preparation condition can be found elsewhere [13]. As-deposited samples were dehydrogenated at 450°C for 1 h and subsequently annealed in pure N2 ambient to precipitate nc-Si at various temperatures (800°C to 1,000°C). Here after, we denoted the as-deposited sample, 800°C, 900°C, and 1,000°C annealed sample as samples A, B, C, and D, respectively. The microstructures of nc-Si/SiO2 multilayers were characterized by cross-sectional transmission electron microscopy (X-TEM) and Raman scattering

spectroscopy. Figure 1 is the X-TEM image of sample D, which is clearly shown that the periodic structures are kept after annealing and nc-Si dots are formed with the size of 4 nm (as shown in the inset of Figure 1). Optical absorption spectra were measured in a spectral range of 300 to 1,000 nm using Shimadzu UV-3600 spectrophotometer (Shimadzu http://www.selleck.co.jp/products/Adriamycin.html Corp., Kyoto, Japan), and the optical bandgap was deduced according to Tauc plots. Room-temperature photoluminescence (PL) was measured under the excitation of He-Cd laser (325 nm). Figure 1 X-TEM micrograph of sample D after annealing at 1,000°C. The inset is the high-resolution TEM image, in which the formed nc-Si dots can be clearly identified. The Z-scan technique [14] was applied to measure the nonlinear optical coefficients of nc-Si/SiO2 multilayers. In this experiment, the excitation laser was a Ti-sapphire laser with 50-fs pulse duration at 800 nm, the repetition rate was 1 kHz.