Tunable Nanosecond and Femtosecond Nonlinear Optical Properties of C–N–S-Doped TiO₂ Nanoparticles

Abstract

Third-order nonlinear optical (NLO) properties of undoped and C–N–S doped TiO 2 nanoparticles (NPs) were investigated using the Z-scan technique in the nanosecond (ns) and femtosecond (fs) regimes. Undoped TiO 2 NPs exhibited reverse saturable absorption (RSA) behavior with both ns and fs laser pulse excitation, whereas C–N–S-doped TiO 2 NPs demonstrated switch over from RSA to saturable absorption (RSA → SA) behavior with fs excitation. Our detailed NLO studies have clearly demonstrated the tunable nonlinear absorption behavior of C–N–S-doped TiO 2 NPs. Such versatile tunability is important for various practical applications. The interesting nonlinear absorption (NLA) properties of these NPs are attributed to the interplay between ground and excited state absorption of the free carriers. The results obtained with ns laser excitation demonstrated that the nonlinear absorption coefficient (β) and nonlinear refractive index ( n 2 ) for these materials varied with input irradiance, thus indicating the possibility of higher order nonlinearity (free–carrier effect) occurrence rather than third-order nonlinearity alone. C–N–S-doped TiO 2 nanoparticles exhibited large magnitude for β and n 2 with values of 2.1 × 10 –10 m/W and 15.4 × 10 –17 m 2 /W, respectively, with ns excitation. Further, the samples were tested for nonlinearity with continuous wave (cw) excitation as heat-induced refractive index changes were observed for the samples with cw laser excitation at 532 nm. The room temperature ferromagnetism (RTFM) was observed in both undoped and C–N–S doped TiO 2 NPs. Undoped rutile TiO 2 exhibited ∼6.9-fold enhancement in RTFM as compared to the anatase phase of C–N–S-doped TiO 2 NPs.