Gold films are widely used for different applications. We present the results of third- and high-order nonlinear optical studies of the thin films fabricated from Au nanoparticle solutions by spin-coating methods. These nanoparticles were synthesized by laser ablation of bulk gold in pure water using 200 ps, 800 nm pulses. The highest values of the nonlinear absorption coefficient (9 × 10−6 cm W−1), nonlinear refractive index (3 × 10−11 cm2 W−1), and saturation intensity (1.3 × 1010 W cm−2) were achieved using 35 fs, 400 nm pulses. We also determined the relaxation time constants for transient absorption (220 fs and 1.6 ps) at 400 nm. The high-order harmonic generation was studied during propagation of 35 fs, 800 nm pulses through the plasma during the ablation of gold nanoparticle film and bulk gold. The highest harmonic cutoff (29th order) was observed in the plasma containing gold nanoparticles.
The polymerization of acrylonitrile was studied in aqueous sulfuric acid with the redox initiator system, ceric ammonium sulfate/glycerol at 15-25 "C. The initiation was effected by the ceric ion and the radical produced from the reaction of Ce4+ with glycerol. At lower concentrations of Ce4+, the rate of monomer disappearance was proportional to [MI',', [GI0,', and [Ce4+]0s5, and the rate of ceric ion disappearance was directly proportional to [Ce4+] and [GI. The results were explained by a kinetic scheme involving the oxidation of substrate by Ce4+ to give the primary radical and further reaction of the latter with Ce4+ to give the final product, and initiation of polymerization both by Ce4+ and the primary radical. The initiation by Ce4+ was found to be negligible as compared to that by the primary radical. The termination step is postulated to be mutual at lower concentrations of Ce4+ and linear at higher concentrations of Ce4+. The induction period and the rate of polymerization was compared with those of other polyol systems, such as sorbitol and mannitol.
A novel tunable whispering gallery modes (WGMs) resonator based on a nematic liquid crystal (LC)-filled capillary and magnetic nanoparticles (MNPs)-coated tapered fiber has been proposed and experimentally demonstrated. Thermo-optic tuning of the WGM resonances has been demonstrated by varying optical pump laser power injected into the MNPs-coated fiber half-taper inside the capillary. The tuning mechanism relies on the change of the effective refractive index (RI) of the nematic LC, caused by the photo-thermal effect of MNPs on the surface of the fiber half-taper inducing a temperature change inside the capillary. Tuning of the WGM resonances with sensitivities of 101.5 ± 3.5 pm/mW and 146.5 ± 3.5 pm/mW and tuning ranges of 1.96 nm and 3.28 nm respectively for the two types of liquid crystals (MLC-7012, MDA-05-2782) has been demonstrated. In addition, the relationship between the optical power of the pump laser and the local temperature of the nematic LC was investigated and the heating rate is estimated as 1.49 °C/mW. The proposed thermo-optic tuning scheme has many potential applications in tunable photonic devices and sensors.
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