Nonlinear Optical Properties of Zinc Oxide Nanoparticle Colloids Prepared by Pulsed Laser Ablation in Distilled Water.

The nonlinear optical properties of zinc oxide nanoparticles (ZnONPs) in distilled water were measured using a femtosecond laser and the Z-scan technique. The ZnONPs colloids were created by the ablation of zinc bulk in distilled water with a 532 nm Nd: YAG laser. Transmission electron microscopy, an ultraviolet-visible spectrophotometer, and atomic absorption spectrophotometry were used to determine the size, shape, absorption spectra, and concentration of the ZnONPs colloids. The nonlinear absorption coefficient and nonlinear refractive index were measured at different excitation wavelengths and intensities. The nonlinear absorption coefficient of the ZnONPs colloids was found to be positive, caused by reverse saturable absorption, whereas the nonlinear refractive index was found to be negative due to self-defocusing in the ZnONPs. Both laser parameters, such as excitation wavelength and input intensity, and nanoparticle features, such as concentration and size, were found to influence the nonlinear optical properties of the ZnONPs.

Design of an optical trap for storing femtosecond laser pulses.

An optical trap for storing femtosecond laser pulses to enhance the interaction effectiveness with optically thin targets is being proposed and investigated. Presently, we studied the trapping of 10-200 fs laser pulses of wavelength 800 nm, 1 µJ energy per pulse, and 10 Hz repetition rate. To compensate the optical losses in the trap, a Ti: Sapphire crystal as an amplifying medium is being considered, which should be synchronously pumped by the second harmonic of the Nd: YAG laser. Due to the propagation of the short pulses through optical trap components, group velocity dispersion introduces a significant broadening in pulse duration. To compensate for this broadening, a chirped mirror with suitable parameters is being proposed. An increase of the average power of the laser pulse in the optical trap that includes an amplifying medium (Ti: Sapphire crystal) by a factor of 805 compared to a single passage of the laser pulse was derived. It should be possible to store the laser pulse in the optical trap for >4 µs with constant power and with a repetition rate of up to 250 MHz.

A linear optical trap with active medium for experiments with high power laser pulses.

A linear optical trap for circulating high power laser pulses and tuning these pulses to high repetition frequency of several tens of MHz has been developed. A ns excimer pumped dye laser pulse has been injected with help of a Wollaston prism and a synchronized Pockels cell into an optical trap formed by two highly reflecting mirrors in a linear configuration. The test was done at λ = 580 nm, but the optical trap can be used without limitations in a broad band of optical wavelengths (400-700 nm). Power considerations give an increase of the efficiency of the optical trap of about 7 times compared to single passage of the laser pulse through the experimental section. The time structure of the trapped laser pulses can be controlled by changing the distance between the two high reflecting mirrors. The efficiency of the optical trap strongly depends upon optical losses. To compensate the optical losses, an amplifying cell was introduced, and the efficiency was about 60 times higher than that by single passage of the laser pulse through the experimental section.