Darja Horvat

Optodynamic observation of double laser-induced breakdown at the water surface

A measurement technique based on the combination of beam deflection probe and photographic detection of optical emission is introduced for the study of laser-induced dielectric breakdown in the vicinity of the air?water interface. The plasma was photographed with a CCD camera and the shock wave generated by the breakdown in the air and inside the water was detected with a beam deflection probe as a function of the focus position. The comparative analysis of the photographic images and the characteristics of the simultaneously recorded ultrasonic signals shows that the amplitudes and the arrival times of the ultrasonic waves can reliably describe the characteristics of the breakdown and its position relative to the water surface. At certain focus positions the existence of double breakdown is evident.

Laser ultrasonics for bubbly flow detection

Laser ultrasonics was used to study flow of air bubbles in still water. An experimental system was developed utilizing a Nd:YAG laser to generate ultrasonic pulses in the MHz range and a He-Ne laser beam to detect echoes from the bubble-water interfaces. Several methods were applied to compare ultrasonic waveform parameters with air flow rates. The results show a correlation between the number and magnitude of amplitudes in the waveform and the bubble production air flow rate.

Using lasers to measure pollutants

In recent years, a large number of linear and nonlinear laser-based diagnostic techniques for detection of pollutions in different environments have been developed. Applications of laser spectroscopy constitute a vast field, which is difficult to cover comprehensively in a review. Due to that here are presented only a few spectroscopic methods, chosen to illustrate the power of applied laser spectroscopy in environmental pollution investigation. The paper also gives a brief presentation of main laser spectroscopy methods.

Green upconversion in Y2O3:Yb nanopowder

Abstract. Green emission lines, in addition to the blue and the red, were observed upon 980 nm excitation in yttrium oxide (Y2O3) nanopowder codoped with Yb3+ and Tm3+, synthesized by the chemical combustion method. Upconversion emission studies suggest that the number and characteristics of the green lines are influenced by the annealing temperature as well as by the Yb3+/Tm3+ concentration ratio, opening possibilities for new customized applications. The chromaticity properties of the upconversion spectra were quantified by the Commission Internationale de l’éclairage coordinate analysis.

Optoacoustic Measurement of Ultrasound Velocity in a Solidifying Polymer

The structure of a polymeric resin is changing in the course of the solidifying process. This is reflected in the change of acoustic properties of the material. A non-contact method using laser light for evaluation of the ultrasonic velocity is reported here. Polyester and vinyl ester resin samples were used. The ultrasonic wave was generated with a pulsed Nd: YAG laser and detected by the deflection of a He-Ne laser beam at two distances in the sample. From the arrival times of the pulses the ultrasound velocity was calculated and the duration of the reaction assessed.

Optical monitoring of the solidification process of polyester and vinyl resins

Abstract Ultrasonic waves were generated and detected by laser light in solidifying polyester and vinyl resins, allowing an evaluation of the extent of reaction by a non-contact method. Commercial polyester and vinyl resin samples were prepared following the standard procedure under room conditions. A liquid adjacent to the sample was irradiated by a pulsed Nd:YAG laser beam. The resulting ultrasonic wave was detected in the sample at two distances from the source by the deflection of an HeNe laser probe beam. The displacements of the probe beam were detected by a position-sensitive photodiode. Arrival times of the ultrasonic pulses at various times during the solidification were measured and the corresponding propagation velocities calculated. The method is aimed at monitoring the final part of the solidification, after the gelation has taken place, and will allow the investigation of the influence of various parameters of the course of the reaction and on the final properties of the material.