Probe Beam Deflection Measurements Research Articles

Spatial confinement of laser-induced plasma by laser-induced and obstacle-reflected shock wave and its effect on optical emission of laser-induced plasma

The laser-induced plasma (LIP) and the shock wave generated by pulsed laser ablation of a graphite target in air and reflected by a flat obstacle were examined by optical emission spectroscopy and probe beam deflection measurements. The interaction between the LIP and the shock wave and its effects on the expansion of the LIP as well as on the optical emission of carbon atoms were studied. The carbon atomic emission can be enhanced or reduced in the situation with a flat obstacle standing in the propagation path of the shock wave. The enhancement or reduction of the carbon atomic emission has a close connection with the shock wave generated by graphite ablation and reflected by the obstacle. The reflected shock wave confines the expansion of the LIP and impedes the travelling of the plasma species. The enhancement was observed at the detection position close to the target and with a short block-target distance. The shock wave thus reflected encounters the luminous LIP at its early expanding stage and confines the expansion of the LIP, resulting in the enhancement in the optical emission of carbon atoms. But at the detection position far from the target and with a longer block-target distance, a reduction in the optical emission due to spatial confinement was observed. The possible mechanisms responsible for the effects of spatial confinement on the optical emission were discussed.

A comparative study of the enhancement of molecular emission in a spatially confined plume through optical emission spectroscopy and probe beam deflection measurements

The spatial confinement effects of shock wave on the expansion of a carbon plume induced by pulsed laser ablation of graphite in air and the enhancement of the plume emission were studied by optical emission spectroscopy and probe beam deflection measurements. A metal disk was set in the way of the ablation-generated shock wave to block and reflect the supersonically propagating shock wave. The reflected shock wave propagated backwards and confined the expanding plume. The optical emission of CN molecules was enhanced in contrast to the case without the block disk and the emission enhancement was dependent on the position of the disk. Based on the results of time-integrated and -resolved optical emission spectroscopy, and the time- and space-resolved probe beam deflection measurements, the processes occurring in the plume were discussed and the mechanisms responsible for the enhancement of molecular emission in the spatially confined plume were investigated.

Dynamics of Ion Exchange between Self-assembled Redox Polyelectrolyte Multilayer Modified Electrode and Liquid Electrolyte

A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.

Photothermal characterization of laser-active Yb: Glasses

To design pulsed laser amplifiers detailed knowledge about beam distortions in the amplifying medium is needed. Time resolved probe beam deflection measurements from different laser glasses have been performed to study pump beam induced optical aberrations. By means of a wavelength tuned probe beam thermally induced variations of the optical path and beam narrowing effects due to the radial gain distribution could be distinguished. The experimental data were theoretically analyzed. Different behavior was found for two types of Yb:glass despite their similar nominative properties.

TEA laser ablation of Upilex polyimide

We report an experimental investigation of the absorption and ablation characteristics of the high-temperature polymer Upilex polyimide, using a line-tunable TEA laser. It is demonstrated that excellent quality through-holes (vias) can be produced in thin films of Upilex when the laser is tuned to a strongly absorbed wavelength as a result of thermal decomposition produced by laser heating of the bulk polymer. For weakly absorbed wavelengths, transmission and probe beam deflection measurements reveal an incubation effect in the ablation which is thought to be related to particulate contamination in the film.

Low noise position sensitive detector for optical probe beam deflection measurements

We report the design of an optical position sensor that uses two discrete photodiodes electrically connected in parallel, with opposing polarities. A lens provides optical gain and restricts the acceptance angle of the detector. The response of the device to displacements of an optical spot is similar to that of a conventional bicell type position sensitive detector. However, the discrete photodiode design enables simpler electronic amplification with inherently less electrical noise than the bicell. Measurements by the sensor of the pointing noise of a focused helium–neon laser as a function of frequency demonstrate high sensitivity and suitability for optical probe beam deflection experiments.

Anomalous CW thermal lens behaviour of uranyl ion in aqueous carboxylic acid media

A study of the thermal lens behaviour of the aqueous uranyl (UO 2+ 2) ion has been carried out using a two beam CW mode mismatched thermal lens spectrometer. Uranyl ion in 1 M HClO 4 exhibits normal thermal lens behaviour, however, with addition of carboxylic acids as part of the solute system anomalous behaviour is observed. The thermal lens slowly decays ( t 1 2 ≈ 3 s) to equilibrium and when the pump beam radiation is removed the thermal lens signal is reversed. Probe beam deflection measurements in the beam interaction volume indicate that this is caused by a refractive index gradient reversal. A possible mechanism for this phenomenon is proposed based on endothermic energy transfer and subsequent photodecarboxylation of the carboxylic acid (acetic acid) which removes energy from the centre of the beam interaction volume and this reverses the temperature gradient and hence refractive index gradient.

Energy relaxations in CS2 and in NO2–N2O4 vapors following pulsed laser excitation at 337.1 nm by probe beam deflection measurements

The pulsed photothermal generation and probe-beam deflection technique was applied to monitor nonradiative relaxation and transport processes in CS2 and in NO2–N2O4 vapors following nitrogen laser excitation at 337.1 nm. A theoretical model is developed to describe the signal shape, and to derive the thermal diffusivity and energy relaxation time. The experiments were performed over a vapor pressure range of typically 1–100 Torr with a time resolution of 1 μs. In CS2 vapor of pressure above 10 Torr at room temperature, a slow heat release component is observed attributed to the photochemical formation of (CS)n particulates (‘‘laser-snow’’). In NO2–N2O4 vapor, the experimental data provide the thermal diffusivity values in the pressure range of 1–100 Torr; these measurements support a ‘‘frozen equilibrium’’ model and indicate that any contribution due to the diffusional transport of chemical enthalpy for this system is small in this pressure range at room temperature. These experiments demonstrate that the photothermal probe-beam deflection technique can provide useful information on heat-release rates, for example, due to particulate formation, as well as measurement of thermal diffusivity, for example, in hostile and dissociative gas systems.