Showing papers by "Richard E. Russo published in 1996"

Invited paper Observation of plasma shielding by measuring transmitted and reflected laser pulse temporal profiles

Abstract: Mass ablation rate increases with laser power density following a power law dependence and a significant change occurs at 0.3 GW/cm $^2$ . A reflected laser temporal profile was measured from a brass sample. When the power density is greater than 0.3 GW/cm $^2$ +, the temporal profile changes. The transmitted laser-pulse temporal profile through a glass sample also was measured. When the power density is greater than 0.3 GW/cm $^2$ , the later part of laser pulse becomes truncated. The power density at which the laser temporal profile changes for each case is same as the power density that the mass ablation rate coefficient changes. The ablated mass can absorb incoming laser radiation through inverse Bremsstrahlung. The mass becomes thermally ionized and opaque to the incident radiation, preventing laser light from reaching the surface. A model based on thermal evaporation and inverse Bremsstrahlung absorption was developed. Calculations show that plasma shielding occurs at approximately 0.3 GW/cm $^2$ . The experiments and model suggest that the significant change observed in mass ablation rate coefficient is caused by plasma shielding.

Nanosecond-time-resolution thermal emission measurement during pulsed excimer-laser interaction with materials

Abstract: A nanosecond-time-resolution pyrometer has been developed for measuring the transient surface temperature of a solid material heated by pulsed excimer-laser irradiation. Fast germanium diodes are employed to capture the transient thermal emission signals in the wavelength range between 1.2 and 1.6 pm. The surface temperature is derived from the measured spectral thermal emission. The directional spectral emissivity is determined in situ by measuring the transient directional spectral reflectivity and applying Kirchhoffs law. The experimental results are in good agreement with numerical thermal modeling predictions. The pyrometric thermal emission measurement also yields the solid/liquid interface temperature during the pulsed excimer-laser-induced melting. The relation between the measured interface superheating temperature and the interface velocity reveals the melting kinetic relation during the high-power, short-pulse laser-induced phase-change processes.

Low noise position sensitive detector for optical probe beam deflection measurements

Abstract: 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.

Laser heating of a cavity versus a plane surface for metal targets utilizing photothermal deflection measurements

Abstract: The effects of a cylindrical cavity in a metal surface on the energy coupling of a laser beam with the solid were investigated by using a photothermal deflection technique. The photothermal deflection of a probe beam over the cavity was measured while the bottom of the cavity was heated with a Nd–YAG laser with a wavelength of 1064 nm. Cavities in three different materials and with two different aspect ratios were used for the experiment. Temperature distributions in the solid and the surrounding air were computed numerically and used to calculate photothermal deflections for cavity heating and for plane surface heating. Reflection of the heating laser beam inside the cavity increased the photothermal deflection amplitude significantly with larger increases for materials with larger thermal diffusivity. The computed photothermal deflections agreed more closely with the experimental results when reflection of the heating laser beam inside the cavity was included in the numerical model. The overall energy c...

Mechanical stress power measurements during high‐power laser ablation

Abstract: Laser‐induced stresses have been studied extensively to understand macroscopic phenomenon during high‐power laser ablation of solids. Recently, a norm of stress times the rate of change in stress, similar to mechanical stress power, was monitored acoustically in the target and ambient medium during high‐power laser‐material interactions, and compared with stress measurements. This study investigates the relationship between stress and the stress powerlike measurements (P*), and their dependence on laser energy, intensity, and spot size. The importance of different components of stress on the measurements is also considered. Results from ablation of aluminum targets by a 30 ns uv excimer laser are presented that show changes in P* with laser energy coupling, and the dependence of P* on laser intensity and stress components. Potential issues are raised for further study of stress power as a diagnostic tool of laser‐material interactions and as a fundamental mechanism of laser‐energy coupling.