Xianzhong Zeng

Plasma diagnostics during laser ablation in a cavity

Abstract The formation of a laser-induced plasma in a cavity and the effects of a cavity on the ablation process were investigated. Cavities were fabricated in fused silica with equal depths and variable diameters to provide aspect ratios (depth/diameter) of 1, 3 and 6. The temperature and electron number density of the pulsed laser-induced plasma in the cavities were determined from spectroscopic measurements. Reflection and confinement effects by the cavity walls and plasma shielding were discussed to explain increased temperature and electron number density with increasing cavity aspect ratio. The temporal variations of the plasma temperature and electron number density sharply decreased inside the cavity. An adiabatic expansion model was not suitable for the laser-induced plasma in the cavity because plasma wall interactions were not included. Properties of laser-induced plasmas in the cavities and on a flat surface were compared.

Laser–plasma interactions in fused silica cavities

The effect of laser energy on formation of a plasma inside a cavity was investigated. The temperature and electron number density of laser-induced plasmas in a fused silica cavity were determined using spectroscopic methods, and compared with laser ablation on a flat surface. Plasma temperature and electron number density during laser ablation in a cavity with aspect ratio of 4 increased faster with irradiance after the laser irradiance reached a threshold of 5 GW/cm2. The threshold irradiance of particulate ejection was lower for laser ablation in a cavity compared with on a flat surface; the greater the cavity aspect ratio, the lower the threshold irradiance. The ionization of silicon becomes saturated and the crater depths were increased approximately by 1 order of magnitude after the irradiance reached the threshold. Phase explosion was discussed to explain the large change of both plasma characteristics and mass removal when irradiance increased beyond a threshold value. Self-focusing of the laser be...

Laser-induced shockwave propagation from ablation in a cavity

The propagation of laser-induced shockwaves from ablation inside of cavities was determined from time-resolved shadowgraph images. The temperature and electron number density of the laser-induced plasma was determined from spectroscopic measurements. These properties were compared to those for laser ablation on the flat surface under the same energy and background gas condition. A theoretical model was proposed to determine the amount of energy and vaporized mass stored in the vapor plume based on these measurements.

Energy deposition and shock wave propagation during pulsed laser ablation in fused silica cavities

Propagation of the shock wave generated during pulsed laser ablation in cavities was measured using laser shadowgraph imaging and compared with laser ablation on a flat surface. The cavities were fabricated in fused silica with the same diameter and variable depths to provide aspect ratios (depth/diameter) of 3 and 6. It was found that outside the cavity, after ~30 ns the radius of the expanding shock wave was proportional to t2/5, corresponding to a spherical blast wave as predicted by the Sedov–Taylor solution. The calculated pressures and temperatures of the shocked air outside of the cavities were higher than on the flat surface. The incident energy driving the shock wave outside the cavity was smaller than that on the flat surface; the greater the cavity aspect ratio, the smaller the energy supporting the shock wave. Plasma–wall interactions are discussed to explain the sharp decrease of the energy driving the shock wave outside the cavity.

Laser-induced plasmas in micromachined fused silica cavities

Cavity formation is a frequent result in many laser ablation applications. Although most theoretical investigations have been devoted to laser ablation on a flat surface, the development of a laser plasma inside a cavity is of both fundamental as well as practical significance. In this study, the temperature and electron number density of laser-induced plasmas in fused silica cavities were determined using spectroscopic methods. The effects of cavity aspect ratio on plasma properties were investigated. The temperature and electron number density of laser-induced plasma were measured to be much higher and to decrease faster for a plasma inside a cavity than on the flat surface. Cavity wall influences on the plasma expansion are discussed.

Laser-induced breakdown spectroscopy: flat surface vs. cavity structures

Cavity formation is a common phenomenon involved in solid-state analysis when repetitive laser pulses are applied to induce breakdown spectra. While previous LIBS investigations have been mostly focused on laser ablation on flat surfaces, the development of a laser-induced plasma inside cavity structures is of both fundamental and practical significance for quantitative chemical analysis using LIBS. In this paper, we attempt to answer the question, to what extent cavity formation would influence the temperature and electron density of laser-induced plasma. We found a significant effect of cavity aspect ratio on plasma characteristics, in particular, the measured high temperature and electron density of laser-induced plasma inside cavity structures.

Ultraviolet femtosecond and nanosecond laser ablation of silicon: Ablation efficiency and laser-induced plasma expansion

Femtosecond laser ablation of silicon in air was studied and compared with nanosecond laser ablation at ultraviolet wavelength (266 nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater. The temperature and electron number density of the pulsed laser-induced plasma were determined from spectroscopic measurements. The electron number density and temperature of fs-pulse plasmas decreased faster than ns-pulse plasmas due to different energy deposition mechanisms. Images of the laser-induced plasma were obtained with femtosecond time-resolved laser shadowgraph imaging. Plasma expansion in both the perpendicular and the lateral directions to the laser beam were compared for femtosecond and nanosecond laser ablation.

Influence of crater dimensions on laser induced plasma properties

When a pulsed, high-powered laser beam is focused onto the surface of a target (solid or liquid sample), a portion of the illuminated mass is ablated into vapor-phase constituents. In many cases, the energy and irradiance of the laser beam is significant to convert this vapor into a luminous plasma above the sample surface. Spectroscopic interrogation of the plasma is a powerful technology for direct multi-elemental chemical analysis; a technology termed laser-induced breakdown spectroscopy (LIBS). LIBS is attractive for the chemical analysis because any material can be ablated without sample preparation, data are measured in real time, and the plasma provides a complete chemical signature of the sample constituents.