No abstract

Phase explosion is a phase change process that occurs during short pulse laser ablation. Phase explosion is a result of homogeneous nucleation of vapor in the superheated melt and results in a rapid transition from a superheated melt to a mixture of vapor and liquid droplets that expand from the surface. The sudden phase transition results in rapid material removal, and if occurring in an ambient gas, causes a shock wave to propagate away from the surface. Measurements of this shock wave are commonly used with the Taylor-Sedov blast wave theory to estimate shock wave pressure and temperature. At low laser fluences the Mach number of the shock wave can be small, resulting in significant errors in pressure and temperature. The paper will demonstrate conditions for which the more general form of the Rankine-Hugoniot relations for thermo-fluid parameters simplifies to the Taylor-Sedov similarity solutions and when the Taylor-Sedov solutions are applicable. The results are compared to experimental shock wave data from the literature to explain why using the Taylor-Sedov blast wave solutions can result in large errors at low Mach numbers.

Phase explosion and Marangoni flow during laser micromachining of thin metal films are studied in this paper. The purpose of this study was to improve understanding of the time scales by which these processes occur. The study was based on a time-resolved reflection imaging method. The method used a nitrogenpumped dye laser to illuminate the surface of the films at a given time after the Nd:YAG laser heats the film. The dye laser irradiation reflected from the surface was then imaged by a CCD camera. The lasers were coupled by a digital pulse-delay generator, allowing the time delay between the two lasers to be controlled by the user. The effects of Marangoni flow and phase explosion can be seen on the starting and ending times of ablation. At all fluences in the study, holes were opened in the aluminum films and the hole formation process was completed in under 350 ns. Ablation of nickel films was very different however, with thin layers of the film surface removed at low fluences, a process which took on the order of microseconds to complete. At higher fluences the nickel films ruptured and the hold opening process was completed in less than 500 ns.

Laser micromachining of an aluminum film on a glass substrate is investigated using a time-resolved transmission imaging technique with nanosecond resolution. Micromachining is performed using a 7 ns pulse-width Nd:YAG laser operating at the 1064 nm wavelength for fluences ranging from 2.2 to 14.5 J/cm 2 . A nitrogen laser-pumped dye laser with a 3 ns pulse-width and 500 nm wavelength is used as a light source for visualizing the transient hole area. The dye laser is incident on the free surface and a CCD camera behind the sample captures the transmitted light. Images are taken from the back of the sample at various time delays with respect to the beginning of the ablation process, allowing the transient hole area to be measured. For low fluences, the hole opening process is delayed long after the laser pulse and there is significant scatter in the data due to weak driving forces for hole opening. However, for fluences at and above 3.5 J/cm 2 , the starting time of the process converges to a limiting minimum value of 12 ns, independent of laser fluence. At these fluences, the rate of hole opening is rapid, with the major portion of the holes opened within 25 ns. The second stage of the process is slower and lasts between 100 and 200 ns. The rapid hole opening process at high fluences can be attributed to recoil pressure from explosive phase change. Measurements of the transient shock wave position using the imaging apparatus in shadowgraph mode are used to estimate the pressure behind the shock wave. Recoil pressure estimates indicate pressure values over 90 atm at the highest fluence, which decays rapidly with time due to expansion of the ablation plume. The recoil pressure for all fluences above 3.1 J/cm 2 is higher than that required for recoil pressure driven flow due to the transition to explosive phase change above this fluence.

Nanosecond laser ablation is studied using a theoretical model combined with experimental data from laser ablation of metal films. The purpose of the research is to obtain the recoil pressure boundary condition resulting from explosive phase change. The ablation experiments are performed using a Nd:YAG laser of 1064 nm wavelength and 7 ns pulse width at full width half maximum. Three samples, 200 and 1000 nm aluminum films and 1000 nm nickel films, are used in the experiments. The transient shock wave positions are obtained by a time-resolved shadowgraph technique. A N2-laser pumped dye laser with 3 ns pulse width is used as an illumination source and is synchronized with the ablation laser to obtain the transient shock wave position with nanosecond resolution. The transient shock position is used in a model for finding the shock wave speed as well as the pressure, temperature, and velocity just behind the shock wave. A power law is used for fitting curves on the experimentally obtained shock wave position. Knowing the shock wave position, the normal shock equations are used to calculate the thermo-fluid properties behind the shock wave. The solutions are compared with the Taylor-Sedov solution for spherical shocks and the reason for the deviation is described. The thermo-fluid property results show similar trends for all tested samples. The results show that the Taylor-Sedov solution under-estimates the pressure behind the shock wave when compared to the normal shock results.