Dynamics of excimer laser‐ablated aluminum neutral atom plume measured by dye laser resonance absorption photography

We report the first dye laser resonance absorption photographs of a single species of aluminum ground‐state neutral atoms in the plume ablated from solid aluminum by KrF excimer laser radiation. Aluminum ground‐state neutral atoms were diagnosed by illuminating the ablated plume with a dye laser tuned to the 32P1/2–42S1/2 transition at 394.4 nm. Measurements have been performed in vacuum as well as in argon and air environments. Streaming velocities measured for neutral aluminum atoms in vacuum ranged from 0.5×106 cm/s at low excimer laser fluences of 1–2 J/cm2 to 3.4×106 cm/s at high fluences of 7 J/cm2. Dye laser resonance absorption photography measurements of ablated aluminum in argon and air showed slower expansion at 50 and 200 Torr, while observations at 760 Torr indicate turbulent mixing of aluminum neutrals near the surface. Differences between data in argon and air may be due to oxidation of neutral aluminum atoms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70489/2/APPLAB-58-15-1597-1.pd

Laser beam deflection as a probe of laser ablation of materials

Helium‐neon laser beam deflection is used to study excimer laser ablation of polymers and a YBa2 Cu3 O7−x superconductor. Density gradients above pulsed laser heated or ablated samples deflect the He‐Ne laser beam and this is measured using a position sensitive detector. The technique permits the determination of the laser fluence threshold for ablation both in a vacuum and in air, and the velocity of the ablation products in a vacuum. A model of the thermal deflection at low fluence was developed which enables measurements of thermal diffusivity of the air.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70263/2/APPLAB-55-23-2435-1.pd

Photoacoustic and photothermal beam deflection as a probe of laser ablation of materials

Photoacoustic and photothermal laser‐beam deflection were applied as diagnostics of the pulsed ultraviolet (UV) laser ablation of a polymer polyethyleneterephthalate. Here, a continuous‐wave (cw) laser beam is passed parallel to the sample, but displaced from it by a few hundred micrometers. A density gradient caused by the pulsed UV laser heating or ablation of the sample deflects the cw laser beam. This deflection is measured directly using a position‐sensitive detector. A quantitative model of the photothermal deflection at low fluence was developed which fits the data very well. This enabled a new method of measuring the thermal diffusivity of the fluid in contact with the sample. Distortion of the photothermal and photoacoustic signal as the excimer fluence is raised through the ablation threshold allowed the determination of the threshold. Also, the velocity of the ablation products was measured through a time‐of‐flight analysis and found to be dependent on the laser fluence used, the nature of the gas above the sample, and the distance above the sample at which the velocity is measured. The beam deflection in a vacuum is used to measure the ablation product velocity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71241/2/JAPIAU-69-3-1330-1.pd

Schlieren and dye laser resonance absorption photographic investigations of KrF excimer laser‐ablated atoms and molecules from polyimide, polyethyleneterephthalate, and aluminum

Hydrodynamic phenomena from KrF excimer laser ablation (10−3–20 J/cm2) of polyimide, polyethyleneterephthalate, and aluminum are diagnosed by schlieren photography, shadowgraphy, and dye laser resonance absorption photography (DLRAP). Experiments were performed both in vacuum and gaseous environments (10−5–760 Torr air, nitrogen, and argon). In vacuum, ablation plumes are observed to expand like a reflected rarefaction wave. As the background gas pressure is increased, shock waves and reduced‐density ablation plumes become visible. Below 10 Torr, the ablation plume follows closely behind the shock wave. Between 20 and 100 Torr, the plume recedes behind the shock wave. Below 10 Torr and above about 200 Torr, both the plume and the shock expand with the same temporal power law dependence. Agreement is found between these power law dependences and those predicted by ideal blast wave theory. The DLRAP diagnostic clearly shows that the ablated material (CN molecule from polyimide and ground state neutral aluminum atoms from laser‐ablated aluminum) resides in the ablation plume. CN molecules are detected in both argon and air environments proving that CN is generated as an ablation product and not by reaction with the background gas. As the background gas pressure and the time after ablation is increased, the film darkening due to the laser‐ablated material begins to fade leaving only the nonresonant shadowgraphy component of the plume. The plume dynamics observed by DLRAP are discussed in terms of gas dynamics, plume chemical kinetics, material diffusion in the plume, and cluster/particulate formation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70260/2/JAPIAU-72-5-1696-1.pd

Laser‐beam deflection measurements and modeling of pulsed laser ablation rate and near‐surface plume densities in vacuum

Quantitative measurements of ablated material from the surface of polyethyleneterephthalate (PET) by 248‐nm excimer laser fluences up to 10 J/cm2 are performed by HeNe laser‐beam deflection in vacuum and by photoacoustic depth profiling in air. HeNe laser‐beam deflection measures the density of gas phase material present in the ablation plume. Photoacoustic depth profiling is a nonintrusive diagnostic that directly measures the etch depths from laser ablation. A hydrodynamic model consisting of a centered rarefaction wave that reflects off the PET surface is shown to describe the laser deflection signals. From these measurements an estimate of the initial temperature of the ablated species is found.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71054/2/JAPIAU-70-2-587-1.pd

Copper vapor laser machining of polyimide and polymethylmethacrylate in atmospheric pressure air

A repetitively pulsed copper vapor laser (510 and 578 nm) is used to machine an opaque polymer (polyimide‐Vespel) and a transparent polymer (polymethylmethacrylate‐Lucite). Lucite is machinable by coating the surface with an ink which is semi‐opaque to the green and yellow laser light. The repetition rate of the laser was 10 kHz with approximately 0.35 mJ/pulse and 3.5 W average power at the copper vapor laser wavelengths for a pulse width of 40 ns. The copper vapor laser thermally loads the target, generating thermal waves and sound waves in the gas which are investigated using HeNe laser beam deflection. The gas adjacent to the target is heated to steady state on the order of 100–400 s. Above the etching threshold, at approximately 10 mJ/cm2/pulse, the target is rapidly machined: 2‐mm‐diam, 2‐mm‐deep holes are drilled in 300 s in Vespel. At higher fluences of 100–150 mJ/cm2/pulse in 760 Torr of air it takes 180 s to bore through a 2‐mm‐thick disk of Vespel. The machined surfaces of the two polymers are very different. Machined Vespel samples are charred and cratered, whereas the Lucite samples show evidence of melting with little charring. The machining of polymers by visible‐light copper vapor lasers is being compared to UV photoablation by KrF excimer laser light in order to study thermal versus nonthermal etching mechanisms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71173/2/JAPIAU-72-7-3080-1.pd

Deflection of carbon dioxide laser and helium‐neon laser beams in a long‐pulse relativistic electron beam diode

Deflection of carbon dioxide and helium‐neon laser beams has been used to measure plasma and neutral density gradients during the operating mode and after the shorting time of a long‐pulse field‐emission electron beam diode. Plasma density gradients of (1014–1015) cm−4 were observed throughout the diode during the final microsecond of the 2–3 μs electron beam pulse. The neutral density gradient was less than 1×1018 cm−4 during the electron beam pulse. Upon diode shorting, neutral density gradients increased to (1018–1019) cm−4 over ∼1 μs, and decayed over many microseconds. Plasma density gradients of ∼1015 cm−4 were also observed after shorting. These experiments demonstrate the value of carbon‐dioxide laser and helium‐neon laser deflection for diagnosing plasma and neutral particles in long‐pulse electron beam diodes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70220/2/RSINAK-62-7-1776-1.pd