David R. Ermer

Explosive vaporization in fused silica initiated by a tunable infrared laser

We present evidence for explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser (FEL) operating in the wavelength region from 2 to 10 μm. The unusual pulse structure of the FEL, which produces 1 ps micropulses at intervals of 350 ps in a macropulse lasting up to 4 μs, makes it possible to test separately the effects of intensity and fluence. We show that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light.

Time-resolved studies of electron-phonon relaxation in metals using a free-electron laser

Here we report recent experiments on ion emission from metals (Ti, Nb, Mg) using a free-electron laser that delivers a train of subpicosecond micropulses in the mid-infrared (IR). The laser intensities for the surface damage and ion emission thresholds are used as markers for the onset of energy transfer to the lattice for ablation. Numerical calculations based on the Anisimov two-temperature model are used to evaluate the electron temperature Te and the lattice temperature Ti. The results are discussed in terms of the Hertz–Knudsen equation for vaporization and a model based on laser-induced multiple electronic transitions.

Resonant infrared laser materials processing at high vibrational excitation density: applications and mechanisms

As laser micromachining is applied to ever smaller structures and more complex materials, the demand for greater control of the laser energy budget, in space and time, grows commensurately. Here we describe materials modification using picosecond resonant laser excitation in the mid-infrared spectral region to create spatially and temporally dense vibrational, rather than electronic, excitation. Examples include ablation of fused silica and machining of crystalline quartz; deposition of functionalized polymers on microstructures, and laser-directed transfer of proteins and nucleotides from a matrix of water ice. The experiments demonstrate that high spatial and temporal density of vibrational excitation can be achieved by ultrafast resonant infrared excitation of selected vibrational modes of these materials. In some cases, resonant infrared materials modification is far more successful than techniques based on ultraviolet excimer lasers. The laser used for most of the experiments was a tunable, high pulse-repetition frequency free-electron laser. However, a comparison of polymer deposition using a conventional nanosecond laser at a wavelength of 2.94 μm shows that the possibility exists for transferring the concept to conventional table-top devices. Mechanistic considerations nevertheless suggest that utlrashort pulses are likely to be more useful than longer pulses for many applications. A figure of merit is proposed for self-consistent comparisons of processing efficiency among different lasers.

Charged-particle emission from dielectric materials initiated by a tunable picosecond mid-infrared laser

In the ultraviolet, visible and near-infrared, single and multiphoton electronic transitions can explain the production and emission of charged atoms, molecules and photoelectrons during laser ablation and desorption. However, the process of charge transfer and ionization during ablation of dielectrics in the mid-infrared is not well understood. Even though significant electronic excitation is unlikely, copious emission of charged particles, e.g. atoms, molecules and electrons, is observed. No evidence of laser plume interactions is observed and inverse Bremsstrahlung (IB) is ruled out as a primary ionization/charge transfer mechanism. By irradiating with an ultrashort pulse-width mid-infrared laser tuned to a vibrational resonance it is possible to generate a high vibrational excitation density in dielectric materials. This high excitation density creates a non- equilibrium state of matter that exists until the deposited energy fully thermalizes. In this paper we report measurements of the kinetic energy of ions and electrons from CaCO3, NaNO3 and dihydroxybenzoic (DHB) acid that are highly non- thermal. This non-thermal energy distribution is evidence that the primary production of charged species occurs while the material is in a non-equilibrium state. The fact that it occurs in quite different materials, and without some of the characteristic signatures of electronically induced desorption and ionization, points toward a new mechanism.

Laser Mass Spectrometry at High Vibrational Excitation Density

Abstract We describe a novel approach to infrared matrix-assisted laser desorption-ionization mass spectrometry using a tunable, picosecond pulse laser to selectively excite specific modes of a solid, thereby creating a high local density of vibrational quanta. The concept is based on recent results from our experiments employing a free-electron laser to explore ‘matrix-less’ mass spectrometry in which an infrared chromophore intrinsic to the sample, rather than an exogenous matrix, is excited by the laser. Examples from both environmental mass spectrometry and a proteomics-driven research project are presented, showing how the principle of selective vibrational excitation can be used to make possible novel and useful ion generation protocols. We conclude with an analysis of possible mechanisms for the phenomena of infrared desorption, ablation and ionization using very short laser pulses. Prospects for achieving similar results with more conventional laser sources are discussed.

Phase explosion and ablation in fused silica initiated by an ultrashort-pulse tunable mid-infrared free-electron laser

Ultrashort laser pulses interacting with brittle dielectrics in the mid-infrared region of the spectrum produce a number of novel effects which are potentially useful in materials processing and analysis. These include the texturing of the surface, the generation of hydrodynamic instabilities, and a surprisingly efficient and gentle ablation behavior. Nevertheless, the mechanism of infrared laser ablation remains somewhat mysterious. Here we present evidence for a mechanism of explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser operating in the wavelength region from 2 - 10 micrometer. The unusual pulse structure of the free-electron laser -- which produces 1-ps micropulses at intervals of 350 ps in a macropulse lasting up to 4 microseconds -- makes it possible to test separately the effects of intensity and fluence. We show in particular that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light.

Thermodynamic and quantum statistic kinetic studies of electron-phonon relaxation by ablation of metals

Electron-phonon relaxation in metals following ultrashort- pulse optical excitation has been studied both in thin films and bulk metals by a number of investigators. Here we report recent experiments on ion emission from several different metals using a free-electron laser which delivers a train of subpicosecond micropulses in the mid-IR. The threshold intensities for the damage threshold together with the ion emission are used as markers for the onset of energy transfer to the lattice for ablation. Numerical calculations based on the Anisimov two-temperature model are used to evaluate the electron temperature Te and the lattice temperature Ti from the incident fluence at the damage threshold and the ion-emission. The result will be discussed in terms of the Hertz-Knudsen equation for vaporization and of a model based on laser induced multiple electronic transitions.

Vibrational Excitation and Relaxation Processes in Insulators Initiated by Ultrashort Mid-Infrared Laser Pulses

Ultrashort-pulse lasers are increasingly being used for laser-induced surface modification, texturing and marking of insulators. Ultrashort pulses interacting with insulators in the vibrational IR produce a number of novel effects of potential utility in materials processing and analysis applications, including the creation of microbumps, microdimples, generation of hydrodynamic instabilities, and creation of smooth ablation craters. This paper describes recent results in the study of ultrashort-pulse laser interactions with surfaces when the irradiation is in the 2- 10 micrometers range. The laser source was a tunable, free- electron laser with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 4 microsecond(s) , with an average power of up to 3W. This unusual pulse structure makes possible novel test of the effects of resonant vibrational excitation, controlling the ratio of absorption depth to thermal diffusion length, and desorption and ionization by resonant excitation. The mechanisms underlying these effects, including vibrational excitation and relaxation dynamics, as well as their implications for materials-modification strategies, are discussed with reference to recent experimental examples.

Wavelength-dependent modification of insulator surfaces by a picosecond infrared free-electron laser

Ultrashort-pulse lasers are at an increasing rate being used for laser-induced surface modification of insulators, including ablation. Ti:sapphire chirped-pulse amplifier systems, with fundamental wavelengths in the near infrared, can produce efficient ablation and other desirable surface modifications with little collateral damage because the laser energy is deposited on a time scale much shorter than thermal diffusion times. Little is known, however, about how ultrashort pulses interact with insulators at wavelengths in the vibrational infrared. This paper describes surface modifications achieved by picosecond laser irradiation in the 2 - 10 micrometer range. The laser source was a tunable, free- electron laser (FEL) with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 4 microseconds, with an average power of up to 3 W. This unusual pulse structure makes possible novel tests of the dependence on fluence and intensity, as well as the effects of resonant vibrational excitation. As model materials systems, we studied calcium carbonate, its isoelectronic cousin sodium nitrate, and fused silica. Particularly intriguing are surface modifications achieved by tuning the laser into vibrational resonances of the target materials, or by tailoring the energy content of the pulse. The mechanisms underlying these effects, and their implications for materials-modification strategies, are discussed.

Surface Modifications in Transparent Dielectrics Induced by a Mid-Infrared Free-Electron Laser

Irradiation of wide-bandgap dielectrics using tunable picosecond infrared pulses at high repetition rate is shown to induce unusually efficient ablation, phase transformations and the evolution of a Kelvin-Helmholtz hydrodynamic instability.