Hee K. Park

IR-MALDI of low molecular weight compounds using a free electron laser

Initial experiments on infrared matrix-assisted laser desorption/ionization mass spectrometry (IR-MALDI) using a free electron laser in the analysis of low-molecular-weight compounds are reported. Mass spectra from samples of ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and a phosphate salt were obtained. Low molecular weight (LMW) compounds are of similar mass as the molecular weight of the matrix compound and interference between matrix and analyte ions can degrade MALDI sensitivity. Under optimal conditions, however, we find little interference and that often the analyte signal exceeds the matrix signal, a result also observed in UV-MALDI of these compounds. In the IR-MALDI of EDTA near 3.0 μm the FEL results are similar to those obtained using a fixed frequency 2.94 μm Er:YAG laser of 200 ns pulse duration. However, the tunable FEL laser can selectively excite particular vibrational modes of the matrix (e.g., an OH or CO stretch) or of residual water contained within the crystalline MALDI sample. This capability was used to explore possible vaporization and ionization mechanisms of IR-MALDI for EDTA.

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.

Processing of polymer and organic materials by tunable, ultrafast mid-infrared lasers

Recent market developments in electronic and opto-electronic devices based on small organic molecules and polymers is driving the search for a variety of new processing tools for thin-film deposition. In this paper, we describe a new vacuum-phase thin-film deposition technique — resonant infrared pulsed laser deposition — that is a true vapor phase deposition technique for thermally labile or photochemically sensitive organics and polymers. Several examples of thin-film deposition, including structural, insulating and conducting polymers, are presented. Specifications for a commercial thin-film deposition based on this technique are also discussed.Recent market developments in electronic and opto-electronic devices based on small organic molecules and polymers is driving the search for a variety of new processing tools for thin-film deposition. In this paper, we describe a new vacuum-phase thin-film deposition technique — resonant infrared pulsed laser deposition — that is a true vapor phase deposition technique for thermally labile or photochemically sensitive organics and polymers. Several examples of thin-film deposition, including structural, insulating and conducting polymers, are presented. Specifications for a commercial thin-film deposition based on this technique are also discussed.

Fabrication of a Multilayer Polymer Light-Emitting Diode by Resonant Infrared Laser Ablation

Multi-layer polymer light-emitting diodes have been fabricated in vacuum by infrared laser ablation of conducting and light-emitting polymers. The spectral output of the devices resembles that of similar spin-coated devices, but shows some fluence dependence.

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.