Kenneth E. Schriver

Effects of the absorption coefficient on resonant infrared laser ablation of poly(ethylene glycol)

We describe experiments on resonant infrared laser ablation of poly(ethylene glycol) (PEG) at two different resonant excitation wavelengths and for different molecular weights of PEG. The two resonant wavelengths correspond to different stretching vibrations of the polymer and have absorption coefficients that differ by roughly an order of magnitude. Ablation via excitation of the O–H terminal group stretching mode at 2.94 μm, the weaker of the two absorptions, is delayed in time by several microseconds with respect to ablation at 3.47 μm, the more strongly absorbing C–H stretching mode of the polymer. Time-resolved plume shadowgraphs along with ablation rate measurements for the two modes reveal that the absorption coefficient strongly affects the physical characteristics of the ejecta and plume, as well as the time scale for material removal. Temperature-rise calculations demonstrate that phase explosion is likely the operative mechanism in ablation at the C–H mode, while normal boiling may play a role ...

Anti-reflective polymer-nanocomposite coatings fabricated by RIR-MAPLE

There is increasing demand for functional polymeric optical coatings for plastic substrates. In the case of anti-reflective (AR) coatings, this is challenging because polymers exhibit a relatively narrow range of refractive indices. We synthesized a four-layer AR stack using hybrid polymer:nanoparticle materials deposited by resonant infrared matrixassisted pulsed laser evaporation (RIR-MAPLE). An Er:YAG laser ablated frozen solutions of a high-index composite containing TiO2 nanoparticles and PMMA, alternating with a low-index solution of PMMA. The optimized AR coatings, with thicknesses calculated using commercial software, yielded a coating for polycarbonate with relative transmission over 94%, scattering less than 5% and a reflection coefficient below 0.8% across the visible range.

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.

Deposition of polymer barrier materials by resonant infrared pulsed laser ablation

We describe resonant infrared pulsed laser deposition (RIR-PLD) of cyclic olefin copolymer, a barrier and protective layer; for comparison, we describe RIR-PLD of polystyrene and poly(ethylene dioxythiophene) about which we already have significant knowledge. Film deposition based on resonant infrared laser ablation is a low-temperature process leading to evaporation and deposition of intact molecules. In this paper, we focus on deposition of this model barrier and protective material that is potentially useful in the fabrication of organic light emitting diodes. The films were characterized by scanning electron microscopy and Fourier-transform infrared spectroscopy. We also compared the properties of films deposited by a free electron laser and a picosecond optical parametric oscillator.

On the mechanism of resonant infrared polymer ablation: the case of polystyrene

We investigate the fundamental mechanisms of resonant-infrared laser ablation of polymers using polystyrene as a model material. Time-resolved plume shadowgraphy coupled with laser-induced temperature-rise calculations indicate that spinodal decomposition of a superheated surface layer is the primary mechanism for the initial stages of material removal. The majority of the ablated material is then released by way of recoil-induced ejection of liquid which proceeds for some tens of microseconds following a ~μs laser pulse excitation. The recoil-induced ejection of liquid material as the dominant ablation mechanism helps to explain previous observations of laser deposition of intact polymeric material.

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.

Mechanism of resonant infrared laser vaporization of intact polymers

Experiments on pulsed laser vaporization of many different kinds of polymers have demonstrated that it is possible to eject intact polymers into the ambient, whether air or vacuum, by resonant pulsed laser excitation, using both neat and matrix targets. Two recent studies of resonant infrared ablation - one on polystyrene, the other on poly(amic acid), the precursor for the thermoset polyimide - show moreover that the ablation process is both wavelength selective and surprisingly non-energetic, especially compared to ultraviolet laser ablation. We propose a wavelength-selective photothermal mechanism involving breaking of intermolecular hydrogen bonds that is consistent with these observations.

Polymer-nanocomposite anti-reflective coating fabricated by resonant IR matrix-assisted pulsed laser evaporation

We demonstrate a multilayer anti-reflective conformal coating for polycarbonate substrates, using a a polymer nanocomposite, fabricated by resonant infrared pulsed laser evaporation. The coating has 97% transmission, and less than 0.6% reflectivity over the visible spectrum.

Thermodynamics of resonant infrared matrix-assisted pulsed laser evaporation of luminescent dendrimers

The successful demonstration of resonant infrared laser ablation and deposition of intact polymers and thermally labile organic molecules raises complex questions about the thermo dynamics of the process. Here we present results of resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) of luminescent dendrimers, successfully deposited as thin films using cryogenic matrices of chloroform and toluene and analyzed by atomic-force microscopy and nuclear magnetic resonance of the films. The properties of the two matrices have definite effects on the dynamics of the ablation process, resulting in relatively rough films with little damage to the dendrimer for the chloroform matrix, and smoother films with more structural damage to the dendrimer in the case of the toluene matrix. Thermodynamic modeling suggests that RIR-MAPLE in the chloroform matrix proceeds via explosive vaporization, while the ablation process from the toluene matrix is normal boiling and vaporization.

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.