We report studies of material processing using the VUV F2 laser which, by virtue of its low threshold, high resolution 'machining' capability, may bring advantage to laser-based optoelectronic and photonic device fabrication. For example, probe beam deflection and etch rate studies of polymethylmethacrylate (PMMA) show this has a low ablation threshold, FT=20mJcm-2, and a large effective absorption coefficient, 1.6 x 105 cm-1, at 157nm, permitting high-resolution etching at modest fluence. The smooth ablated surfaces and low degree of thermal damage obtained with this laser make it well suited to machining structures such as relief gratings in PMMA. We also describe new results on producing fiber Bragg gratings with the 157nm laser. It is shown that these gratings can be written in a non-sensitized single mode fiber (Corning HI 980) with a low fluence and low total dose.
Excimer laser polymer ablation has been an active field of research and development for some twenty years now. We briefly review basic mechanistic aspects of the interaction, practical considerations related to polymer processing by ablation and applications in micromachining.
The 157nm F2 laser wavelength is strongly absorbed by glasses, even those with high silica content, making it potentially well suited for machining these materials by ablation. This is of interest for fabricating micro-optics and micro-devices in glass, provided crack-free surfaces with minimal laser-induced stress and surface roughness can be produced. Experimental studies are reported on the ablation threshold, ablation rate and surface quality of N-BK7 and soda lime glass for exposure with the VUV F2 laser. Optical probe techniques and etching are employed to determine the ablation threshold and removal rate and scanning electron microscopy to assess the surface quality of the glass following laser exposure. The interaction is discussed within the framework of a thermal vaporization model and the surface thermal loading is used to make a preliminary assessment of resolution attainable in micro-feature definition.
Theoretical and experimental studies of the surface quality in 157 nm F2 laser-ablated glasses are reported. Limitations set by statistical fluctuations in the multi- mode beam and by stationary beam non-uniformity are explored together with materials issues such as laser-induced surface cracking. Experimental work on ablating polymethyl methacrylate, used as a low threshold medium for recording of the VUV beam, and soda lime glass are described. Use is made of the probe beam deflection technique to determine ablation thresholds, and a variety of methods adopted for characterizing and assessing the quality of ablated surfaces e.g. scanning-electron microscopy, mechanical and optical interference profiling and atomic force microscopy. Preliminary roughness measurements are compared with theoretical expectations and the implication for glass micromachining with the F2 laser discussed.
Thin films of 4-cyano-4'-pentylbiphenyl (5CB) and E7 liquid crystals have been fabricated by pulsed laser deposition. The suitability of different lasers (ArF, KrF, XeCl and CO2) has been investigated over a range of fluence using visible-UV and infrared absorption and optical polarizing microscopy to characterize the films. High performance liquid chromatography and matrix assisted laser desorption ionization mass spectroscopy were used to assess the extent of decomposition of the films. The high photon energy of ArF and KrF excimer lasers produce severe and partial decomposition of the deposited films respectively, whilst films deposited using the CO2 laser also present partial degradation, most likely related to thermal processes during the laser-target interaction. Films with near identical structure to that of the starting LC target and good textures were obtained by XeCl laser deposition up to fluences of 130 mJ/cm2.
A brief review of laser ablation mechanisms and applications is presented. Topics covered include polymer ablation, thin film deposition, tissue ablation, and chemical analysis.
The excimer laser provides the necessary optical resolution and sufficiently high fluence to permit rapid micro- structure patterning of polymers and glasses by ablation. Micro-scale gratings and structures formed in this way have potential applications in the fields of opto-electronic devices, display technologies and environmental sensors. Conventional broad-band excimer lasers of poor spatial and temporal coherence can be used to write sub-micron gratings with an appropriate silica phase mask in proximity mode. This simple technique has been used to fabricate fiber Bragg gratings and relief gratings on polymers. The proximity of the mask and target increases the likelihood of damage to the mask during ablation. An alternative approach using Talbot re-imaging is attractive as the mask can be remote from the samples and undesirable orders are rejected. We describe the design of a Talbot interferometer in which the zero and first order beams from a grating are recombined and experiments using this with 193 nm ArF laser illumination to form submicron gratings on polymers and in fibers.
Measurements of thermoelastic and ablative stress transients generated by pulsed lasers in tissue
samples have been made using fast time-response (nanosecond) piezoelectric film transducers. Studies
of the ablation of cornea using excimer and CO2 lasers, and of vascular tissue using excimer, dye
and solid-state lasers are described. It is shown that useful information on the pulsed laser
interaction can be gained from these measurements.
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