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The small-signal absorption coefficients of 193- and 213-nm nanosecond laser pulses in bovine corneal tissue have been studied. The absolute reflectance of a planar quartz-cornea interface was measured at various angles of incidence for low-intensity laser irradiation (i.e., pulse fluences 3 orders of magnitude below the ablation threshold). The reflectance-versus-angle data were analyzed by the use of Fresnel theory to estimate the effective complex index of refraction of the tissue. This analysis indicated corneal absorption coefficients of 39,900 ± 9800 cm(-1) at 193 nm and 21,400 ± 4900 cm(-1) at 213 nm.

A theoretical description of the ultraviolet laser etching process is developed. The threshold for laser ablation is reached when the density of absorbed photons is approximately equal to the density of chromophores in the material. Saturation of the absorption coefficient, absorption by the plume of ablated products, and multiphoton effects are considered. Agreement with all available experimental etch data, including femtosecond ultraviolet laser ablation, is found. The description is based on an analysis of the radiation transport at high intensities and is independent of the question as to whether ultraviolet laser ablation is photochemical or thermal.

The energetics of 308-nm excimer laser irradiation of human aorta were studied. The heat generation that occurred during laser irradiation of atherosclerotic aorta equaled the absorbed laser energy minus the fraction of energy for escaping fluorescence (0.8-1.6%) and photochemical decomposition (2%). The absorbed laser energy is equal to the total delivered light energy minus the energy lost as specular reflectance (2.4%, air/tissue) and diffuse reflectance (11.5-15.5%). Overall, about 79-83.5% of the delivered light energy was converted to heat. We conclude that the mechanism of XeCl laser ablation of soft tissue involves thermal overheating of the irradiated volume with subsequent explosive vaporization. The optical properties of normal wall of human aorta and fibrous plaque, both native and denatured were determined. The light scattering was significant and sufficient to cause a subsurface fluence (J/cm2) in native aorta that equaled 1.8 times the broad-beam radiant exposure, phi o (2.7 phi o for denatured aorta). An optical fiber must have a diameter of at least 800 microns to achieve a maximum light penetration (approximately 200 microns for phi o/e) in the aorta along the central axis of the beam.

Autofluorescence spectroscopy of arterial surfaces provides information about the distribution and composition of atherosclerotic plaques. The

Transmission experiments on thin polyimide films under laser ablation conditions using 248 nm KrF laser radiation have been performed. The transmitted temporal pulse shapes and the transmittted intensity show fluence-dependent absorption as predicted by a recent theoretical description of the pulsed ultraviolet laser ablation process.

Abstract. Experiments that demonstrate quantitatively the importance of laser absorption dynamics for ultraviolet laser ablation of organic materials are presented. Laser pulse transmission measurements have been performed on 0.1 jam spin-coated polyimide films at three ultraviolet wavelengths ( 193 nm, 248 nm, and 355 nm) over the fluence range l0 -3-10 J/cm 2. Target transmission is observed to increase with increasing fluence by a factor of --,5 at 193 nm, and a factor of ~ 10 at 248 nm. In contrast, transmission decreases by approximately one half during 355 nm target irradiation. These results are analyzed theoretically with a two-level model of chromophore absorption. This theory is also applied to reported pulsed UV-laser polyimide ablation data. It is shown that an accurate description of the fluence-dependent film absorption leads to a prediction of the etch depth versus pulse fluence relationship in good agreement with experimental data. PACS : 42.10, 81.60, 82.50 Pulsed ultraviolet-laser ablation has been shown in a multitude of studies to etch organic targets precisely, which in turn has led to widespread application of UV lasers in materials processing, microelectronics, and medicine. In order to optimize these procedures, as well as to assess possible risks of clinical laser use, the laser/material interaction must be well understood. One aspect of photoablation that has only lately become apparent is that the absorptive properties of the target can change during the intense laser irradiation [1][2][3][4]. Recently, two of the current authors presented a theoretical description of such dynamic optical properties [1,5] showing that these absorption changes could account for the commonly noted substantial discrepancy between experimental data and the etch depth versus laser fluence relationship predicted by Beer's law (i.e., the simple "blow-off" model [6] described below).However, other processes such as thermal diffusion could also affect the ablation depth/fluence behavior [7]. A recent theoretical analysis concluded that dynamic target optics were probably more important than thermal diffusion in affecting photoablation [8], yet this issue has not been conclusively resolved. The absolute magnitude of the optical effect cannot be deduced from the reports cited above because those studies were either qualitative in nature or limited in terms of incident laser fluence. For these reasons we have conducted the experiments described here, quantifying the change in absorption for thin films of polyimide (a common photoablation substrate) over a wide range of laser fluences at three distinct ultraviolet wavelengths. This paper thus provides the first quantitative experimental test of the ideas formulated in [1,5], i.e. that proper treatment of the radiation transport to include chromophore saturation and excited-state absorption is extremely important for determination of the etch depth per pulse in organic materials.

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