Author(s): Vogel, Alfred; Venugopalan, Vasan | Abstract: The mechanisms of pulsed laser ablation of biological tissues were studied. The transiently empty space created between the fiber tip and the tissue surface improved the optical transmission to the target and thus increased the ablation efficiency. It was found that the structure and morphology also affect the energy transport among tissue constituents.

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Mechanisms of pulsed laser ablation of biological tissues.

The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

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Ultrafast lasers—reliable tools for advanced materials processing

The excimer laser offers entirely new corneal surgery procedures. In this paper, we describe conditions for one such procedure, photorefractive keratectomy, the direct reshaping of the cornea's central optical zone using tissue ablation with far ultraviolet radiation. We present equations for the required tissue ablation to achieve required refractive corrections. Conditions for beam uniformity are presented and a method of achieving better beam uniformity using beam integration by rotation is demonstrated. The healing of rabbit corneas after area ablation was observed. Five days after treatment, the surface quality of deep cuts was very irregular; the shallow cuts produced more regular results, as predicted by our calculations of beam characteristics.

Photorefractive keratectomy: A technique for laser refractive surgery

A single diagram, encompassing most medical applications for all types of laser in current use, forms the basis of this review of recent medical developments. Emphasis is placed on the physical processes that govern different microscopic mechanisms of laser-tissue interaction. Four distinct photophysical groups are considered in a general classification of these specific modes of interaction: for continuous wave exposure, the photothermal and the photochemical transformations; and, for pulsed irradiations, the electromechanical and the photoablative processes.

Photophysical processes in recent medical laser developments: A review

Laser myopic keratomileusis (photorefractive keratectomy) was performed on 29 rhesus monkey corneas with an argon fluoride (193-nm) excimer laser and a computer-controlled, moving slit delivery system. The 4-mm-diameter central ablation zone ranged in depth from 11 microns (-2 diopters effect) to 46 microns (-8 diopters effect). Corneas were studied for the 9 months postoperatively by clinical slit-lamp microscopy, and periodically with light and transmission electron microscopy. By 6 weeks, mild to moderate subepithelial haze was apparent in 93% of the corneas, with considerable variability in density. Progressive clearing occurred so that by 6 to 9 months 12 of 13 surviving corneas (92%) were either completely clear (4 corneas) or trace hazy (8 corneas). The epithelium was thickened at 21 days after ablation and returned to normal thickness by 3 months. At 3 weeks, subepithelial fibroblasts were three times the density of normal keratocytes and returned to nearly normal numbers by 9 months. We concluded that the anterior monkey cornea demonstrated a mild, typical wound healing response after excimer laser keratomileusis.

Wound healing after excimer laser keratomileusis (photorefractive keratectomy) in monkeys.

Excimer Laser Surgery of the Cornea

Pulsed laser radiation at 193, 248, or 308 nm can etch films of polyimide (DuPont KaptonTM). The mechanism of this process has been examined by the chemical analysis of the condensible products, by laser‐induced fluorescence analysis of the diatomic products, and by the measurement of the etch depth per pulse over a range of fluences of the laser pulse. The most important product as well as the only one condensible at room temperature is carbon. Laser‐induced fluorescence analysis showed that C2 and CN were present in the ablation plume. At 248 nm, even well below the fluence threshold of 0.08 J/cm2 for significant ablation, these diatomic species are readily detected and are measured to leave the polymer surface with translational energy of ∼5 eV. These results, when combined with the photoacoustic studies of Dyer and Srinivasan [Appl. Phys. Lett. 48, 445 (1986)], show that a simple photochemical mechanism in which one photon or less (on average) is absorbed per monomer is inadequate. The ablation proces...

Ultraviolet laser ablation of polyimide films

The effect of pulse width on the ablation of polymers has been extended to ultrashort pulses (160 fs) of 308 nm wavelength. Polymethylmethacrylate has negligible absorption at this wavelength for one‐photon excitation. With the ultrashort pulse clean etching without any sign of thermal damage can be achieved at fluences as little as 0.2–0.3 J/cm2. This is the first demonstration that the high power of ultrashort pulses of ultraviolet light can produce photochemical etching by taking advantage of multiphoton excitation to dissociative states.

Ablation and etching of polymethylmethacrylate by very short (160 fs) ultraviolet (308 nm) laser pulses

The formation of C2 in the ablation of polymethyl methacrylate (PMMA) by 193- and 248-nm laser pulses was studied by laser-induced fluorescence. The formation of C2 is observable at a fluence at 248 nm that is well below the fluence threshold where significant etching takes place. The opposite is true at 193 nm. The velocity distribution of the diatomic form was also measured and found to approach but not exactly fit a Maxwell–Boltzmann equation. Average translational energies as high as 6 eV were recorded even at the fluence threshold for this product. The other products of the laser ablation of PMMA at 193 or 248 nm are methyl methacrylate (MMA) and a solid that is a low-molecular-weight fraction of PMMA. Although the products are the same at both wavelengths, the mix is quite different. At 193 nm, 18% of the ablated polymer is MMA, whereas at 248 nm less than 1% of the polymer appeared as MMA. A semilog plot of the mass of material removed versus the fluence shows three distinct regions. The central portion at each wavelength (80–300 mJ/cm2 at 193 nm; 600–2000 mJ/cm2 at 248 nm), which corresponds to rapid etching, is identified as ablative photodecomposition. At lower fluences, the etching efficiency falls off rapidly, indicating that there is a threshold fluence. At fluences above the range for ablative photodecomposition, the etching levels off, probably because of the secondary absorption of the incoming photons by the products. The mass of material ablated per joule of energy absorbed in the ablated volume was remarkably similar at both wavelengths. It is suggested that ablative photodecomposition involves both a one-photon process that produces MMA and low-molecular-weight polymeric fragments and a many-photon process that gives rise to products such as C2 with high translational energy. In order to establish the role of the many-photon process, samples of PMMA doped with acridine were exposed to laser pulses. The presence of as little as 2% of acridine lowers the threshold for significant etching at 248 nm from 400 to 90 mJ/cm2. The absorption characteristics of the doped polymer at this wavelength show that the acridine molecules absorb >10 photons each, whereas the absorption of the polymer changes little. The drop in the threshold energy for etching should therefore be attributed to the many-photon decomposition of the acridine and the consequent ablation of the polymer as a whole.

Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies

Irradiation, i.e.bombardment, with 193 nm laser pulses having an energy fluence of 2.5 J cm 2 and a duration of ~12 ns leads to rapid sputtering with Au, Al2O3, MgO, MgO. Al2O3, SiO2, glass and LaB6, relatively slow sputtering with MgF2 and diamond, and mainly thermal-stress cracking with W. Scanning electron microscopy (SEM) suggests that the mechanism for the sputtering of Au in either vacuum or air is one based on the hydrodynamics of molten Au, while an SEM-derived surface temperature estimate confirms that thermal sputtering (which might have been expected) is not possible. SEM with W shows that the near total lack of material removal is due to the thermal-stress cracking not leading to completed exfoliation, together with the surface temperature being too low for either hydrodynamical or thermal processes. Corresponding SEM with Al2O3 shows, in the case of specimens bombarded in vacuum, topography of such a type that all mechanisms except electronic ones can be ruled out. The topography of Al2O3 or other oxides bombarded in air through a mask is somewhat different, showing craters as for vacuum bombardments but ones which have a cone-like pattern on the bottom.

Bodil Braren

Papers

Excimer Laser Surgery of the Cornea

Ultraviolet laser ablation of polyimide films

Ablation and etching of polymethylmethacrylate by very short (160 fs) ultraviolet (308 nm) laser pulses

Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies

Laser sputtering: Part I. On the existence of rapid laser sputtering at 193 nm