808-nm Laser Irradiation Research Articles

FORMATION OF C3N4 CLUSTERS BY LASER ABLATION OF NITRIDED GRAPHITE

Clusters of C 3 N 4 have been detected from laser ablation of nitrided graphite, The nitrided target was prepared by irradiating a graphite specimen with 248-nm laser radiation in the presence of N2. Auger electron spectroscopy of the irradiated graphite indicated the formation of a thin layer with a composition of CN 0.29. Ablation of this nitrided layer results in the ejection of carbon cluster as well as CN 4 and C 3 N 4. These results indicate that the C 3 N 4 cluster ion is stable. They also suggest that laser ablation within the presence of a reactive gas may be a generally applicable method of preparing targets for the subsequent formation of unique cluster species.

Laser-enhanced gas surface reaction of chlorine with aluminum polycrystalline surface

The supersonic molecular beam technique is used to investigate the dynamics of photochemical gas-surface reactions of Cl 2 with polycrystalline Al surface irradiated by 355- and 1064-nm laser photons. The mass and velocity distributions of desorbed reaction products are measured by time-resolved mass spectrometry. Results show that the main products are Al + and AlCl x (x=1,2,3) produced with the irradiation at 355 nm; only AlCl 3 is produced under 1064-nm laser irradiation

Characterization of thiol self-assembled films by laser desorption Fourier transform mass spectrometry

Self-assembled alkanethiol monolayers on gold films have been studied using the technique of laser desorption coupled with Fourier transform mass spectrometry. The authors demonstrate that thiol molecules can be desorbed intact using 193-nm laser radiation. Information on the self-assembled films at a molecular level is achieved. In addition to the well-known thiolate species, alkane sulfonates resulting from air exposure of the self-assembled film are also detected. No phase segregation {ge}30 {mu}m is observed from the laser desorption of self-assembled films made of alkanethiol mixtures. 37 refs., 8 figs.

Laser-induced reactions of semiconductor surfaces

Laser-induced reactions of Ge(111), Si(111), GaAs(100), and InP(100) surfaces with chlorine under 355-, 560-, and 1064-nm laser irradiation have been investigated using a supersonic beam technique and time-resolved mass spectrometry. It has been found that the reaction yields depend not only on the laser fluence and wavelength, but also on the translational energy of the incident chlorine molecules. A possible mechanism of laser-induced reactions is proposed.

Use of temporally coincident ultraviolet laser pulses of two different wavelengths to ablate organic polymers

The etching of polymethyl methacrylate (PMMA), by pulsed ultraviolet laser radiation [20-ns full width at half maximum (FWHM), 193 nm] can be increased more than threefold by overlapping both spatially and temporally laser pulses of 308 nm (20 ns FWHM) along with the 193 nm pulses. At a constant fluence of 193 nm, increasing the fluence of the 308 nm pulses progressively increases the etch depth/pair of pulses even though the fluence of the 308 nm radiation is kept below the value at which it would etch the polymer by itself at 1 Hz repetition rate. When a time delay of +300 to −400 ns is introduced between the two pulses, the 193 nm pulse being set as time zero, the enhancement due to pairing the pulses is found to be a maximum at nearly total overlap. A similar enhancement in the etching of PMMA by pulsed 248-nm laser radiation can be achieved by mixing 308 nm pulses with the 248 nm, and in the etching of polyethylene terephthalate, by mixing 351-nm laser pulses with 308-nm laser pulses. Chemical analysis of the solid polymeric product from the laser ablation of PMMA by 248 and 308 nm alone, and by a mixture of 248 and 308 nm laser radiation shows that the mixed laser radiation etches in a way that is chemically similar to pulsed 248-nm laser light and the effect can probably be attributed to an enhancement of the photoablation process.

DNA Damage in Cultured Human Skin Fibroblasts Exposed to Excimer Laser Radiation

Ultraviolet excimer lasers are being considered for use in a variety of refractive and therapeutic procedures, the long-term biologic consequences of which are unknown. The effect of sublethal doses of 193-nm laser radiation on cellular DNA was examined in cultured human skin fibroblasts. In contrast to 248 nm, treatments with the 193-nm laser radiation below 70 J/m2 did not cause significant pyrimidine dimer formation in the skin cells. This was indicated by the lack of excision repair activities (unscheduled DNA synthesis assay), and further demonstrated by direct analysis of pyrimidine dimers in DNA from irradiated cells. However, a low level of unscheduled DNA synthesis could be detected following irradiation at 193 nm with 70 J/m2. Both the 193-nm and 248-nm radiation were able to induce chromosomal aberrations, as indicated by a micronucleus assay. A dose-dependent increase in micronuclei frequency was observed 48 and 72 h after laser irradiation. These results indicate that exposure of actively replicating human skin fibroblasts to sublethal doses of either 193- or 248-nm laser radiation can result in genotoxicity.

Microstructural studies of epitaxial Ge films grown on [100] GaAs by laser photochemical vapor deposition

Ge films grown on [100] GaAs by laser photochemical vapor deposition (LPVD) in parallel geometry at temperatures (Ts) ranging from ∼240 to 415 °C have been examined by transmission electron microscopy. For 285 ≤Ts≲330 °C, a thin (250–700 Å) epitaxial film is grown initially but a switch to amorphous material is subsequently observed. At higher substrate temperatures (Ts ≳400 °C), thicker (≳800 Å) epitaxial Ge films are grown before the transition to polycrystalline material takes place. In the absence of external 193-nm laser radiation (i.e., growing by conventional low-pressure chemical vapor deposition), the Ge films are completely amorphous (285 ≤Ts≤330 °C) or heavily defected polycrystalline (Ts∼400 °C). The >100 °C temperature reduction for the growth of epi Ge films made possible by LPVD is attributed to the direct production of a species (GeH3) by the laser which is collisionally converted to Ge2H6 en route to the substrate. Upon reaching the [100] GaAs surface, the digermane is pyrolyzed. Experiments with [100] substrates tilted 3° toward [110] yielded thinner (∼120 Å), but smooth, epitaxial films which is attributed to the higher density of available nucleation sites. Films grown at 280–330 °C on [111] oriented GaAs were completely amorphous which appears to arise from reduced adatom mobilities on [111] surfaces. These results clearly demonstrate the feasibility of photochemically generating a species which migrates to the surface and alters the chemistry at a substrate (which is itself not illuminated by the optical source) so as to permit the growth of epitaxial semiconductor films.

Optical switching of a dc discharge using an excimer laser

The effects of unfocused 193- and 248-nm laser radiation of 3–30 mJ/cm2 on dc discharges of vinylchloride in helium are described. Laser-initiated striking and quenching of the low-current discharges are observed at pulse repetition rates of 10 to 0.001 s−1 over a range of operating conditions. The apparent paradox of the coexistence of strike and quench conditions in the discharge gap for consecutive laser pulses is discussed.

Ultraviolet-Laser Ablation of Skin

We report on the use of pulsed ultraviolet-laser irradiation at 193 nm from an argon-fluoride laser and at 248 nm from a krypton-fluoride laser to ablate skin. In vitro, both wavelengths performed comparably, removing tissue precisely and cleanly, and leaving minimal thermal damage to the surrounding tissue. In vivo, the 193-nm laser radiation failed to remove tissue after bleeding began. The 248-nm radiation, however, continued to remove tissue despite bleeding and left a clean incision with only minimal thermal damage. The krypton-fluoride excimer laser beam at 248 nm, which should be deliverable through a quartz optical fiber, has great potential as a surgical instrument.