Showing papers on "Excimer published in 1993"

Fabrication of refractive microlens arrays by excimer laser ablation of amorphous Teflon.

Abstract: The fabrication of refractive microlens arrays by the technique of excimer laser ablation of doped amorphous Teflon combined with the subsequent annealing and melting of the produced polymer islands is described. The microlenses obtained are optically clear from the far UV (190 m) to the near IR (2000 nm) and are of good optical quality. They vary in size from 50 to 385 μm in diameter with numerical apertures between 0.2 and 0.3. Utilization of these microlenses for material processing by excimer lasers at 193 nm is demonstrated, and possible applications are discussed.

Myopic keratomileusis with the excimer laser: one-year follow up.

Abstract: BACKGROUND The possibility of using an argon fluoride excimer laser to perform the refractive incision in keratomileusis spurred us to develop the present surgical technique of using a microkeratome to make a primary corneal incision that yields a corneal disc, then making the refractive excision of stroma with excimer laser ablation, and finally suturing the corneal disc in place. METHODS We describe the procedures and a prospective study of 30 consecutive eyes from 22 consecutive patients with high myopia (range, -11.20 D to -24.50 D) who underwent excimer laser myopic keratomileusis. A plano corneal disc was cut with a BKS 1000 microkeratome, followed by argon fluoride excimer laser ablation of the stroma with the Summit Eximed UV 200LA laser (Summit Technology, Inc, Waltham, Mass), either of the resected disc (28 eyes) or in situ on the stromal bed (two eyes). RESULTS By 3 weeks after surgery, 83% of the corneas were considered clear. In 17 eyes (57%), the difference between the intended and achieved refraction was < 1.00 D; in 9 eyes (30%), the difference was 1.00 to 3.00 D; in 4 eyes (13%), the difference was 3.00 to 6.00 D. At 12 months after surgery, uncorrected visual acuity was 20/40 or better in 3 (10%) eyes, and 20/50 to 20/100 in the remaining 27 (90%) eyes, in contrast to 100% of eyes that saw worse than 20/200 before surgery. Two eyes had a very deep ablation of the lenticule that led to irregularities in Bowman's layer. CONCLUSIONS Excimer laser myopic keratomileusis is an effective way to correct high myopia and may be more accurate than other methods of keratomileusis. A new nomogram for the procedure is presented.

Excimer laser photorefractive keratectomy for myopia: results in 1165 eyes

Abstract: BACKGROUND We report the results of a prospective study conducted in Italy to evaluate the efficacy, safety, predictability, and complications of excimer laser photorefractive keratectomy for the correction of myopia. METHODS. Photorefractive keratectomy was performed on 1236 myopic sighted eyes in 16 centers, using the Summit Excimed 193 nm excimer laser. The attempted correction ranged from -0.80 to -25.00 diopters (mean, -7.83 +/- 3.88 D). The population was divided into three groups of attempted correction: between -0.80 and -6.00 D, between -6.10 and -9.90 D, and between -10.00 and -25.00 D. We report the data of 1165 eyes at 1 month, 970 eyes at 3 months, 752 at 6 months, and 330 at 12 months. At each visit, we evaluated (1) the refractive changes over time; (2) the difference between attempted and achieved correction; (3) uncorrected and best spectacle corrected visual acuity; and (4) haze. RESULTS Twelve months after surgery, the spherical equivalent refractive error in 146 eyes with attempted correction of -0.80 to -6.00 D was -0.52 +/- 1.04 D; 104 eyes (71.2%) were within +/- 1.00 D of attempted correction. In 145 eyes with attempted correction of -6.10 to -9.90 D, the spherical equivalent refractive error was -1.66 +/- 1.88 D; 50 eyes (34.5%) were within +/- 1.00 D of attempted correction. The spherical equivalent refractive error in 39 eyes with attempted correction of -10.00 to -25.00 D was -1.86 +/- 3.47 D; 11 eyes (28.2%) were within +/- 1.00 D of attempted correction. Eight eyes (2.4%) lost two or more Snellen lines of best spectacle corrected visual acuity. None of the treatments caused severe postoperative complications, or scarring. CONCLUSIONS Photorefractive keratectomy proved safe and effective, but highly predictable only in the correction of myopia up to -6.00 D.

Ground- and excited-state interaction in di-1-pyrenyl-substituted oligosilanes

Abstract: Excimer formation is reported for 1,2-di(1-pyrenyl)tetramethyldisilane (P2P), 1,3-di(1-pyrenyl)hexamethyltrisilane (P3P), and 1,4-di(1-pyrenyl)octamethyltetrasilane (P4P), while 1,6-di(1-pyrenyl)dodecametbylhexasilane (P6P) shows a σ(Si-Si)→π * charge-transfer fluorescence. The 1 H-NMR spectra of the bichromophores were measured and compared with the reference compounds 1-(trimethylsilyl)pyrene (P1), 1-pyrenylpentamethyldisilane (P2), 1-pyrenylheptamethyltrisilane (P3), and 1-pyrenyltridecametbylhexasilane (P6). A strong shielding is observed for the aromatic protons of the bichromophoric compounds, which suggests that a fraction of the molecules are present in a conformation in which the two pyrene groups interact intramolecularly

Excimers of organic molecules

Abstract: Studies devoted to excimers in organic systems are surveyed. Processes leading to the excitation of the excimer luminescence are examined. Examples of the excimer-like glow of dimers are described. The relation between the structure of the complexes and their fluorescence properties is demonstrated. The bibliography includes 218 references.

Micromachining Using Excimer Lasers

Abstract: Excimer lasers can be used to machine metals, ceramics and polymers. UV radiation with short pulses of several 10 ns and intensities much higher than those of other sources allow machining with minimum thermal influence. The high photon, energy of excimer laser radiation facilitates a direct photolytic interaction with the material. Besides this thermal and pyrolytic interaction mechanisms also appear. Concidering on this, the removal mechanisms of certain materials are discussed. The application of excimer laser radiation in a multiaxis CNC-micromachining unit with modular design is presented. Special features of this micromachining center are the beam handling system with integrated flexible mask and energy control. The workpiece handling and all process parameters are controlled by a host computer with ergonomic user interface. The unit can be used to generate microstructures in ceramic and polymer surfaces. Furthermore, it is possible to produce micromechanical parts of these materials, also in fibre reinforced plastics.

Molecular conformation effects on the relaxation processes in the excited state of naphthalenophanes

Abstract: The photophysical processes of naphthalenophanes having different molecular structures have been studied by measurements of emission spectra, emission quantum yields, intersystem crossing (ISC) quantum yields, and lifetimes. The molecular conformations of naphthalenophanes are divided into three groups by means of MM2 calculations: (a) parallel forms (1 and 2), (b) antiparallel form (3), and (c) tilted forms (4 and 5). The singlet excimer state 1 EX * formed via the Franck-Condon excited singlet state 1 (R * -R) or 1 (R * ΛR) is produced very rapidly. Intramolecular excimer emissions are observed significantly for naphthalenophanes except for the antiparallel form 3

Probing solute-entrainer interactions in matrix-modified supercritical carbon dioxide

Abstract: Binary supercritical fluids composed of CO[sub 2] and small amounts of acetonitrile or methanol (entrainers) are studied as a function of fluid density, using pyrene as the solute probe. These experiments show how preferential entrainer clustering effects pyrene excimer formation. To this end, steady-state and time-resolved fluorescence spectroscopy are used in concert to probe the ground and excited states of pyrene. Results show that entrainers enhance solute-solvent clustering and slow the, traditionally diffusion-controlled, excimer formation reaction. Clustering also helps shield the excimer from non-radiative deactivation processes as evidenced by a longer decay time in the near-critical region. All data over the concentration range are consistent with a homogeneous pyrene ground state. There is no evidence for solute-solute (pyrene-pyrene) interactions prior to excitation. However, once in the excited state, excimer formation is facilitated due to the decreased fluid viscosity compared to liquids. 65 refs., 10 figs., 4 tabs.

Measurement of Gas Temperature Profile in Discharge Region of Excimer Laser with Laser Schlieren Method

Abstract: Shock waves are generated by pulse discharges in the cavity of excimer lasers. The shock waves cause arcing, nonhomogeneous excitation of laser gas and limitation of repetition rate of a high-repetition-rate excimer laser. Distribution of temperature rise by pulse discharge is an essential factor for generation and propagation of shock waves. Gas temperature profiles in the discharge region of the excimer laser cavity are measured by a laser schlieren method for single-pulse operations. The results show that the temperature distribution depends on the xenon concentration. In the cases of pure helium and higher xenon concentration, the temperature distributions are steeper than those in the cases of lower xenon concentration.

Scanning electron microscopy and thermal characteristics of dentin ablated by a short‐pulse XeCl excimer laser

Abstract: The interaction of a short pulse XeCl excimer laser radiation with human dentin was investigated. The dependence of surface temperatures and temperature gradients into the treated teeth on laser parameters such as fluence (0.5J/cm2-7J/cm2), pulse repetition rate (1Hz-35Hz), and spot size (0.004cm2-0.12cm2) was studied. Additionally, the effect of fluence and pulse repetition rate on dentin microstructure was studied using scanning electron microscopy (SEM). It is demonstrated that this “cold ablation” excimer laser can result in significant thermal modification in the dentin surfaces. Changes include the formation of melted dentin grains, which uniformly cover the surface and the exposed dentin tubules. Maximum temperatures of the ablated surfaces, however, remained relatively low at most laser parameters used. Also, the immediate neighborhood of the root canal was essentially undisturbed at most laser parameters. These observations suggest that with the appropriate choice of parameters XeCl lasers can be effective in producing surface structures that may prove useful in enhancing bond strength or other applications in dentistry, without exposing tooth pulp to significant temperature elevation. © 1993 Wiley-Liss, Inc.

The yttrium oxide chemiluminescence from the 308 nm excimer laser ablation of YBa2Cu3O7−X, Y2O3, and YCl3

Abstract: The yttrium oxide emission produced in the photoablation of Y2O3, YCl3, and YBa2Cu3O7−X in oxygen is investigated. Chemiluminescence from the B 2Σ‐X2Σ and the A 2Π‐X 2Σ YO electronic states, produced by the reaction Y+O2, is measured. The intensity of the A 2Π3/2 band is monitored as a function of pressure, distance from the target, and laser fluence. Hydrodynamic effects have a significant influence on the spatial and temporal evolution of the YO emission in the plume, which is governed by the dynamics of the shock wave generated in the excimer ablation. This shock wave compresses the ambient oxygen and leaves a trough of rarefied oxygen in its wake.

High-Performance Poly-Si Thin-Film Transistors with Excimer-Laser Annealed Silicon-Nitride Gate

Abstract: We report, for the first time, that ArF excimer laser annealing can improve silicon-nitride film properties. It is shown that the 15-nm-thick top region of the laser preannealed film had a lower hydrogen content and a much lower etching rate than the as-deposited film. The laser preannealed film is very useful as the gate insulator of high-performance bottom-gate thin-film transistors fabricated with the laser-recrystallized poly-Si film. The field-effect mobility of electrons was as high as 200 cm 2 /V-s, while the mobility was as low as 40 cm 2 /V-s without preannealing the silicon-nitride film

Physical problems of excimer laser cornea ablation

Abstract: ArF excimer laser ablation of corneal tissues ( in vitro , pig) is investigated. Scanning electron microscopic analysis shows that the spatial resolution of the ablation is better than 1 μm. The etching rate and etched depth is found to be proportional to the logarithm of the laser fluence and the number of shots, respectively. To investigate the dynamic processes a laser-based, ultrafast photographic arrangement was constructed having a temporal resolution of 1 ns. The maximum speed of the shock wave generated during ablation is found to be as high as 4000 m/s. The ejection of the ablation plume with plume-front velocities in the range of 600 m/s is also observed. A new type of surface wave generated by the recoil forces of the ablated material is found. The amplitude of this surface wave can be as high as 0.4 mm.

Addition of oxygen to a gas mix for use in an excimer laser

Abstract: The present invention relates to discovery that oxygen may be used to stabilize the efficiency and performance of an excimer laser Therefore, oxygen is not a contaminant to be minimized but rather a necessary part of an excimer lasing composition

Interactions of ionic surfactants with gelatin in fluid solutions and the gel state studied by fluorescence techniques, potentiometry, and measurements of viscosity and gel strength

Abstract: The photobehavior of pyrene (fine structure of the monomeric fluorescence band; excited state lifetimes; excimer/monomeric fluorescence intensity ratios and excited state quenching rates by oxygen) in fluid solutions of gelatin and in the gel state, both in the absence and presence of ionic surfactants, has been examined. Surfactants considered were sodium dodecylsulfate and dodecyltrimethylammonium bromide. Potentiometric measurements performed by using surfactant selective electrodes allowed for the determination of the binding capacity of the surfactants onto gelatin in the gel state. When complemented with viscosity and gel strength measurements, the sesults obtained allow for a discussion of the effects of the surfactant-gelatin interactions on microscopic properties of the solutions or gels and their relation with macroscopic properties.

Convolution analysis of the pyrene excimer formation in membranes

Abstract: The mechanism of excimer formation in rigid and fluid membranes is analyzed from the fluorescence dynamics of both monomer and excimer emissions. We analyze the kinetics of the excimer emission using a convolution formalism. In the rigid state of distearyldimethylammonium chloride bilayers, we observed a discrepancy between the decay of the monomer and the excimer populations. This shows that a fraction of the pyrene probes is isolated and is not submitted to excimer formation. The convolution analysis enables us to estimate with good precision the fraction of isolated monomers. We have analyzed the monomer emission using transient diffusion models

Concentration dependence of pyrene excimer formation in doped polyethylene

Abstract: Pyrene-doped low density polyethylene films were studied by steady state and time-resolved fluorescence emission spectroscopy at room temperature for dopant concentrations up to about 1.5×10−2 mol dm−3. The excimer fluorescence observed in these films for a high pyrene concentration (about 10−2 mol dm−3) was proposed to result either from pyrene molecular pairs close to the excimer configuration or from pyrene aggregates which satisfy the geometrical requirements for excimer formation.

Excimer laser micromachining

Abstract: The machining of several materials such as polymers, metals and ceramics using excimer lasers (193, 248 and 308 nm) was investigated. By photoablation, micrometer resolution can be achieved for polymers if the wavelength and fluence are chosen properly. High-definition processing of metals is complicated by the occurrence of surface melting. An interaction between the ablation plume and the walls of the laser-drilled holes leads to hole widening in the case of polymers at high fluences. For small structures (<100 μm) all materials investigated show a size-dependent ablation rate. This effect has to be taken into account if a precise ablation depth is required for a pattern with structures of different sizes.

Pyrene excimer formation in individual oil droplets dispersed in gelatin matrixes: space- and time-resolved fluorescence spectroscopy

Abstract: Pyrene excimer formation in individual liquid paraffin droplets dispersed in aqueous gelatin matrices was studied by three-dimensional space- and time-resolved fluorescence spectroscopy. The pyrene excimer formation efficiency in the droplets was constant irrespective of the droplet size. The parameters of the excimer formation were determined for each droplet and compared with those observed in bulk pyrene/liquid paraffin solution. The results were discussed on the basis of the depth profiles and the dynamics of pyrene fluorescence in individual liquid paraffin droplets

Design and Development of a New 193-Nanometer Excimer Laser Surgical System

Abstract: BACKGROUND The 193-nanometer excimer laser is used for photoablative reprofiling of the cornea to correct refraction errors and to treat corneal pathologies. To use the unique properties of the excimer laser, some technical features should be provided by the excimer laser system. A new excimer laser system is presented. METHODS The instrument has a fluence sensor for precise measurement of the actual fluence just before surgery, a new delivery system including an integrator system to homogenize the laser beam, a steelband with apertures and a zoom optic to get the desired form of central reprofiling, and an alignment system to reduce the possibility to decenter the optical zone. RESULTS The fluence before surgery can be measured very precisely with the fluence sensor. The ablation zone size is at least 8 mm in diameter with a homogenous laser beam profile resulting in a smooth surface of the treated material. Different forms of ablations can be created using the steelband with different forms of apertures. The center of the ablation zone can be found using the alignment system. CONCLUSION The new excimer laser system fulfills some important technical features for optimal use of the excimer laser in corneal surgery.

Observation of a near-IR absorption band of the fluorene excimer by photodissociation spectroscopy

Abstract: Photodissociation behaviors of the fluorene excimer, produced from the electronic excitation of the corresponding van der Waals dimer, have been investigated by photodepletion and photofragmentation techniques. The excimer exhibits a strong absorption band in the near-IR region, with an intensity maximum at about 850 nm. This excimer absorption is assigned to an intervalence transition between the lowest excimer state and an upper state of repulsive character, both arising from the exciton resonance interaction of the monomer S 1 state. Excitation of this band leads to a rapid dissociation of the excimer into monomer fragments, one of which is electronically excited. The photodissociation dynamics of the fluorene excimer is compared with that of the naphthalene excimers

Photophysics of N-ethylcarbazole in fluid solution. Evidence for solvent dependence and triplet excimer formation

Abstract: The photoexcitation of N-ethylcarbazole (NEC) in fluid solutions of cyclohexane, N,N-dimethylformamide (DMF), ethanol, paraffin oil, and mixtures of glycerol with either ethanol or DMF has been carried out using 308-nm pulses from a XeCl excimer laser. Emission spectra recorded at 70 [mu]s or longer after the excitation pulse depend upon the solvent. In ethanol and DMF red-shifted, structureless bands appear at these longer delay times which are attributed to delayed excimer fluorescence. No such bands are found using cyclohexane. In glycerol/ethanol or glycerol/DMF (90/10 (v/v)) monomeric delayed fluorescence is found at delay times of 70 [mu]s or less, but at delay times from 700 [mu]s to 3.8 ms a prominent band at 500 nm is observed which is attributed to emission from triplet excimers. Transient absorption spectra of NEC in cyclohexane recorded at delay times from 1 to 20 [mu]s after the excitation pulse are entirely attributable to the carbazolyl triplet state. In ethanol and DMF, however, additional bands corresponding to radical cations and anions of NEC are also found. A broad band observed near 500 nm is provisionally assigned to the triplet excimer. Kinetic decays of the absorption signals due to triplets were fit to concurrent first-order and second-ordermore » processes whereas those due to the radical anion were fit to a biexponential decay. It is proposed that delayed excimer fluorescence arises from recombination of cation dimers and radical anions and that triplet excimers are formed by recombination of geminate ion pairs to form a monomeric triplet and a ground-state partner, followed by interaction of the triplet and the same partner. The primary mechanism for ion formation is thought to involve electron transfer from multiply excited molecules to ground-state species. 34 refs., 10 figs., 1 tab.« less