Alexander A. Serafetinides

Picosecond and subpicosecond visible laser ablation of optically transparent polymers

Abstract The ablation rates, as a function of the laser fluence, of the optically transparent polymers, Nylon-6,6 and PMMA, are reported using picosecond and subpicosecond laser pulses, obtained from a Regenerative Amplified Nd:YAG laser system. The laser pulses had a duration of 100 ps at 1064 and 532 nm wavelengths and 0.8 ps at 595 nm. The ablation rate results indicate a strong saturation behaviour for both polymers in the investigated irradiation conditions. The material removal is 2–3 times higher in the case of the visible (532 nm) picosecond laser ablation experiments. The surface topology of the polymers was also studied. The obtained Atomic Force Microscopy images reveal no mechanical damage in the inner ablation crater wall. The qualitative analysis of the ablation mechanism for ultrashort pulse laser irradiation reveals a combination of photochemically induced direct bond dissociation and a photothermal process due to the relaxation of the excited polymers within the vibrational levels of the ground state.

Ultra-violet and Infra-red Laser Ablation Studies of Biocompatible Polymers

Polytetrafluroethylene (PTFE) and other biocompatible polymers have been extensively used for sutures, vascular grafts and bone, and other hard tissue replacements. The use of surgical lasers for intervention on teflon-tissue interfaces has attracted a great deal of interest, as both the high intensity pulsed lasers and prosthetic biomaterials are in increasing use. The study of the ablational behaviour of PTFE films with three surgical lasers (CO2, Nd-YAG and XeCl) have been undertaken for assessing the optimal laser parameters for ablation (e.g. the absorption coefficient and the relevant threshold fluence) from ablation rate measurements.

Ablation of nylon-6,6 with UV and IR lasers

Abstract In this paper we study the basic phenomenology of nylon-6,6 ablation with XeCl, CO2 and Nd:YAG lasers and quantify on its parameters. The material degradation temperature calculated for pulsed laser heating predicts melting of the polymer surface observed by scanning electron microscopy. The results obtained assist in the elucidation of the polymer degradation mechanism at 308 nm which is important for the manufacturing of plastic optical components.

Laser-Induced Fluorescence and Reflectance Spectroscopy for the Discrimination of Basal Cell Carcinoma from the Surrounding Normal Skin Tissue

The object of this study was to investigate whether laser-induced skin autofluorescence (LIF) and/or light reflectance spectra could provide a useful contrast between basal cell carcinoma (BCC) tissues and the surrounding healthy skin. Unstained human skin samples, excised from humans undergoing biopsy examination, were irradiated with a nitrogen laser (λ = 337 nm) for excitation of autofluorescence and a tungsten halogen lamp for the reflectance measurements. The ex vivo spectroscopic results were correlated with the histopathology images to distinguish the areas of BCC from those of the surrounding health skin. A simple spectral analysis technique was also applied for better skin diagnosis. In conclusion, it seems that LIF and reflectance spectra could be used to differentiate neoplastic from normal skin tissue using an appropriate classification model analysis.

The effect of voltage pulse polarity on the performance of a sliding discharge pumped HF laser

The influence of the applied voltage pulse polarity on the performance of a HF laser pumped by a sliding surface discharge is investigated. Details are presented for the polarity dependence of the discharge properties as well as for their relation to the physical processes that affect the sliding discharge evolution. The output energy and efficiency were higher when the initiating or stressed electrode was in relative positive potential, irrespective of which electrode was grounded. The experimental results indicate that for positive polarity the discharge was more uniform, self terminating in type, with reduced electron density and temperature. The increase in output energy and efficiency is attributed to the improvement of the spatial uniformity of laser pumping, to a more efficient utilization of discharge input energy due to an increase in the steady state discharge time phase, as well as to the reduced electron temperature and to the increased steady state voltage which may also result in more efficient laser pumping.

A detailed analysis of the MIG spectrum for the development of laser-based seam tracking sensors

This paper presents a detailed series of measurements of the spectrum of the light emitted from a MIG welding arc. This work was done in the framework of a larger project concerning the development of a seam tracking sensor. Detailed measurements and analysis of the spectrum produced from welding arcs have been performed. The measurements extend from the ultraviolet region of the spectrum (150 nm) to the near infrared region (970 nm) and have revealed the presence of many strong emission lines in the spectrum. The results are of great importance for the design of any optical or vision system working close to a welding arc, because the wavelength of the optical system can be selected to be close to a value where the spectrum of the arc causes the minimum interference to the laser light.

Characteristics of doubling circuits used in gas laser excitation: application to the N/sub 2/ laser

The electrical characteristics of a doubling circuit (or LC-inversion circuit) used to excite a TEA (transversely excited atmospheric) N/sub 2/ laser have been studied, and a theoretical model for the voltage and current oscillations in the circuit is proposed. The theoretical predictions are in excellent agreement with the experimental data, and thus the shape of the voltage and current waveforms in doubling circuits can be predicted from the circuit parameters. With this method, the nonmeasurable quantities of inductance and resistance of the laser gas discharge and their dependence on the electrode gap can be determined. The theoretical treatment described can be easily applied to any type of gas using doubling-circuit excitation, such as excimer lasers. >

Optical rectification and photon drag in n-type gallium phosphide

Optical rectification and the photon-drag effect have been studied in n-type gallium phosphide at various wavelengths between 1.06 and 10.8 mu m. A pronounced peak occurs in both quantities near 3 mu m, corresponding to transitions between the lowest point in the conduction band, near X1, and the minimum at X3. It is shown that the optical rectification is primarily due to free electrons and the spectrum explained in terms of the band structure of GaP proposed by Lawaetz. At this peak, the optical rectification coefficient of n-type GaP containing 2.4*1016 electrons cm-3 is the largest ever observed. The results also represent the first observation of a resonance effect in optical rectification and the first in which free carriers play a major role.

Tea gas lasers excited by a sliding discharge along the surface of a dielectric

Abstract A high current sliding discharge has been used to pump directly a N 2 :He mixture at an overall atmospheric pressure and produce laser output at 0.3371 μm and 0.3577 μm. In addition to that the sliding discharge has been used, to the best of our knowledge for the first time, to pump directly Ar and Xe and produce laser action at 1.79 μm, 1.27 μm and 0.912 μm from Ar:He mixtures and at 1.73 μm and 0.98 μm from Xe:He:Ar mixtures. The performance parameters of these lasers are given and among them their charging voltage, pressure and SF 6 addition dependence.

Evaluation of trapping efficiency of optical tweezers by dielectrophoresis

A relatively new method for measuring optically induced forces on microparticles and cells, different from the conventional Brownian motion and viscous drag force calibration methods widely used, is introduced. It makes use of the phenomenon of dielectrophoresis for the calibration of optical tweezers through the dielectrophoretic force calculations. A pair of microelectrodes is fabricated by photolithography on a microscope slide and it is connected to a high-frequency generator. The calibration of the optical tweezers setup is performed by the manipulation of polystyrene beads and yeast cells. Calibration diagrams of the transverse forces versus power are deduced for different cell radii and numerical apertures of the objective lenses. The optical system and the related technique provide a fast and easy method for optical tweezers calibration.