Constantine Skordoulis

Amplification characteristics of a discharge excited F2 laser

Abstract The small signal gain coefficient and the saturation intensity of a molecular-fluorine pulsed-discharge laser at 157 nm have been measured using two discharge devices in the oscillator-amplifier configuration. At 3 atm total gas pressure and 1·5 cm electrode spacing the small signal gain coefficient and the saturation intensity of the 3Π2g → 3ΠI2u transition were measured to be 5·2% cm−1 and 5 MW cm−2 respectively.

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.

What Conceptions do Greek School Students Form about Biological Evolution

In Greece, since 2000, the teaching of evolutionary theory is restricted solely to lower (junior) high school and specifically to ninth grade. Even though the theory of evolution is included to the 12th grade biology textbook, it is not taught in Greek upper (senior) high schools. This study presents research conducted on the conceptions of Greek students regarding issues set out in the theory of evolution after the formal completion of the teaching of the theory. The sample comprised 411 10th grade students from 12 different schools. The research results show that the students appear to have a positive view of the idea of evolution, the evolution of man, and the common origin of organisms. However, they have retained many alternative views, or else they are completely in ignorance of basic issues in evolutionary theory regarding: what is considered evolution in biology, the main mechanism of evolutionary changes in what is considered natural selection, what the theory of evolution actually explains, and what the word theory means in science. At least in Greece, these views still prevail because the theory of evolution is marginalized in the teaching of biology in Greek schools, and biology education does not help students formulate overall conceptual structures to enable them to understand the question of biological change.

The Inability of Primary School to Introduce Children to the Theory of Biological Evolution

A great number of research papers in the English literature of science education present difficulties pupils have in understanding natural selection. Studies show that children have essentialist and teleological intuitive ideas when dealing with organisms and that these biases hinder their ability to understand the theory of evolution by natural selection. Consequently, it is interesting to ascertain if and how the school education offered today deals with the problem, i.e., helps the children confront these biases. To that purpose, this study answered the two following research questions: (a) How is biological evolution presented—from the past to the present day—in the official documentation of primary school education, namely the science curricula and the textbooks of Greece? and (b) what are the conceptions held by Greek primary school teachers of the concepts of evolutionary theory and relevant issues that they have to teach? Our research found that not only are the intuitive ideas not “confronted” but they are also “affirmed” in Greek primary education. This phenomenon, as some other international studies have shown, must not be only a Greek one. A drastic change in the content and structure of primary school curricula and the training of educators is necessary in order to improve and facilitate the teaching of biological evolution.

Laser induced fluorescence (LIF) of Hg2 and Hg3 via dissociation of HgBr2 at 157 nm

Laser induced fluorescence of the mercury clusters Hg2 and Hg3 in the spectral range between 300 nm to 510 nm has been obtained from the dissociation of HgBr2 at 7.88 eV (157.5 nm) with an F2 molecular laser, together with fluorescence from mercury atomic transitions from highly excited states. The excitation process involves two photon absorption which dissociates the molecule at 15.76 eV total photon energy with the subsequent formation of the metallic clusters.

VIS and UV laser ablation of polymers

Pulsed laser ablation is well established as a universal tool for surface processing of organic polymer materials. The polymer ablation efficiency of different laser wavelengths, from 355 nm to 532 nm, is studied for short laser pulses, of nanosecond pulse duration. Ablation rates of polymers have been measured by irradiating the polymers using a pulsed Nd:YAG laser. This laser source produced pulses at the second and third harmonic simultaneously at (lambda) equals 532 nm and (lambda) equals 355 nm. Effective ablation was also observed by irradiating the polymer samples with the Raman-shifted Nd:YAG laser, at (lambda) equals 397.1 nm, in order to investigate comparatively the ablation efficiency at different laser settings. For transparent materials, in the corresponding ablation wavelengths and pulse width, the dependence of the ablation rate on the laser spot diameter and the laser energy fluence has been investigated. In general, the polymers used in this work show a more or less pronounced increase of the ablation rate with decreasing spot diameter for the same fluence, especially in the case of visible laser irradiation.

A unit on deterministic chaos for student teachers

A unit aiming to introduce pre-service teachers of primary education to the limited predictability of deterministic chaotic systems is presented. The unit is based on a commercial chaotic pendulum system connected with a data acquisition interface. The capabilities and difficulties in understanding the notion of limited predictability of 18 pre-service teachers have been investigated, using a teaching experiment design. Our results show that students could be guided to significant insight into the limited predictability of deterministic chaotic systems.

XeCl laser ablation of biocompatible PTFE studied by photothermal beam deflection.

Laser beam deflection is a useful tool for studying the dynamics of the pulsed laser ablation mechanism, and has the advantage of being used as an in situ laser damage monitoring technique for both bulk and thin film polymeric and biological tissue samples. This work employs the photothermal deflection technique to study possible photo-acoustic phenomena during the excimer laser ablation of biocompatible polytetrafluoroethylene (PTFE) and to examine the crater surface quality by video microscopy.Although photo-acoustic phenomena are detected in the laser fluence threshold region, ablation is mainly the result of the photothermal process.