Classical field theory methods in laser physics. III. Propagation of laser radiation in vacuum outside a resonator

The third Part of our work continues the analysis of the problem considered in Parts I and II [J. Russ. Laser Res.,17, 205 (1996);18, 2 (1997)]. We continue the search for the answer to the question: What is laser radiation from the viewpoint of the classical theory of wave fields? Here, we give one of the possible interpretations of propagation of laser radiation in vacuum outside a resonator from both the nonrelativistic and relativistic points of view. The relation between the metric and the radiation field structure is noted. Moreover, the third Part contains the conclusion concerning the work as a whole. It presents the main observations that may be useful for further studies of the problem.     

Key methods for intensifying soft X-ray emission from a laser plasma for lithography

A two-pulse two-wave (Nd and CO₂ lasers) scheme is proposed for irradiating a laser target, which ensures the highest factor of laser radiation conversion to the X-ray range (13.5 nm ± 1%). Analytic estimates are obtained for parameters of pulses and of the target made of Xe or Li. Numerical optimization is performed for X-ray emission from a spherical Xe target exposed to a CO₂ laser pulse. The maximal factor of conversion of laser radiation to X rays is ～1%. Angular and spectral characteristics of X-ray radiation are obtained. The flux of fast Xe ions ejected from the target and damaging the Mo/Si coating of X-ray mirrors is estimated.     

强场X射线激光物理

相干X光,特别是X射线自由电子激光技术的发展提供了一种新的产生超强光场的途径.由于其较高的光子能量、高峰值功率密度与超短的脉冲长度,有望将强场激光物理从可见光波段推进到X光波段.目前,基于X射线的非线性原子分子物理已取得了初步进展,随着X射线光强的提升,相互作用将进入相对论物理、强场量子电动力学(quantum electrodynamics,QED)物理等领域,为激光驱动加速与辐射、QED真空、暗物质的产生与探测等带来新的科学发现机会.本文对强场X射线激光在固体中的尾场加速、真空极化、轴子的产生与探测等方面进行介绍,旨在阐明X射线波段强场物理在若干基础前沿与关键应用方面的独特优势,并对未来的发展方向进行展望.     

Classical field theory methods in laser physics. I. Radiation field in empty resonators and resonators with inhomogeneous active medium

One of the possible interpretations of laser radiation inside a resonator is given. It is based on the method of normal coordinates. In the course of presentation, the method is gradually complicated. This is accompanied by refinement of the interpretation of laser radiation in a resonator.     

Risk assessment and laser safety

The basic tenets of risk assessment have always been applied in laser safety during the development of safety standards. For example, statistical methods were used in the probit analysis of the threshold of ocular injury; concepts of risk analysis were employed in the development of hazard classes, where the increased risk of exposure and potential for injury from increasing laser output power led to assignment of an increasing hazard class. In recent years, however, there has been a number of attempts to apply statistical probability analysis in the risk assessment of actual use conditions. However, once the hazard classification has been assigned, how should one further apply the techniques of risk assessment in the determination of hazard control measures, or does this lead to a potential controversy of what is the risk? Risk analysis is the evaluation of potentially hazardous exposure conditions coupled with a realistic assessment of actual human exposure. The maximum permissible exposure values for laser radiation coupled with the laser hazards classification scheme, already permit realistic health hazard evaluations. However, in determining effective hazard control measures, one must perform a risk analysis. A risk analysis must consider aspects of human behaviour and how behaviour affects exposure. This is frequently the area of greatest controversy in the derivation of safety standards; however, it is this aspect where standards are most needed.     

Optical methods of laser radar field processing

The book describes modern methods of receiving laser signals, viz., by interferometry, by nonlinear optics, by laser amplification, and by heterodyne mixing. The theory of these methods is considered as applied to the problem of laser radars. Investigated in detail are features such as the resolving power of interferometry and nonlinear-optics method, the efficiency of parametric conversion of laser signals, the sensitivity of laser amplifiers, and problems of optimal heterodyne detection. Methods of speckle interferometry and adaptive optics are described. The book is aimed at a large circle of scientists and engineers active in the problem of recording of laser signals, as well as to students in advanced courses of the corresponding specialties.N. D. Ustinov — General Editor.Translated from Metody Obrabotki Opticheskikh Polei v Lazernoi Lokatsii, pp. 3–269, 1983.     

Fast microstructuring of silica glasses surface by NIR laser radiation

The glass surface microstructuring technology using laser radiation with NIR wavelength (λ=1.064 μm) was revealed in this work. Glass plates were placed on the cellular graphite surface. Focused laser radiation passed through the glass plate and interacted with cellular graphite. The radiation heated the graphite surface and thus the high temperature influenced the back side of the glass plate. After consecutive laser scans, having certain periods and interruptions of laser radiation, the microstructures with depth ~0.5 μm were formed. Besides, in this work we suggested the method to calculate optical characteristics of formed elements. It was experimentally shown that these microstructures could be used to form phase diffraction gratings (PDGs) and random phase plates (RPPs). We experimentally demonstrated the possibility of these elements being used as RPPs which are suitable for multimode laser radiation homogenization and as PDGs which are suitable for laser simultaneous processing of metal films.     

Development and characterization of Yb-Er laser glass for high average power laser diode pumping

A new strong erbium laser glass (SELG) based on a boro-alumo-phosphate composition is reported. We discuss the synthesis and chemical properties together with spectroscopic and thermo-mechanical data. The new glass composition shows excellent laser performance and withstands high-average power pump radiation. We present laser results at 1.54 μm from flashlamp and laser pumping. In tests with laser-diode pumped Q-switched Er-Yb microchip lasers, we have achieved up to 150 mW of average output power and generated 1.2 kW in peak power. Co²⁺:MgAl₂O₄ was here used as the saturable absorber. Received: 21 December 2001 / Revised version: 14 April 2002 / Published online: 8 August 2002     

Multiphoton detectors of laser radiation

The paper is devoted to the problem of using multiphoton detectors to measure the statistical characteristics of multifrequency laser radiation affecting the nonlinear interaction between radiation and matter. Section 1 gives the characteristics of laser radiation determining the probability of power-type nonlinear interaction with a medium. Section 2 briefly characterizes multiphoton detectors. Sections 3–6 are devoted to the problems of measuring the so-called statistical radiation factor by means of multiphoton detectors and to the analysis of the corresponding experimental data. The possibility of using multiphoton detectors to measure other radiation characteristics is discussed in Sections 7 and 8.Based on material presented at the first All-Union Conference on the Problems of Laser Radiation Parameter Control, Tashkent, 1978.     

Direct laser patterning of self-assembled monolayer using elliptical laser beams: A theoretical parametric study

A theoretical quantitative analysis of processing parameters for application of an elliptical laser beam to achieve maximum patterning area is the focus of this study. Direct laser patterning (DLP) of self-assembled monolayers (SAM) is achieved by localized heating of the sample above the SAM desorption temperature. Through use of elliptical laser beams in the present work, three goals are achieved by analyzing the heat diffusion model and related thermo-kinetics model: (1) optimal working conditions (combination of laser power, scanning velocity and aspect ratio) for DLP to produce maximum feature size, or highest processing velocity at a given power; (2) identification of conditions that reduces the potential thermal damage to the substrate; (3) shedding light on issues related to uniformity or homogeneity of heating a substrate using an elliptical laser beam. A heat diffusion model is employed to provide the resulting surface temperature caused by elliptical laser beams, and the coupled thermo-kinetics model is used to determine the final SAM coverage generated by DLP. Parametric analysis revealed that 70–150 mW can be used to pattern feature sizes in the range of 2–10 times of equivalent circular beam size. It is also found that each elliptical laser beam has a unique optimal aspect ratio to result in the widest feature size for a given laser power and scanning velocity. The edge transition width increases with an increase of the aspect ratio. Keeping the aspect ratio of elliptical laser beam small (i.e. β<20), a sharp edge definition could be obtained; if an aspect ratio larger than 30 is used, a surface with gradual edge definition could be obtained.     

XRD and XPS analysis of laser treated vanadium oxide thin films

In this work, we present X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis of laser treated vanadium oxide sols. The films were also observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to reveal how the original xerogel structure changes into irregular shaped, layer structured V₂O₅ due to the laser radiation. XRD revealed that above 102 W/cm² the original xerogel structure disappears and above 129 W/cm² the films become totally polycrystalline with an orthorhombic structure. XPS spectra showed O/V ratio increment by using higher laser intensities.     

Inverse problem method in laser physics

Applications of the method of ill-posed inverse problems in laser physics are considered. Means of calculating the level cross sections of physical-chemical reactions with subsequent calculation of the reaction rate constants are developed. Mathematical and numerical aspects for the realization of ill-posed inverse problem are considered in detail using the concrete example of level cross sections of dissociation.     

Nonlinear optical laser devices using GaSe

Tunable second-harmonic generation of CO₂ laser radiation and tunable upconversion of infrared radiation from CO₂-laser radiation in a GaSe crystal are demonstrated. The capability of low infrared-signal detection in this crystal is also explored and compared with the AgGaS₂ crystal.     

A primer for black hole quantum physics

The mechanisms which give rise to Hawking radiation are revealed by analyzing in detail pair production in the presence of horizons. In preparation for the black hole problem, three preparatory problems are dwelt with at length: pair production in an external electric field, thermalization of a uniformly accelerated detector and accelerated mirrors. In the light of these examples, the black hole evaporation problem is then presented.

The leitmotif is the singular behavior of modes on the horizon which gives rise to a steady rate of production. Special emphasis is put on how each produced particle contributes to the mean albeit arising from a particular vacuum fluctuation. It is the mean which drives the semiclassical back reaction. This aspect is analyzed in more detail than heretofore and in particular its drawbacks are emphasized. It is the semiclassical theory which gives rise to Hawking's famous equation for the loss of mass of the black hole due to evaporation dM/dt − −1/M². Black hole thermodynamics is derived from the evaporation process whereupon the reservoir character of the black hole is manifest. The relation to the thermodynamics of the eternal black hole through the Hartle-Hawking vacuum and the Killing identity are displayed.

It is through the analysis of the fluctuations of the field configurations which give rise to a particular Hawking photon that the dubious character of the semiclassical theory is manifest. The present frontier of research revolves around this problem and is principally concerned with the fact that one calls upon energy scales that are greater than Planckian and the possibility of a non unitary evolution as well. These last subjects are presented in qualitative fashion only, so that this review stops at the threshold of quantum gravity.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

Soot-visualization strategies using laser techniques

Strategies for spatially resolved soot volume-fraction measurements have been investigated in sooting laboratory flames with known soot characteristics. Two techniques were compared: Laser-Induced Fluorescence in C₂ from Laser-Vaporized Soot (LIF(C₂)LVS), and Laser-Induced Incandescence of soot (LII). The LII signal is the increased temperature radiation from soot particles which have been heated to temperatures of several thousand degrees as a consequence of absorption of laser radiation. The LIF(C₂)LVS technique is based on the production of C₂ radicals from laser-vaporized soot which occurs for laser intensities ≥10⁷ W/cm². A laser wavelength is chosen such that besides vaporizizng the soot, it also excites the C₂ radicals, and the subsequent C₂ fluorescence signal is detected. The signals from both techniques showed good correlation with soot volume fractions in the studied flame. The dependence of the signals on experimental parameters was studied, and the influence of interfering radiation, such as background flame luminosity and fluorescence from polyaromatic hydrocarbons, on studied signals was established. The potential of the two techniques for imaging of soot volume fractions in laboratory flames was demonstrated. Advantages and disadvantages of the studied techniques are discussed.     

Glass modification by continuous-wave laser backside irradiation (CW-LBI)

We report on a permanent change in the physical properties inside glass that is rapidly heated and quenched with a continuous-wave (CW) laser beam. The absorption of the glass was enhanced by laser heating, and the heated spot moved by thermal radiation and conduction. To trigger the heating, an absorbent material was placed on the backside of a glass plate and irradiated through the glass. The laser beam can modify borosilicate glass with a high aspect ratio (∼100:1) at a rate of ∼130 mm/s. The modified zone consists of two concentric cylindrical zones and is crack-free.     

Photochemical xenon-oxide laser

Theoretical and experimental investigations are described, aimed at the develo pment, construction, and study of the features of a photochemical xenon-oxide laser whose action is based on the photodecomposition of nitrous oxide by vacuum ultraviolet radiation of an open hlgh-current discharge with formation of oxygen atoms followed by oxidation of the xenon. The theoretical problems connected with the determination of the mechanism and the calculation of the kinetics of the physicochemical processes in an XeO laser are considered. A numerical computation of the pumping is carried out, and the inversion and gain on the laser transition are calculated. The construction of the laser is described, as well as an experimental procedure for obtaining the optimal temperatures, composition, and pressure of the working-mixture components. The spectral composition of the laser radiation and the energy characteristics of the laser are investigated. The heat released in the course of the photolysis, the constants of the quenching rates of the upper laser state of XeO (2¹Σ⁺), and the vibrational relaxation of the lower state of XeO (1¹Σ⁺) are estimated. The internal losses in the active medium of the photochemical XeO laser are theoretically investigated. The possibility of increasing the volume of the active medium using a plane-parallel cavity is estimated.     

Pumping of titanium sapphire laser

Two methods of Ti:Sapphire pumping for the generation of tunable laser radiation in the visible region were studied. For coherent pumping, the radiation of the second harmonic of a Nd:YAP laser was used and a maximum output energy ofE _out=4.5 mJ was reached from the Ti:Sapphire laser. For noncoherent pumping, two different lengths of flashlamp pulses were used and a maximum ofE _out=300 mJ was obtained. Preliminary estimations of the wavelength range of tunability were made.     

Classical theory of the molecule alignment in a laser field

In the classical mechanics framework the solution is given for the problem of the two-atomic molecule alignment in a field of linearly polarised laser pulse. The numerical results are presented for the Cl₂ molecule, together with the analytical results in the limit of a very short and strong laser pulse. The width of the angular distribution for molecular axes in field is demonstrated to depend on the radiation intensity as S ^?1/4, the alignment along the polarisation direction is possible as well as in the transversal direction. The analysis of some experimental data is given.