Transparent Optical Materials Research Articles

Special Issue: Nonlinear Optics

Before the advent of lasers, transparent optical materials were assumed to be essentially passive, unaffected by light traveling through them. The high powers of laser beams made it possible for the first time to observe that the presence of light can indeed affect the medium. Intense light can, for example, change the refractive index or absorption of an optical material.

Automated spatially scanning ellipsometer for retardation measurements of transparent materials

A highly sensitive and automated technique has been developed for measuring the birefringence in transparent optical materials. The spatially scanning modulated transmission ellipsometer maps the birefringence of a transparent material by probing it with a polarization-modulated He-Ne laser beam. Computer-controlled voltage biasing of a Pockels cell permits self-calibration and background subtraction of the system retardance. The technique is capable of resolving differential retardances as small as 0.1 nm (λ/6328) through a range of ±λ/2, where λ = 632.8 nm. Samples typically range in size from 50 µm to 10 cm in diameter within the sample plane and as much as 400 mm along the optical axis.

A versatile low-cost Czochralski crystal growth system for nonlinear optical organic materials

A versatile low-cost Czochralski system for pulling crystals from melt has been described. It is designed for low melting, transparent, and nonlinear optical materials. One of the most important novel feature of this crystal growth system is that the entire growth process including the solid–liquid interface can be viewed from any direction. Another is the use of an after-heater to reduce excess heat loss from the surface of the melt.

Photolithography at 193 nm

Photolithography at 193 nm is a natural continuation of the progression from 436 to 365 to 248 nm in lithography, dictated by the requirement for continually higher resolution. It is anticipated that 193-nm lithography will enable 0.25-μm patterning in volume production with conventional masks, and 0.18-μm resolution with phase-shifting masks. The main issues related to lithography at this new wavelength are being addressed. It has been shown that highly transparent optical materials are available at 193 nm. Also, they are damaged by the laser radiation at a slow enough rate that high-quality projection optics are expected to perform within specifications for ten years of full-time operation. Consequently, a 193-nm step-and-scan system is being constructed, and it has been designed to attain 0.25-μm resolution over a 22 by 35 mm field. A range of 193-nm photoresist schemes has been demonstrated. They include semitransparent single-layer resists, positive-tone surface imaging (silylation), and negative-tone bilayers using ultrathin silicon-based polymers. In most instances we have demonstrated sub-0.25-μm resolution, high photosensitivity, good exposure–defocus latitude, and very low levels of etch residue. In sum, the first successful steps towards a fully engineered 193-nm photolithography have been taken, and no major obstacles are anticipated.

Brewster and Pseudo-Brewster Angle Technique for Determination of Optical Constants

The analytic solution of the pseudo-Brewster angle, for the case of electromagnetic waves incident from a transparent optical material onto an absorbing material, has been obtained. We have developed a unique measurement method, called the pseudo-Brewster angle method, for obtaining the optical constants of thin films. By this method, the reflected intensity is measured at any angle of incidence. The optical constants are calculated from the pseudo-Brewster angle at the minimum value of the reflection coefficient. As a result, the pseudo-Brewster angle method is very useful for measuring the optical constants of a thin film.

Laser-Induced Damage And The Role Of Self-Focusing

We review the influence of self-focusing on the measurement of bulk laser-induced-damage (LID) thresholds in normally transparent optical mate-rials. This role is experimentally determined by measuring the spot size and polarization dependence of LID and by observing beam distortion in the far field. Utilizing these techniques, we find that by using a tight focusing geometry in which the breakdown power is below P2, the effects of self-focusing can be practically eliminated in an LID experiment. P2 is the so-called second critical power for self-focusing, and P2 = 3.77P1, where P1 = cX2/327r2n2, where c is the speed of light in vacuum, X is the laser wavelength and n2 is the nonlinear index of refraction. This is in accordance with numerical calculations by J. H. Marburger [in Progress in Quantum Electronics, J. H. Sanders and S. Sten-holm, eds., Vol. 4, Part 1, pp. 35-110, Pergamon, Oxford (1975)]. With this knowledge we determine that damage is only partially explained by avalanche ionization and that the initiation of damage is strongly influenced by extrinsic processes. We therefore conclude that we are measuring extrinsic LID.

Refractive index determination using an orthogonalized dispersion equation

An orthogonal form of a general three-term dispersion equation is presented which is useful for determination of the refractive index of transparent optical materials from measured spectral data. The orthogonal basis functions are dependent on the spectral range of the data and whether the spacing of the data is uniform with respect to wavelength or wavenumber. The application of the technique to the reduction of ellipsometric measurements of a single layer thin film of zirconia on a fused silica substrate is presented. The measured data were fit to a Conrady dispersion equation and to the corresponding orthogonal basis set. The convergence to the solution was an order of magnitude faster for the orthogonal form of the dispersion equation.

Numerical simulation of laser damage to an optical material with defects

Laser damage was simulated numerically for two different defect distribution functions in terms of the breakdown initiation thresholds in order to study the statistical dependences. Various methods of obtaining dependences of the breakdown probability and its density on the laser radiation intensity are discussed. A numerical experiment is used to show that the method of the inverse problem of the laser breakdown statistics can be used for the diagnostics of microinclusions initiating damage in transparent optical materials.

Pulsed photoacoustic technique to measure the absorption coefficients of the highly transparent optical materials

A new technique of measuring the low optical absorption coefficient is described by using pulsed photoacoustic calorimetry. The frequency dependence of the photoacoustic signal is different owing to its main origin, either the bulk absorption or the surface absorption. The Fourier transform of the pulsed photoacoustic signal, generated by a pulsed CO 2 laser, is obtained as the frequency response of the photoacoustic signal. The ratio of the bulk absorption coefficient and surface absorption coefficient is acquired as 1:1.5.

Effect of UV irradiation on evaporated ZnS films

Evaporated ZnS films used as a component in reflectance enhancing or decreasing multilayer coatings for mirrors, transparent optical materials, and vacuum UV reflecting optics, are investigated with reference to the reflectance loss under UV irradiation and the formation of ZnO as a result of the decomposition of ZnS in the presence of oxygen. Reflectance measurements over a broad wavelength range reveal that the UV induced reflectance losses are generally restricted to wavelengths shorter than 4000 A and are most severe in the vacuum UV region. After 134 hr of UV exposure initially polycrystalline film of 150-200 A thick is completely converted to amorphous ZnO. The results demonstrate that caution should be exercised in employing ZnS as the outer layer in an optical coating system designed for use in the UV and vacuum UV regions.

Laser induced damage in optical materials: eleventh ASTM symposium

The eleventh Symposium on Optical Materials for High-Power Lasers (Boulder Damage Symposium) was held at the National Bureau of Standards in Boulder, Colorado, 30-31 October 1979. The symposium was held under the auspices of ASTM Committee F-1, Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced Research Projects Agency, the Department of Energy, and the Office of Naval Research. About 150 scientists attended the symposium, including representatives of the United Kingdom, France, Canada, Japan, West Germany, and Denmark. The symposium was divided into sessions concerning transparent optical materials and the measurement of their properties, mirrors and surfaces, thin film characteristics, thin film damage, considerations for high-power systems, and finally theory and breakdown. As in previous years, the emphasis of the papers presented at the symposium was directed toward new frontiers and new developments. Particular emphasis was given to materials for high-power apparatus. The wavelength range of prime interest was from 10.6 microm to the UV region. Highlights included surface characterization, thin film-substrate boundaries, and advances in fundamental laser-matter threshold interactions and mechanisms. The scaling of damage thresholds with pulse duration, focal area, and wavelength was discussed in detail. Harold E. Bennett of the Naval Weapons Center, Alexander J. Glass of the Lawrence Livermore Laboratory, Arthur H. Guenther of the Air Force Weapons Laboratory, and Brian E. Newnam of the Los Alamos Scientific Laboratory were cochairpersons. The twelfth annual symposium is scheduled for 30 September-1 October 1980 at the National Bureau of Standards, Boulder, Colorado.

Laser induced lattice strains and damage threshold limitation due to lattice defects in transparent optical materials

The interaction of a high power laser beam with the lattice of transparent optical materials induces internal strains in the latter. Attention is focused on optical materials of zinc blende structure and excitation at the 10.6-microm wavelength of the laser. A lattice dynamical technique, which utilizes the imperfect lattice Green's functions, is used to calculate the expected magnitude of the actual strains induced in real crystals taking into account the effect of lattice point defects. The information on the elastic strength of these materials then enables the quantitative evaluation of laser induced mechanical damage thresholds in very good agreement with the available state-of-the-art figures.

Refractive-index dependence of pulsed-laser-induced damage

A parametric study of the threshold for pulsed-laser-induced damage with refractive index for surfaces and thin films of several transparent optical materials is reported. An empirical analysis of the experimental results obtained using 1.06-μm radiation in ~40-nsec pulses, together with other published data, yields, to first order, a 1/(n2 − 1) dependence for the threshold optical electric field. The proposed scaling law describes quite well the variation of damage threshold for optical surfaces ranging in refractive index from 1.38 to 2.49. Similar agreement is noted for homogeneous thin films deposited on a fused silica substrate; however, deviations are noted for inhomogeneous films.

Laser-induced electric breakdown in solids

A review is given of recent experimental results on laser-induced electric breakdown in transparent optical solid materials. A fundamental breakdown threshold exists characteristic for each material. The threshold is determined by the same physical process as dc breakdown, namely, avalanche ionization. The dependence of the threshold on laser pulse duration and frequency is consistent with this process. The implication of this breakdown mechanism for laser bulk and surface damage to optical components is discussed. It also determines physical properties of self-focused filaments.