Thomas P. Seward

193-nm excimer-laser-induced densification of fused silica

We report the densification of fused silica as a function of exposure to pulsed 193-nm excimer-laser irradiation. Defining a dose as the number of pulses N times the square of fluence I per pulse, we find that densification follows a universal function of dose, a x (NI(2))(b), where a and b can vary somewhat according to glass preparation. Densification is measured with interferometry and birefringence, interpreted with a finiteelement elastic model. Wave-front distortion for a typical photolithographic lens element in typical use conditions is described.

Elongation and spheroidization of phase-separated particles in glass

Abstract When certain phase-separated glasses are elongated by redrawing near their softening point, the spherical second-phase particles are found to elongate as well. The elongation process is discussed, and the conditions under which high particle elongations are possible are estimated. Annealing the glass tends to spheroidize cylindrical-shaped particles by an ‘ovulation’ process. The spacing of the resulting spheroidal particles is compared with theoretical predictions.

Densification of synthetic fused silica under ultraviolet irradiation

Fused silica undergoes densification upon prolonged exposure to high energy radiation and explanations in terms of changes in the silica tetrahedral ring structure have been offered. Under laser irradiation, the densification is often less than 10 parts per million, necessitating sensitive optical techniques for its measurement. This paper reviews some of the key literature, with emphasis on recent studies involving 193 nm excimer laser irradiation, and presents recent evidence that the UV laser-induced densification involves optically-induced weakening of bonds, via a two-photon absorption process, and subsequent structural relaxation of the glass.

Effects of glass forming conditions on the KrF-excimer-laser-induced optical damage in synthetic fused silica

Abstract The optical damage induced by exposure to KrF (248 nm) excimer laser light was examined in synthetic fused silicas made under a variety of processing conditions. The SiCl 4 -based glass exhibited increased UV absorption, increased red fluorescence and compaction after prolonged 248 nm exposure. The induced damage was most severe in glasses made with oxidizing deposition flames or in glasses with reduced levels of molecular hydrogen. The damage did not correlate with the OH level. Fused silica produced using a hydrogen/oxygen flame showed the same damage resistance as glass made with a natural gas/oxygen flame. Fused silica made with the chlorine-free precursor, octamethylcyclotetrasiloxane (OMCTS), exhibited the same general damage behavior as that seen in SiCl 4 -based glass except that the absorption damage shifted to a saturated condition without passing through the absorption transition prevalent in the SiCl 4 -based glass.

Thermally darkenable photochromic glasses

Thermally darkenable photochromic (TDPC) glasses have been produced by precipitating copper−doped silver halide crystallites in certain lanthanum borate base glasses. A TDPC glass is characterized by a darkened state which is stable at room temperature but which can be bleached by visible light. The glass will return to the darkened state with the passage of time. The rate of darkening and the color of the darkened state are affected by temperature. These TDPC glasses are further characterized by the vanishing of the darkened color above a certain high temperature and redarkening of the glass below a certain somewhat lower temperature. These temperatures are believed associated with the melting and crystallization of the silver halide particles.

Interpretation of induced color in polychromatic glasses

The variable colors produced in polychromatic glasses by an optical/thermal treatment are analyzed in terms of a specific geometric model. The calculations, based on small-particle scattering theory, involve the specific shape of the silver on the sodium halide microcrystal that is formed in the process and account for the color shift with the optical/thermal treatment. In addition, absorption calculations were made to explain the formation of the microcrystalline phase.

Photoinduced optical anisotropy and color adaptation in silver‐containing glasses

This paper reports photoinduced optical anisotropy (dichromism and birefringence) and color adaptation in oxide glasses containing silver and halogens. The effects are found in both photochromic and fixed‐tint glasses. In certain of the photochromic glasses there is memory, such that after thermal fading redarkening by uv radiation reestablishes the anisotropic or color‐adapted state. Dichroic ratios of 4 and birefringences of Δn=10−4 have been measured. The effects, similar to the Herschel and Weigert effects in silver halide photography, are explained on the basis of optical bleaching of anisotropically shaped submicroscopic aggregates of silver in contact with a silver halide.

Lightguiding in photochromic glasses.

Lightguiding in ion-exchanged photochromic glass layers is reported. The guides were formed by ion exchange of Ag(+) for Na(+) or by outdiffusion of F. Guide attenuation was less than 0.2 dB/cm in many cases. The attenuation in the guide was varied over a range of Deltaalpha = 30 dB/cm by illuminating the guide with (1) uv radiation to darken the guide or order of seconds. (2) visible radiation to bleach the guide. Switching times were in the order of seconds.

Glass polarizers containing silver

There are at least two general categories of absorptive-type glass polarizers which involve silver metal in their polarizing mechanism. Glasses of the first category, permanent polarizers, contain elongated, submicroscopic particles of silver metal aligned along a common axis. Their polarizing capacity is due to a resonant absorption of the silver conduction electrons, the spectral location of which depends on the extent of silver particle elongation and the particle orientation with respect to the applied electromagnetic (light) field. These polarizers have been made with dichroic ratios (optical density ratios) in excess of 40. The best performance is obtained in the long wavelength visible and near infrared regions. They are useful in applications similar to those of plastic sheet polari-zers. They have better optical quality and better thermal, mechanical, and chemical durability. Polarizing glasses of the second category are optically alterable. That is, their color and degree of polarization can be changed by intense visible or near infrared irradiation. These glasses owe their polarizing properties to small, nonspherical specks of silver metal in contact with silver halide crystallites within the glass. The change in color and polarization is believed due to the growth and dissolution of differently shaped and oriented specks during irradiation. This second category of glass polarizer shows much lower dichroic ratios than the first, generally less than 5, but has the advantage that the orientation of the polarizing axis can be changed, or made to vary from one region of the glass to another, by suitable high intensity irradiation.

Compaction of fused silica under low-fluence/long-term 193-nm irradiation

The compaction (densification) of fused silica under low fluence (< 1 mJ/cm2/pulse), long term (hundreds of millions of pulses) 193 nm irradiation has been studied. With the use of a finite element analysis, the unconstrained densification, (delta) (rho) /(rho) , is extracted from the experimentally determined wavefront distortion. We find that the densification of silica in the low fluence exposure regime corresponds to what is predicted from the behavior described by: (delta) (rho) /(rho) equals 0.000117 (NI2)0.53 where N equals number of pulses, I equals intensity (mJ/cm2/pulse). Results are presented of high and low intensity studies that establish the utility of the above form as a predictive tool for densification in fused silica.