Coloration and optical anisotropy in silver-containing glasses

Journal of Non-CrystallineSolids 40 (1980) 499-513 © North-Holland Publishing Company

COLORATION AND OPTICAL ANISOTROPY IN SILVER-CONTAINING GLASSES T.P. SEWARD, III Research and Development Laboratories, Coming Glass Works, Coming, New York 14830 US4

1. Introduction Silver has been used as a colorant in glass since ancient times (1-3). Generally, it is added to the glass batch like other metal oxide ingredients or by ionic diffusion, from a suitable silver-ion-containing bath or paste, into sub-surface layers of the final glass product in exchange for alkali ions [3]. Silver ions do not absorb in the visible portion of the electromagnetic spectrum. Likewise, reduced (atomic) silver does not color the glass, provided it remains in solution. However its presence can be detected by visible fluorescence under UV irradiation [4]. If the silver atoms are caused to coagulate or precipitate as a colloidal suspension of silver metal particles in the glass, light absorption results. The color and appearance of the glass depends on the size, shape and number of the precipitate particles. If the particles are small (less than about 20 nm diameter) the glasses tend to be transparent (very little haze or light scattering) and are colored yellow, brown or red-brown. If the particles are larger in size, light scattering by the particles can become of comparable magnitude with their absorption. Dull, opaque olive or gray colors generally result, although so can opaque yellows, browns, greens and blues. Thin sections of such glasses still generally appear brown or red-brown in transmitted light. Opaque or translucent silver-containing glasses have found occasional use in glass art objects. In the past, silver, as a glass coloring agent, has perhaps been most used as a yellow stain for glass decoration or in stained glass windows [3,5]. Currently, silver is used as the colorant in several technologically and scientifically interesting glasses. These include certain photosensitive and photochromic glasses and the recently announced polychromatic [6] and photo-adaptive colored, dichroic, birefringent [7] glasses. These glasses will be defined and reviewed later in this paper. For reasons of space and clarity, the review concentrates on transparent glasses, the colors of which are essentially the same whether viewed in reflected or transmitted light. This is not an overly restrictive approach, since the transparent 499

500

T.P. Seward, III / Coloration and optical anisotropy

colored glasses are technologically the most interesting and the most investigated. gated.

2. Photosensitive glasses The coloration in silver-containing glasses is generally controlled by adding to the batch a silver salt or oxide and oxides of certain polyvalent ions such as tin, antimony, arsenic, bismuth, lead, iron, selemium, tellurium, etc. [8,9]. These oxides play the role of thermo-reducing agents. The melting conditions are adjusted so that much of the polyvalent ion, for example tin, is reduced to its lower oxidation state, Sn 2+, while the silver remains oxidized as Ag÷. On lowering the temperature of the glass, the oxidation-reduction (redox) equilibrium shifts and the thermo-reducing agent tends to oxidize at the expense of the ionic silver. For the silver-tin combination this shift usually occurrs at temperatures below about 900°C, according to the equation 2Ag+ + Sn 2+ "-~2Ag ° + Sn 4+ .

(1)

If the temperature of the glass is held between about the thermo-reduction temperatures and the anneal point, the reduced silver atoms can aggregate and grow particles of silver metal, producing a colored glass. It was learned that the precipitation of colloidal copper [10,11] and silver [12] from glass could be enhanced by exposure to short wavelength radiation prior to, or simultaneously with, the precipitation heat treatment. Stookey and Armistead successfully developed this photosensitization process for copper-[13], gold-[14] and silver-J15] containing glasses to the degree that resolution and contrast sufficient for good photographic reproduction was possible. An essential element in their photosensitization process is polyvalent cerium. If the melting conditions are adjusted so that Ag÷ and Ce a÷ are present when the glass is cooled to low temperatures (anneal point or below), irradiation at wavelengths within the UV absorption band of Ce a÷ will photosensitize the glass [16]. The following type of reaction was proposed [17]: Ag+ + Ce 3+ + hu ~ Ago + Ce 4+.

(2)

Photosensitization, in general, involves the reduction of silver (or some other metal) by electrons which are photo-released from some ionic site in the glass by short wavelength radiation (UV, X-ray, "r-ray, etc). In the case of glasses containing cerium, Ce 3+ is the source of the electrons. Whether the electron directly reduces the silver, which then migrates atomically to the precipitation site, or the electron is trapped at a future precipitation site and is subsequently neutralized by the migrating silver ion is still in question. The kinetics of photosensitized gold colloid precipitation in glass have been studied [18,19], but no similar work has been reported for silver.

T.P. Seward, 111/ Coloration and optical anisotropy

501

There are other types of photosensitive glass, all of which require, as the initial pattern determining step, the photosensitized precipitation of colloidal metal; gold, silver or copper. They are briefly mentioned here to provide background for the discussion of polychromatic glass to follow in section 4. The metal colloid acts as nucleation sites for subsequent phase changes in the glass. For example, an opalizing phase such as sodium fluoride may be precipitated to give a controlled opal pattern within the glass [ 17]. Also, chemical machining of glass is made possible by the photosensitized precipitation of lithium disilicate crystals from lithium silicate based glasses [20]. Several authors have used photosensitive glasses as systems in which to study the optical properties of small metal particles; gold [21] and silver [22-24]. Their theoretical treatments follow the same general approach as used in the next section.

3. Absorption band calculations The amount of light absorbed by particles suspended in glass may be calculated from light scattering theory [25]. For the case of isolated (non-interacting) absorbing particles with dimensions much smaller than a wavelength of light, the absorption cross section per particle may be written in terms of the complex dielectric constant, ec = c1 + ie2, of the particle: 27rVN 3

CABS - L2X

C2

[e, + N ~ ( I / L - 1)12 +e~'

(3)

where V is the volume of the particle, No is the refractive index of the glass matrix, 5` is the wavelength in the free space, and L is the electric depolarization factor appropriate for the particle geometry and orientation with respect to the applied electric (light) field. If one approximates the particle geometry by a general ellipsoid, the L values can be determined analytically [25,26]. The complex dielectric constant for silver metal can be writtent in the approximate form CC = CO -- A ~ 2 + i B 5 ` 3 .

(4)

The constants A and B can be approximated by calculations from the free-electron theory [27]. Alternatively, they can be fitted to experimental data for buLk silver. The author and co-workers [28] have taken the latter approach and found a good fit to the combined data of refs. [29-34] to be 61b = 5 -- 555` 2 ,

(5a)

e2b = 0.06 + 275` exp(--29 5`2) + 1.65,3 ,

(5b)

where X is expressed in #m and the subscript b refers to bulk data. The first terms in elb and e2b can be interpreted as the wavelength-independent

T.P. Seward, III / Coloration and optical anisotropy

502

contribution of bound electrons; the second term in %, important only in the ultraviolet, represents the onset of interband transitions; and the last term in each is the free-electron contribution. When the particle size is smaller than the electron mean-free-path in the metal, a correction to the imaginary part of the dielectric constant is necessary. There are a number of approaches to this correction (e.g. refs. [22,23,35]) but they all assume an equivalent form: 6 2 = e2b +

C/a,

(6)

where a is the radius of the metal particle. This author favors the Kawabata-Kubo [35] form of the correction which is e2 = %8 + (2.7/a)g(u),

(7)

where g(~) is a quantum mechanically determined function defined by eq. (4.13) of ref. [35]. For the purpose of the present review, this particle size correction can be ignored. Substitution of eq. (4) into eq. (3) gives

2rrVN 3 CABS -

L2

[% - A X 2

BX 2 +N~(1/L-

1)12 +B2X 6 '

(8)

which, when plotted as a function of X, gives a Lorentzian line shape. The resonant absorption occurs when the first term of the denominator goes to zero at XR =

( I L L - A1)N~o + Go)_ '/z

(9)

For a spherical particle in a glass of refractive index 1.5, L = ~ and with G0 = 5 and A = 55 [from eq. (5a)], XR is found to be 416 nm. Thus we find that the free-electron absorption of bulk silver has been shifted into the blue region of the visible spectrum because the electrons are confined to a small, bounded region. It is this blue absorption band which gives most silver-containing glasses their yellow or brown color. We note that the location of the absorption depends on the refractive index of the matrix glass. For a 1.8 index glass, XR would be 457 nm. If the particles are not spheres, for example ellipsoids of revolution (prolate or oblate), the L-factor depends on the orientation of the particle with respect to the electric field of the light, i.e. the polarization direction. Hence, such an ellipsoidal particle wiU have a different absorption cross-section, depending on the polarization direction of the incident light. In fig. 1 XR values are shown calculated for prolate and oblate ellipsoids of silver metal (% = 5, A = 55) in a glass of index 1.5. From fig. I we fred that prolate particles with an aspect ratio of 0.4 (2.5 : 1) would have absorption maxima at 596 nm and 380 nm for the parallel and perpendicular directions, respectively. These effects have been verified experimentally. Stookey and Araujo [36]

T.P. Seward, 111 / Coloration and optical anisotropy 800[-

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Fig. 1. Wavelengths of maximum absorption calculated according to eq. (9) for eUipsoidal silver particles in glass: Solid curve - prolate, dashed curve - oblate. II and .L refer to the electrical field orientation with respect to the particle symmetry axis. Fig. 2. Transmittance spectra of polarizing glasses selected from table 1. The non-symmetrical curve shape is attributed to a particle aspect ratio distribution, arising in part from a particle size distribution. Table 1 Color, wavelength of maximum absorption and elongation stress measured for a series of stretched, silver-containing glasses. Particle aspect ratios are estimated from hma x and fig. 1. (Light polarized parallel to stretch axis.) Color (Eli)

Measured hma x (rim)

Measured stress (PSI)

Est. particle aspect ratio

Yellow Yellow Orange Rose-magenta Violet Blue-violet Blue

420 445 475 515 540 575 640

None 5 0 0 - 800 1200 1600 1900-2100 2500-2700 3200

1.0 1.3 1.5 1.8 2.0 2.2 2.7

504

T.P. Seward, III / Coloration and optical anisotropy

stretched glasses containing silver metal particles at temperatures near the glass softening point in such a manner as to elongate and align the particles and observed the predicted absorption band shifts. Seward [37] reported experiments showing that, for a given glass, the resulting silver particle aspect ratios depend on the stress applied during stretching. Some of the reported data are shown here in table 1 and fig. 2. From the data it is apparent that a wide range of colors, blue, purple, red, orange and yellow, result when the glass is viewed in light polarized parallel to the stretch axis.

4. Polychromatic glass In the polarizing glasses, all the elongated particles are aligned. If they were randomly oriented in the glass, each particle would act as a crossed polarizer for some particles lying in the layers below it. The glass would be colored, as determined by the particle aspect ratios, even if viewed with natural (not polarized) light. AnalyticaUy, the effective absorption cross-section per particle can be expressed as CABS = ~CABs I 2C g a s (for L±). (for LII) + ~

(10)

Non-spherical particles of random orientation can sometimes be generated without mechanical deformation of the glass if a second precipitated phase is present. This is the explanation [6,38] of the coloration in Coming Glass Works' recently announced polychromatic glasses [6]. Polychromatic glasses are a special class of photosensitive materials (in our above defined sense) in which a variety of colors (all hues) can be produced by a series of controlled thermo-optical treatments. The mechanism of variable controlled coloration in polychromatic glass has been proposed by Stookey et al. [6] to involve a photosensitized formation of metallic silver on a sodium halide microcrystalline phase which itself had been photosensitively precipitated from the glass. The sequence of steps leading to the colored state first involves a UV activated, Ce a÷ sensitized precipitation of small silver particles within the glass matrix. The silver then serves to nucleate the growth of sodium halide microcrystals during the thermal treatment which follows the UV exposure. The resulting sodium halide crystal shape has been shown [6] to be a cubic base with varying degrees of pyramidal protrusion from one cube face. The concentration of pyramidal microcrystals is controlled by the first UV exposure (intensity × time). A second UV exposure and heat treatment produces silver on the tips of the pyramidal extensions; the extent of silver formation, in particular the aspect ratio, correlates with the induced color [6, 38]. The color control comes primarily with the first exposure as follows: a weak exposure leads to relatively few initial silver particles which nucleate correspondingly few sodium halide crystals. A stronger first exposure produces more silver nuclei and more halide crystals. The second exposure provides a relatively fixed

T.P. Seward, III / Coloration and optical anisotropy

505

amount of silver to be distributed on the tips of the halide crystals. This fixed quantity of silver, distributed on a few crystal sites tends to produce larger, longer (higher aspect ratio) specks than when distributed over more sites. It is the aspect ratio, see fig. I and 2, which controls the color.

5. Photochromic glasses Photochromic refers to materials which darken or change color under irradiation, and which generally reverse (clear) if removed from the activating source. Although there are many types of photochromic materials [39] and photochromic glasses [40,41] we are concerned here with those containing a fine dispersion of copper-doped silver halide crystallites as their active agent. The silver halide crystallites in most commercially available photochromic glasses are about 10 nm in diameter and about 0.1 to 0.5/am apart. These crystallites are precipitated thermally with no prior exposure to radiation being required. (Hence, they are not photosensitive in our earlier definition of the word.) Darkening can be likened to latent image formation in silver halide photography [42], although the analogy is not exact. Exposure to light of wavelengths within the absorption band of the silver halide particles darkens the glass. The absorbed radiation creates electron-hole pairs. Electrons are successively trapped at some site, probably at the particle-glass interface, where they neutralize (or are neutralized by) interstitial silver ions. The holes are trapped at Cu ÷ sites: This role of Cu ÷ has been verified by ESR measurements [43,44]. There are, of course, competing trapping and recombination mechanisms, but repetition of this sequence leads to the growth of small specks or aggregates of silver which provide the darkened state absorption. Because the glass confines the reaction products (trapped species) in close proximity, the reaction reverses and the silver is reabsorbed into the silver halide after the irradiation ceases. The darkened absorption band is generally broad, with its maximum located between 500 and 1000 nm. The darkened color of the glass is generally a brown, pink, magenta or blue hue with low saturation. As we have seen, spherical silver particles generally color glass yellow because of a strong absorption band near 400 nm. Considering eq. (9), this absorption band would shift to about 500 nm if the silver specks were fuliy imbedded in the silver halide (refractive index about 2.1). This is not sufficient to account for the magenta and blue darkened colors. We could attempt to explain the band locations (at longer wavelengths) by appealing to larger silver particle sizes. For metal colloid sizes greater than 10 nm or so, the more complete Mie theory of light scattering [25,45] predicts that the wavelength of maximum absorption will increase as the particle size increases. Rohloff [46] has published curves for silver in silver chloride which show, according to the Mie theory, that an absorption maximum at 580 nm requires colloid sizes about

506

T.P. Seward, 111 / Coloration and optical anisotropy

60 nm. The two simplest arguments against this explanation are that the required silver particle sizes are larger than the silver halide particle from which they must form, and that at such large particle sizes, the reflected and transmitted colors of the glass would be different [25], which is contrary to experiment. Moser et al. [47] credited Duboc with suggesting, perhaps on the basis of Rohloff's work [46] with deformed bulk silver halide, that the observed color of darkened silver halide crystals and emulsions arises from the non-spherical shape of the silver specks. Seward [48] proposed this same explanation for the darkened color of silver-halide type photochromics. Since then, Nolan and co-workers have calculated photochromic absorption bands based on specific assumed colloid geometries, e.g. ref. [49]. It should be noted that the silver speck shapes and locations in photochromic glasses have never been observed directly, or indirectly. The experiment is very difdicult to do, even using an electron microscope with a cold stage. The silver specks are expected to be very small, with dimensions near 1-5 nm (10-50 A). Even if the silver forms as a continuous coating on the silver halide particle, the coating thickness would be small. In addition, the silver halide tends to decompose in the electron beam [50], the resulting silver specks confusing any interpretation of the micrographs. Possibly the strongest support for the non-spherical silver colloid model for photochromic absorption bands comes from optical bleaching experiments.

6. Optical bleaching Most silver halide-type photochromic glasses darken when exposed to radiation in the near UV and blue spectral regions and fade (clear) when irradiation ceases. When exposed to intense visible radiation the rate of fading is often enhanced, an effect called optical bleaching. There are some photochromics which fade slowly or negligibly in the dark, but can be optically bleached to original clarity [51,52]. The sensitivity to optical bleaching is composition dependent. Optical bleaching occurs throughout a wavelength band which generally overlaps the darkening sensitivity band (some wavelengths will darken a clear glass, but also bleach a darkened glass) and extends to a long wavelength limit, usually in the near IR. Portions of optical bleaching sensitivity curves have been reported [53]. Optical bleaching is believed to involve photo-ejection of electrons from the silver speck into the silver halide followed by a release of silver ions (to restore local charge neutrality); a process like that proposed by Mott and Gurney [54] to explain the Herschel effect [55] in silver halide photography. The current theoretical explanation [56,57] of the photo-ejection step is that the bleaching light excites surface plasmon modes of the silver specks which decay via single electron excitations [58]. If the electron has sufficient energy it escapes into the conduction band of the silver halide. Fig. 3 schematically represents the

T.P. Seward, Ill / Coloration and optical anisotropy

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Fig. 3. Energy band diagram at silver-silver chloride interface: (A) optical excitation of silver plasma oscillation; (B) decay of plasmon to single-electron excitation; (C) escape of the electron into conduction band of silver-chloride. From refs. [56,58].

process. Such photo-ejection has been experimentally observed in other materials [591. Surface plasmon excitation requires light of wavelengths within the speck absorption band. Hence, bleaching wavelength limits are imposed by this band. In addition, at short wavelengths bleaching is further limited by competition with UV darkening and at long wavelengths by decreased efficiency of photo-ejection. For AgC1, with the Fermi level assumed midway in the forbidden gap, the upper wavelength threshold for bleaching should be about 780 nm (Eg/2). BorreUi and Young [56] reported a decrease in bleaching sensitivity of three orders of magnitude for wavelengths greater than 800 nm. The broad absorption band of a darkened photochromic glass is not characteristic of each individual speck within the glass. Rather, it is a superposition of many different absorptions, each characteristic of a particular speck geometry. Thus, if a photochromic glass is bleached by a narrow wavelength band of light, portions of its absorption band can be selectively bleached. Only those silver specs are bleached (returned to solution) which have their shape-shifted resonant absorption strongly overlapping the wavelength band of the bleaching light. This effect is demonstrated in fig. 4. Similar optical bleaching curves have been reported for bulk silver chloride crystals [60]. The change in the absorption spectrum accompanying selective bleaching represents a change in the color of the glass: it tends to take on the color of the bleachLug light. This is called color adaptation. The evidence for the non-spherical silver speck model for photochromic darkening is even greater when the effects of optical treatment with polarized light, as discussed in the next section, are considered.

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and then separate sample areas were bleached with light of different wavelengths. Bleaching wavelengths and exposures, in J c m - 2 , are labelled. The experimental technique is described in ref. [ 5 3 ] .

7. Photo-induced optical anisotropy As reported by Borrelli and Seward [7], a recent patent [61] teaches that if optically bleachable photochromic glasses are partially bleached with polarized light, an absorptive-type light polarizer results. That is, the glass has a different absorption coefficient for plane polarized light oriented with its electric vector parallel to that of the bleaching light than for light of a different polarization (e.g. electric vector rotated by 90°). Several characteristics of the reported [7,58,61] photo-induced anisotropy can be described with the aid of fig. 5. If the glass is simultaneously UV darkened and bleached with broad-band visible polarized light, fig. 5a, the glass transmittances, as measured for light of two orthogonal polarization directions (one parallel to that of the bleaching light), become different. The dichroic ratio, defined as the ratio of the absorption coefficients for the two directions, increases with time of exposure. The dichroism (polarization) is often to a large extent retained after bleaching is stopped, even under continued UV darkening', fig. 5b, If the glass is allowed to thermally fade toward clarity, it becomes essentially non-polarizing because it no longer significantly absorbs light of any polarization, fig. 5c. When the glass is

T.P. Seward, III/ Coloration and optical anisotropy

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Fig. 5. Qualitative representation of perpendicular and parallel transmittances for different exposure conditions: (a) combined UV and polarized bleaching; (b) UV only; (c) no exposure (dark fading); and (d) re-exposure to UV. For example, see fig. 5 and 17 of ref. [58]. redarkened (with UV radiation), fig. 5d, it regains some or all ot its pre-fading anisotropy. This cycling can be repeated many times. Among other reported observations are that color adaptation, resulting from selectively bleaching only a portion of the spectrum, also tends to be remembered through many darkening-fading cycles, and that the polarizing direction of the glass can be changed by UV darkening and re-bleaching with polarized light of a new orientation. The effects of pulsing or varying the intensities of the darkening and bleaching exposures are discussed elsewhere [58,61] as are the effects of glass composition and heat treatment. Dichroic ratios as high as 4 have been reported. These induced anisotropy effects are understood in terms of the optical bleaching model discussed above. The light absorption which causes bleaching depends on the silver speck shape and its orientation with respect to the incident light electric field. The only specks bleached are those with shape and orientation which permit them to significantly absorb the polarized bleaching light. The photo-induced effects involve an interplay between the darkening process, which tends to generate and grow silver specks of random orientation, and the polarized bleaching, which tends to remove those of a certain orientation. This can be visualized with the help of fig. 6. Although, for reasons discussed above, the silver specks in photochromic glasses have never been observed, they are here portrayed schematically as disks, or partial spherical caps, at the interface between the silver halide particle and the glass. BorreUi et al. have argued [58] on the basis of a comparison of dichroic ratios measured after opucal treatments with non-polarized light directed perpendicularly to

510

T.P. Seward, III / Coloration and optical anisotropy A UV

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Fig. 6. Schematic representation of photo-induced anisotropy and memory effect.

the viewing direction, and with polarized light directed colineafly with the viewing direction, that oblate shapes are more likely than prolate. As the glass is first darkened, the silver specks are randomly oriented in space; a uniform distribution N(¢) versus ~, fig. 6a. During bleaching with light polarized in the vertical direction, those particles so oriented as to absorb this polarization of light are removed from the system. Simultaneous UV exposure tends to precipitate more specks at orientations less subject to bleaching, fig. 6b. Borrelli et al. [58] have developed a simple mathematical model to predict the time evolution of the dichroism. To explain the memory effect we postulate [58] that in the faded state not all the silver redissolves into the silver halide, i.e. there are residual silver specks or atomic clusters as in fig. 6c. This is not unreasonable since many photochromic glasses exhibit an effect, called "hang-up", where after once being darkened they do not completely fade back to the virgin (undarkened) transmittance. For some glasses this is a very small (but still measurable) effect, for others it is quite large. The residual specks of silver remember the locations, and therefore the goemetrical orientations (the local particle/glass interface orientations) of the original specks, so that upon re-darkening, fig. 6d, replicas of the original specks are created.

T.P. Seward, 111 / Coloration and optical anisotropy

511

As discussed in ref. [7] and [62], photo-induced anisotropic effects are also found in non-photochromic silver halide-containing glasses; i.e. glasses which do not rely on UV radiation to generate the visible absorption. In these glasses the color develops during their preparation, apparently as a result of excess silver being precipitated in contact with the silver halide particles. This coloration is believed to be analogous to additive coloration in bulk alkali halides [63-65] and silver halide [66] for which suitable treatments generate colloidal specks of excess silver at defects within, or at grain boundaries between, the crystals. Two general approaches to the preparation of these optically alterable, additively colored glasses are reported [7,62]. The first primarily involves the control of the composition, melting conditions and heat treatment of the glass. For some compositions (e.g. ref. [53]) a certain range of Ag/C1 ratios, in excess of stoichiometry for AgC1, is required. For others controlled redox conditions in the melt or the addition of thermo-reducing agents to the batch are helpful. The glasses are sometimes colored as melted and formed, but they often require a subsequent heat treatment to develop the silver and silver halide phases. The colors developed are generally ruby red or purple, although other colors are reported [62]. It is Suspected that the silver ruby glasses of Forst and Kreidl [67] were of this type. The other approach to additive coloration is to treat certain glasses containing silver halide precipitates either in a silver ion-containing bath (silver staining) at temperatures between 200°C and the glass softening point, or by subjecting the glass to a strongly reducing atmosphere at similar temperatures. The additive color develops in a surface layer. The resulting colors are generally purple, red, orange or yellow. Exposure to strong colored light causes color adaptation of the glass. Blues and greens are produced in addition to the above mentioned colors. Exposure of the glass through a colored transparency produces a similarly colored image [7]. The image is not permanent in that it can be further altered by exposure to strong light. The dichroic ratios measured for photo-induced polarization in these addivitely colored glasses are comparable with those reported for the photochromics, the maximum being about 4. In ref. [62] energy densities effective for optically altering the absorption bands are reported. Reports on more detailed studies are in preparation for publication. Although not strictly a coloration effect, birefringence is also observed when the glasses are optically treated with polarizing light [58,7,61,62]. The optical axis coincides with the polarization direction of the bleaching light. The induced birefringence in the additively colored glasses is generally higher than for the photochromic types and extends to longer wavelengths. A value of An = 10 -~ at a wavelength of 850 nm has been reported. Color adaptation and photoinduced anisotropic effects have been reported for bulk silver halides and film emulsions [55,68,69]. Because of the similarity of the active agents, silver in conWct with silver halide, the mechanism in those materials

512

T.P. Seward, III / Coloration and optical anisotropy

would appear to be the same as is operative in the glasses reported here. Unlike for the bulk crystals and emulsion films, the photo-induced effects in the glasses can be erased and repeated without deforming or destroying the sample. It should also be pointed out that the polychromatic glasses discussed in section 4 are not optically alterable in the sense discussed here. They apparently do not satisfy one or both of the following requirements: (a) the halide (or other crystalline phase) must have a sufficiently narrow forbidden gap that electrons can be photo-ejected from the silver metal by light in the visible or infrared spectral regions; and (b) the silver ions must have sufficient room temperature solubility and mobility in the halide phase that they can diffuse away from the silver speck.

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