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On the optical properties of glasses
240
Journal of Non-Crystalline Solids 123 (1990) 240-249 North-Holland
ON THE OPTICAL PROPERTIES OF GLASSES G.T. P E T R O V S K I I State Optical Institute, Leningrad 199034, USSR
Intrinsic features of electron-vibrational interaction in glasses and in the 'glass-microcrystal inclusions' systems are considered. Changes in optical properties of glass by means of an intensive light flux is stressed. A new method to record optically information using radiative color centers is suggested. Peculiarities of the optical breakdown of glasses in a super-small spot are described. The studies of optical properties of a modified glass surface are indicated to be promising.
1. Introduction Glasses are widely used mainly because they are transparent in the visible spectral range. We are now entering an era when it has become at least as significant to process and transmit informarion rapidly as to manufacture materials with prescribed properties. It is even possible to say that one of the main factors that govern with h u m a n development in the near future will be the degree of research activity in the optical frequency range of the electromagnetic field. So, glass, with all the varieties of optical properties, will occupy one of the leading positions in h u m a n civilization. Hence, a very large n u m b e r of research papers on optical properties of glasses is in evidence.
takes place due to a transition into an excited state by the transfer of an electron from an oxygen ion to a metallic ion, e.g., 3s-states of sodium are excited. The absorption b a n d of a glass matrix clearly manifests itself in the series of absorption spectra of glasses containing various concentrations of iron (fig. 1). With a decrease of the iron content, a b a n d with an exponential dependence of the absorption index on the energy of a photon appears. The shape of the b a n d and its position do not depend on the concentration and the type of admixtures in the glass and stay as they are under a variation of the redox conditions. When being excited within the limits of this band, hole color
/~/C,'n "/ 2. Self-absorption and impurity absorption of light in #asses The limit of the fundamental boundaries of light transmission is one of the most important problems in question. In sodium silicate glass, the low-energy fundamental absorption band is caused b y anisotropic luminescence centers called Lcenters. Their structure is represented by a fragment - S i - O - - N a +. An excitation does not travel from one center to another because of the disordering, i.e., r a n d o m orientation of anisotropic luminescence centers. An inter-center absorption
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Fig. 1. Absorption spectra of sodium-calcium-silicate glass synthesized under neutral conditions. The upper curve corresponds to 5 × 10-2% of FeqO3, the lower one - to 0.5 × 10-5% of FeqO3.
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G.T. Petrovskii / On the optical properties of glasses
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Fig. 2. Absorption spectra of Na20 3SiO2 glass (dotted line, 1') and of the same glass with the addition 0.03 tool% of PbO (solid line, 1).
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towards the shortwave region; introduction of Pb, Ba, Na cations first produces the same effect, and then at a 20-30% molar concentration the boundary moves to the longwave range. This significant observation indicates some under-exploited possibilities in the design of optical glasses of the phosphate flint types with high transparency in the shortwave spectral range. Such glasses may be useful for new generations of UV objectives for microphotolithography. In the majority of real cases, the absorption and luminescence properties of glasses are controlled by admixtures of elements of the transition series or rare earth group present in glass.
3. Effect of excitation conditions on optical spectra
centers, i.e. ionized centers of the matrix, are formed. Strong absorption in the ultraviolet range may also be caused by the ions in which electron transitions between their orbitals occur. The latter, first of all, are the mercury-like ions T1+, Ge 2÷, Pb2+, Sb3+ and As 3+. An optical center with a two-valence lead ion has frequently been studied [1,2]. Optical transitions of 6s-electrons happen between the fundamental 1S0 level and the excited Sp states of the 3P1 configuration. Figure 2 shows an absorption band of the transition and a fundamental absorption boundary of the sodium silicate matrix formed by an electron charge transfer from oxygen to sodium. The value of the energetic gap 1S0-3p1 for a free lead ion is 8 eV (160 rim). In the figure a shift of the absorption band is seen, which is due to the interaction of lead with oxygen ligands, partial transfer of oxygen negative charge to lead and an enhancement of 6s electron screening from the lead nucleus. This swelling, called a nepheloxic effect, makes a transition of the 6s electron to an excited level energetically easier. In the paper by Wong and Angell [3], attention is given to a non-standard cation behavior in glass-like P205, the ultraviolet transmission boundary of which lies at 5.23 eV (273 nm). Introduction of cations with high polarization ability (lithium, beryllium) shifts the boundary
When observing luminescence under monochromatic laser excitation inside an inhomogeneous band, only those centers are excited whose transition energy is equal to the energy of the exciting quanta. When there is no energy transfer among the centers (small concentration), only laser excited centers are luminescing and, instead of the wide inhomogeneously broadened contour, a narrow line in the spectrum is observed. Under energy transmission in high concentration materials, besides the narrow line, the spectrum contains a broad component emitted by centers which undergo excitations as a result of the energy transfer process. In the first work [4] accomplished in this field, the luminescence of europium glasses excited by mercury lamp lines of 576.9 and 579.1 nm, situated in the same spectral region as the reasonance line 5Do-7Fo of europium, was investigated. In this case, instead of the described band, two lines, the positions of which coincide with the excitation lines, are observed in the spectrum (fig. 3). A uniform bandwidth reflects specific features of the low-frequency vibrational spectrum of the glass matrix. With a decrease of temperature, the vibrational spectrum of glass behaves in a different way than the spectrum of crystals. One recognizes two types of relaxation of the excited centers caused by an interaction with matrix vibrations. Similar
G.T. Petrooskii / On the optical properties of glasses
242
e
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Fig. 3. Resonanceband of luminescence(1% weight concentration in glass) for various methods of excitation: 1, nonselective excitation; 2, excitation by the line of the mercury spectrum; 3, the spectrum of the mercury lamp.
tO the terms of EPR-spectroscopy, they are called longitudinal and transverse relaxations. Relaxation of the first type reveals itself in the change of populations of energy levels towards the values corresponding to the thermodynamic equilibrium. This relaxation is called 'longitudinal' and is characterized by a time Tz. Relaxation of the second type, called 'transverse' is characterized by a time T2, is the result of the destruction of wavefunction coherence.
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2, crystal of yttrium orthoaluminate.
The temperature dependence of a homogeneous bandwidth 8Phom of the luminescence line 5Do7Fo of europium in the interval 1.7-100 K has been investigated by Seizer et al. [5]. A dye laser (rodamin) with a bandwidth of only 50 MHz was the excitation source. Under such conditions, the line became 105 times narrower. Values of 8Vhom(T ) do not coincide with those for activated crystals (fig. 4). The value of 81,hom for glasses exceeded corresponding values for crystals of similar composition by almost two orders of magnitude under the same conditions. These data are evidence for the electron-vibrational interaction of rare earth ions in glass. They correspond with the anomalies of other glass properties, in which low-frequency vibrations are exhibited, namely, specific heat, IR and Raman scattering spectra, and the rate of nuclear spin relaxation. All the anomalies are assigned to peculiar vibrational states of glasses - the 'disorder modes', which complement the usual Debye spectrum because of the non-zero entropy of glass at zero temperature. This leads to the presence in glasses of a specific type of low-energy excitations being the excitations of a 'two-level system'. The existence of such systems in glasses is associated with the possibility for some particular atoms or groups of atoms to reside in a number of almost energetically equivalent states. It is worthwhile to note that the theory of two-level systems is mostly a phenomenological one. There exist about ten theoretical models providing the temperature dependence of a homogeneous bandwidth in glasses. They differ in the way in which they account for the interaction of electron states of an optical center with two-level systems and phonons. Thus, a linear dependence of the broadening on T2 or quadratic law of broadening is explained. However, there are cases of broadening proportional to T 1"3 which are badly explained. A generalized approach is suggested in which both absorbing centers and the matrix have two-well adiabatic potential. The distortion of phonons under tunneling and additional phonon broadening due to the interaction of an optical electron with tunnel degrees of freedom are incorporated. This provides one with various forms of
G.T. Petrovskii / On the optical properties of glasses
dependence of the broadening on T2 or TEE with either one or two points of inflection [6]. Some peculiarities of optical properties of glasses with rare earth ions have recently been exploited in a non-traditional way for a modification of the spectral parameters under selective light action on glass. Selected by monochromatic excitations, centers differ from the others only during a period when they are in exited electron state. After relaxation to the fundamental state the centers become indistinguishable. One can achieve long-term spectral selection by means of photoburning of spectral holes, i.e. by the fight transformations of a center, after which it does not drop down to an initial state and, thus, loses the ability to absorb light of a certain frequency. This transformation may happen according to various mechanisms - purely photochemical, non-photochemical, consisting in the change of interaction between the center and its surrounding, and purely photophysical burning of a spectral hole by means of a complete saturation of one of the electron transitions. The general principle of information recording is the correspondence of one item of information to a certain place of the bulk or the surface. Light diffraction phenomenon limits focusing of a light ray over an area smaller than 2k2. Therefore, for a spectraUy non-selective binary recording (enlightened-not enlightened) for ~ - 500 nm the limiting surface density of recording is 4 × 108 b i t / c m 2. The photobuming of spectral holes provides a new way to perform recording, when one bit of information corresponds to a hole in the inhomogeneously broadened spectrum and the density of recording is limited by the total possible number of holes in an inhomogeneous band. Approximate estimates show that a density 1014 b i t / c m 2 is theoretically accessible. A stable burning of spectral holes has been obtained for the first time in inorganic glasses by Macfarlane and Shelby [7]. The authors, for a temperature interval 1.6-22 K, obtained burnt out spectral holes in alkali-silicate glasses activated by three-valent europium (0.25%) or praseodymium (0.1%).
243
4. Effect of high energy excitation Let us consider yet another type of color center formation in glasses, when high energy radiation acts as its primary generator. Ionization of a glass matrix takes place under the influence of electromagnetic radiation with high energy photons. The released charge carriers are captured into new local states forming color centers with the properties depending on the interaction between the centers and their surroundings. The centers themselves, while interacting with surrounding atoms, change the state of the latter; in the first place, they distort the distances between the atoms near color centers. The optical properties of radiatively colored multicomponent glasses are extremely complicated due to a variety of color center types and their strong parameter dispersion inside the disordered structure. The difficulties in experimental data interpretation are strongly increased by the fact that any action on glass, as a rule, gives rise to a simultaneous response of a number of types of color centers. Their decoloration has already been attempted for the optical recording of information with the help of radiative color centers (RCC). This kind of information recording is not reversible since, in such a situation, the recombination phenomenon implies an essential decrease of RCC concentration. The anisotropy of RCC in glasses enables us to introduce a reorientation process under the influence of optical radiation related to the recapture of delocalized charge carriers into a new state. Alumosilicate glasses processed by ~,-radiation have been investigated. Under the action of linearly polarized optical radiation, the absorption of light polarized identically with the activating radiation results in a decrease of the value (a~) over all the observed spectral range. Near 2.8 eV the absorption of light polarized perpendicular to the direction of activating radiation, polarization (a~) increases. In fig. 5 the dependence of the variation of a ~_ (curve 1) and a~ (curve 2) at 2.8 eV on the duration of activating radiation are plotted. Since radiation with h~, = 2.8 eV is not sufficient to ionize the glass matrix, the growth of a± indicates processes of RCC reorienP
244
G.T. Petrovskii / On the optical properties of glasses
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Fig. 5. Photostimulatedreorientationof hole color centers in alkali-silicateglass under the action of linearlypolarized optical radiation. 1, a.L, ' • 2, all.'
tation. The research showed that at the initial stage of action by linearly polarized light, RCC concentration does not change. One should stress I that at the same time the values a_L and all undergo substantial changes. This change means that in these glasses situations are implemented when the contribution of recombination processes is negligibly small compared with the background of the reorientation processes. Due to optical radiation, reorientation of the hole RCC, responsible for absorption in the band with the maximum at 2.8 eV, takes place. (An example of a unique use of radiatively colored glass is one suggested by Czechoslovakian technologists. They proposed decorating a new building of the National Theatre in Prague by glass slabs made up of a high-silica containing glass, Simax [8]. Five thousand 60 × 80 cm2 slabs were irradiated by a gamma-ray source to produce an original smoked amber-brownish color for the facade.) Let us now consider some not so ordinary phenomena accompanying interaction of a glass with ionizing photons [9,10]. The formation of color centers is accompanied by the appearance of mechanical stresses produced by volume changes in the irradiated part of the glass. The mechanism for this phenomenon is of a non-thermal nature. Variation of glass volume is caused by deformation of the surroundings of
local centers in the process of capturing charge carriers excited by ionizing radiation. The variation in the volume of one center is 6 × 10 -24 cm3, and the corresponding strain energy of its surroundings is about 1 eV, the relative variation of the volume of the irradiated zone is 5 × 10 -6, which corresponds to a value of 8 × 104 Pa (0.8 k g / c m 2) for stresses at the boundary of the zone and to a change in refraction index of approximately one part in 106.
5. Optical breakdown of glass In the discussion above we have not associated optical properties of glasses with the intensity of light flux. One of the most significant laws of optics - the law of light absorption, as has been established by P. Buger as early as in 1729 means physically that the process of photon losses does not depend on the light beam flux; in other words, the number of absorbing centers is independent of light intensity. The change comes, however, with high intensity light beams, for instance, due to energy absorption at the front of a pulse and re-irradiation into its back, or simply due to the saturation of populations of electronic states. Non-linear photostimulated processes also incorporate such an important phenomenon as the optical breakdown of glass placed in a field of powerful monochromatic radiation. An optical breakdown of materials under the action of passing radiation presents a serious obstacle to the power enhancement of laser systems. In addition to the usual optical properties of glasses, such as refraction index, dispersion and transparency range, a requirement of high radiation strength is now necessary. All the variety of processes governing the optical breakdown of transparent dielectrics reduces to the following three groups of phenomena. First, it is an admixture breakdown originating because of the interaction between laser radiation and admixture centers. Second, non-linear phenomena, such as self-focusing multiphoton absorption and forced scattering implying sub-threshold distortions of spatial-temporal characteristics of radiation within the interaction bulk, occur. Third,
G.T. Petrovskii / On the optical properties of glasses
optical breakdown may be caused by the interaction of laser radiation and the matrix without any subthreshold distortions of spatial-temporal structure in the interaction volume. Thus, in order to observe a true optical breakdown and to study its properties, one needs to apply the focusing of a single-frequency laser radiation, with quantum energy less than the halfwidth of the gap, to the bulk of a homogeneous optical material [11]. The true optical breakdown threshold is a material constant. This parameter has been measured for quartz glass and K-8 glass. The breakdown threshold value is about 10 a3 W / c m 2. A new conjecture regarding optical breakdown is essentially as follows: under the action of laser radiation, no individual ionization of atoms occurs, but a step-wise restructuring of electron states takes place which results in the disappearance of the dielectric gap. Under such circumstances, glass is transformed to a quasi-metallic state. Radiation absorption by this quasimetallic particle leads to the fast formation of a plasma cloud and to the forthcoming destruction [12]. Note, that for the first time a laser pulse of nanosecond duration was focused on a spot with a diameter less than 1 ~tm in our research. In a number of papers other authors report spots with diameters of about 3 ~tm and thresholds lower by two orders of magnitude. In the research of Moscow Physical School, an interpretation based on the dynamics of restructuring of the electron-phonon system of glass during the pulse was given [13]. They assumed that the rate of electron transfer from the irradiated spot is higher than the rate of the avalanche ionization. In the field of the laser pulse front, heated electrons more to the periphery of the spot. A hole in the electron density is formed at its center, the avalanche has no time to develop here, while in the periphery it cannot grow because of the exponential decay of the field.
6. Non-linear optical properties A set of optical phenomena in glass becomes complicated due to the collective (cooperative)
245
interaction of a system of emitters. One of them, for instance, is a superradiation phenomenon, when a system of excited atoms emits radiation during a time period much shorter than the radiation time of an individual atom, The atoms in such a system maintain mutual influence by means of a specific type of interaction between the particles, namely, with the help of their common radiation changing, suddenly, the optical properties of the material [14]. Phase transitions in quantum electronics systems originate mainly due to the re-irradiation dipole field interaction. The intensity of an exterior light wave serves as a varying exterior parameter analogous to temperature. At a certain critical value of the intensity, an order parameter appears spontaneously in the system as, for example, a step-wise variation of difference in populations. The appearance of the order parameter, in its turn, provides the changes of the spectroscopic characteristics of the material. The earmark of these phase transitions is their pronounced non-equilibrium nature: they occur either in the system inverted by an external pumping or under the action of a light wave changing, drastically, populations of electronic transitions. As an example of the cooperative behavior of an atomic system interaction via radiation field, one can point out the bistability of optical transrnitting revealed in the existence of two steady-state values of the output for a given output intensity Pm of the light wave incident on the system. One observes optical bistability when transmitting light through a material with saturable absorption. To obtain bistability, one has to have some non-linear medium with a refraction index or transmission coefficient depending on the intensity of the incident radiation. A facility is optically bistable when its o u t p u t - i n p u t optical power dependence is hysteretic. By its manifestations (step-wise variation of the transmission, hysteresis) optical bistability is the analog of a phase transition of the first kind. A curve depicting the dependence of transmission on Pm is similar to the van der Waals isotherm for the vapor-liquid phase transition. It is of extreme interest that this, new enough, optical phenomenon may be realized in the most
G. 7". Petrovskii / On the optical properties of glasses
246
27
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Fig. 6. Dependence of the transmission boundary position of glass on the size of cadmium selenide crystals (after ref. [15]).
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[18]). simple way by using familiar glasses tinted by cadmium sulpho-selenides. Actually, only a new level of understanding of process is these systems is required. Spectral-optical properties of glasses which contain semiconductor microcrystals are determined by the character of the energy spectrum of a semiconductor particles - activators. A semiconductor microcrystal inside a transparent dielectric glass matrix forms a potential well for nonequilibrium fight-produced charge carriers. For small microcrystals, their energy is specified by a dimension quantification of translational motion of non-equifibrium electron-hole pairs coupled by Coulomb interaction. This effect introduces a strong dependence of the spectral-optical properties of such heterogeneous glasses on the size of the particle activators [15,16].
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In fig. 6, as an example, we plot the experimental dependence of the spectral position of the transmission boundary of glasses containing CdSe microcrystals against the size of the latter. It can be seen that the strong dependence of the width of the semiconductor microcrystal gap on the size, caused by the quantum-dimensional effect, allows one to control the spectral position of the transmission boundary of such glasses within the limits of practically all the visible range. The first experimental observations of non-finear optical absorption in quasi-zero-dimensional structures, being microcrystals inside a glass-fike matrix, were described in 1987 [17,18]. In fig. 7 a dependence is shown of the value of transmission at h = 676.4 nm on the density of pumping measured at T = 80 K. Evidently, a substantial enlightenment is achieved at a comparatively small density of pumping radiation, i.e. P 1 M W / c m 2. In fig. 8, extracted from the paper by Yumoto et al. [18], a phenomenon of optical bistabifity is demonstrated.
0,¢, 7. Refraction index gradients in glass 0
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So far we have discussed the optical properties of homogeneous glasses or systems with inhomogeneities of a microcrystalline nature. We now
G.T. Petrovskii / On the optical properties of glasses
discuss the optical properties of glasses with optical inhomogeneities in the form of a refraction index gradient. As early as in 1886, Exner [19] noted that a cylinder made up of a translucent material with refraction index axially symmetric and parabolically dropping towards the axis forms an image similar to a lens. A ray propagating in such a cylinder happens to be sinusoidal and the optical force is a periodic function of the length of the element. In other words, the element may produce a real, imaginary, inverse or direct image of a subject depending on its length. One of the most fundamental elements of integral optics is a planar light guide - a thin layer of material whose refractive index differs from that of a substrate sufficiently for light to propagate only inside the layer due to total internal reflection. The waveguide effect in such glass films was studied in detail for the first time in the paper of Osterberg and Smith in 1964 [20]. A thorough study performed recently by us [21] shows that the change of refractive index under low-temperature ion exchange is controlled by three mechanisms, each of them having an absolute value larger than the resulting variation. In fig. 9, the variation of refractive index, determined by the total action of all three mechanisms, is shown. One can see that a positive contribution to the variation is given only by an increase of polarizability of K by replacing Na + (fig. 9, curve 1). The variation of refractive index
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Fig. 9. Influence of an exchanged potassium ion concentration on the refractive index variation according to various mechanisms. Curve 4 shows the experimentally determined total growth (see text for description of curves 1-4).
247
due to this mechanism may be as large as 250 × 10 -4 . Real values of refractive index variation happen to be two orders of magnitude smaller (curve 4) due to a joint negative contribution of the other two mechanisms - a concentration one, that is due to a decrease of the bulk concentration of particles because of the expansion of glass (curve 2), and one due to stress (curve 3). The absolute value of the first contribution grows more than linearly with the concentration of the diffusing ion, due to the non-proportionality of the volume change of glass with the concentration of K ÷ ions. From fig. 9 it can be seen that the variation of the refractive index caused by the action of microstresses in the investigated range of K ÷ concentrations (curve 3) remains approximately constant. This may be understood if one accounts for the fact that the influence of microstresses on the refractive index is determined by the deformation of each ion present in the glass as well as by their total number. With a high initial content of N a in glasses, the structure is open. It may be assumed that the deformation of K ÷ ions in such glasses would decrease. Therefore, since the concentration of the diffusing ion grows with decreasing Na concentration, the total effect on microstresses changes only slightly.
8. Glass as a prospective material for integral optics Films of terbium-containing glasses possess magneto-optical properties. These properties were also observed in glasses with three valent cerium and with cobalt ferrite microcrystals [22]. There exists the possibility of their integration with a radiation source, since waveguides on substrates made up of laser glasses have already been developed [23]. There also exists a possibility of their integration with a receiver of radiation, because any photosensitive layer has a good contact with glass. The possibility of integration with electronic circuits is excellent as well, since an ohmic buffer between the input and the output is provided. As for manufacturing, technological difficulties are here smaller than those when dealing with monocrystals.
248
G.T. Petrovskii / On the optical properties of glasses
Thus, only for the versions for which large electro-optical effects are required are there no satisfactory glass-like structures. On the other hand, the property of photosensitivity has only been implemented in waveguides on glass substrates. For waveguides on photochromic glasses, we have demonstrated [24] the possibility of the direct control of light flux by the action of another light beam. In waveguides formed by ion exchange diffusion in substrates of photochromic glass, an increase of photosensitivity with the growth of the guiding layer depth is detected, which is related to the departure of copper from the surface layers of glass into the salt melt under diffusion processing. Waveguides in photochromic glasses demonstrate simultaneously the quantum nature of light in the processes of color center formation and the wave nature in the processes of the discrete propagation of eigenmodes. Field maxima of waveguide modes with different numbers are essentially displaced one against the other: the main maxima of modes with large numbers are situated at greater depth. Under the action of exterior activating radiation, an absorbing (negative) mask is formed in the waveguide with the absorption profile corresponding to the photosensitivity gradient. When wavegide modes of different numbers travel through such an apodization mask, the higher number modes are absorbed more strongly than the lower number ones since the maxima of their field lie in the domain of maximum absorption. Such an interaction of modes with the absorbing mask leads to the selection of lower order waveguide modes (fig. 10). One of the most important problems is the creation of optical materials for optical frequency multipliers. To have an efficient generation of the second harmonics of optical radiation it is necessary to have phase synchronism, that is, equality of the first and the second harmonic velocities in a non-linear material. Since in any material propagation velocities of the first and second harmonics differ because of the dispersion, phase synchronism is usually achieved in anisotropic crystals if the difference of the refractive indices of ordinary and extraordinary rays is larger than the disper-
k
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Fig. 10. Waveguide mode selectors on photochrome glasses formedby externalradiation.
sion of the material. In view of this fact, the choice of materials for second harmonic generation is limited. We have demonstrated the possibility of phase synchronism under the conditions of the non-linear interaction of waves in a gradient waveguide, i.e., the achievement of the equality of refractive indices of the same waveguide [25]. The limiting condition for at least one condition of phase synchronism to exist is that the maximum refractive index in the waveguide for the first harmonic has to be greater than the refractive index of the substrate for the second harmonic. Let us consider the possibility of the practical implementation of phase synchronism of the first and the second harmonics of a neodymium laser in a planar waveguide on optical K8 glass obtained by diffusion of silver. The parameters of the waveguide are for X = 1.06 ~tm, c1 = 1.5062, B 1 = 1.5262; for X = 0.53 ~tm, c 2 = 1.5192, B E --1.5393. The dependence of the model refractive indices on the thickness of the waveguide is shown in fig. 11. One is able to see that at certain thicknesses the modal curves of the first and second harmonic have an intersection. This means that for those thicknesses the propagation velocities of the harmonics coincide, i.e. the phase synchronism condition is fulfilled. As a conclusion the author would like to express his hope that the above exposition demon-
G.T. Petrovskii / On the optical properties of glasses
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l/
.--/i
/
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Fig. 11. Dependence of the modal refractive indices in a gradient waveguide of K8 glass on the thickness of the waveguide for h =1.06 ~m (solid lines) and h=0.53 ixm (dashed lines).
strates to some extent the unbounded possibilities associated with the study and use of the optical properties of an old and eternally young material: glass.
References [1] J. Duffy, J. Non-Cryst. Solids 76 (1985) 391. [2] E. Raaben and M. Tolstoy, Fiz. Khim. Stelda 14 (1988) 66. [3] J. Wong and C. AngeR, Glass Structure by Spectroscopy (Dekker, New York, 1976). [4] Y. Denisov and V. Kizel, Opt. Spectrosc. 23 (1967) 472. [5] P. Selser, D. Huber, D. Hamilton, W. Yen and M. Weber, Phys. Rev. Lett. 36 (1976) 823.
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[6] J. Osadko, Zh. Eksp. Teor. Fiz. 90 (1986) 1453. [7] R. Macfarlane and R. Shelby, Opt. Commun. 45 (1983) 46. [8] Z. Prasil and M. Pesek, Sklar Keram. 36 (1986) 33. [9] L. Glebov, V. Dokuchaev and G. Petrovsky, Dokl. Akad. Nauk SSSR 274 (1984) 568. [10l L. Glebov, N. Niconorov and G. Petrovsky, Fiz. Khira. Stekla 12 (1986) 345. [11] O. Efimov and L. Glebov, Trudy Opt. Inst. 69 (1988), 1. [12] L. Glebov, O. Efimov, M. Libenson and G. Petrovsky, Dokl. Akad. Nauk SSSR 287 (1986) 1114. [13] A. Kozneev, V. Osadchiev and S. Pozdnyakov, Preprint Ing. - Fiz. Inst. N 024, Moscow (1987). [14] A. Andreev, V. Emelyanov and Y. llinsky, Koop. Javi. Opt. Moscow (1988). [15] A. Ekimov, A. Efros and A. Onushchenko, Sofid State Commun. 56 (1985) 921. [16] N. Borrelli, D. Hall, H. Holland and D. Smith, J. Appl. Phys. 61 (1987) 5399. [17] Y. Vandyshev, V. Dneprovsky and A. Ekimov, Zh. Eksp. Teor. Fiz~ Pisma 46 (1987) 393. [18] J. Jimoto, S. Fukushima and K. Kubodera, opt. Lett. 12 (1987) 831. [19] S. Exner, Repet. Phys. 22 (1986) 299. [20] H. Osterberg and L. Smith, J. opt. Soc. Am. 54 (1964) 1074. [21] L. Glebov, V. Dokuchaev, S. Evstropev and G. Petrovsky, Fiz. Khim. Stelda 14 (1988) 79. [221 G. Petrovsky, T. Sarubina and J. Edelman, Opt.-mech. Prom. 11 (1987) 33. [23] M. Babikova, V. Berenberg, L. Glebov and G. Petrovsky, Kvant. Elektron. 12 (1985) 1973. [24] G. Petrovsky, K. Agafonova and N. Nikonorov, Kvant. Elektron. 8 (1981) 2266. [25] L. Glebov, J. Morozova and G. Petrovsky, Dokl. Akad. Nauk SSSR 261 (1981) 589.
Journal of Non-Crystalline Solids 123 (1990) 240-249 North-Holland
ON THE OPTICAL PROPERTIES OF GLASSES G.T. P E T R O V S K I I State Optical Institute, Leningrad 199034, USSR
Intrinsic features of electron-vibrational interaction in glasses and in the 'glass-microcrystal inclusions' systems are considered. Changes in optical properties of glass by means of an intensive light flux is stressed. A new method to record optically information using radiative color centers is suggested. Peculiarities of the optical breakdown of glasses in a super-small spot are described. The studies of optical properties of a modified glass surface are indicated to be promising.
1. Introduction Glasses are widely used mainly because they are transparent in the visible spectral range. We are now entering an era when it has become at least as significant to process and transmit informarion rapidly as to manufacture materials with prescribed properties. It is even possible to say that one of the main factors that govern with h u m a n development in the near future will be the degree of research activity in the optical frequency range of the electromagnetic field. So, glass, with all the varieties of optical properties, will occupy one of the leading positions in h u m a n civilization. Hence, a very large n u m b e r of research papers on optical properties of glasses is in evidence.
takes place due to a transition into an excited state by the transfer of an electron from an oxygen ion to a metallic ion, e.g., 3s-states of sodium are excited. The absorption b a n d of a glass matrix clearly manifests itself in the series of absorption spectra of glasses containing various concentrations of iron (fig. 1). With a decrease of the iron content, a b a n d with an exponential dependence of the absorption index on the energy of a photon appears. The shape of the b a n d and its position do not depend on the concentration and the type of admixtures in the glass and stay as they are under a variation of the redox conditions. When being excited within the limits of this band, hole color
/~/C,'n "/ 2. Self-absorption and impurity absorption of light in #asses The limit of the fundamental boundaries of light transmission is one of the most important problems in question. In sodium silicate glass, the low-energy fundamental absorption band is caused b y anisotropic luminescence centers called Lcenters. Their structure is represented by a fragment - S i - O - - N a +. An excitation does not travel from one center to another because of the disordering, i.e., r a n d o m orientation of anisotropic luminescence centers. An inter-center absorption
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Fig. 1. Absorption spectra of sodium-calcium-silicate glass synthesized under neutral conditions. The upper curve corresponds to 5 × 10-2% of FeqO3, the lower one - to 0.5 × 10-5% of FeqO3.
0022-3093/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Honand)
G.T. Petrovskii / On the optical properties of glasses
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241
towards the shortwave region; introduction of Pb, Ba, Na cations first produces the same effect, and then at a 20-30% molar concentration the boundary moves to the longwave range. This significant observation indicates some under-exploited possibilities in the design of optical glasses of the phosphate flint types with high transparency in the shortwave spectral range. Such glasses may be useful for new generations of UV objectives for microphotolithography. In the majority of real cases, the absorption and luminescence properties of glasses are controlled by admixtures of elements of the transition series or rare earth group present in glass.
3. Effect of excitation conditions on optical spectra
centers, i.e. ionized centers of the matrix, are formed. Strong absorption in the ultraviolet range may also be caused by the ions in which electron transitions between their orbitals occur. The latter, first of all, are the mercury-like ions T1+, Ge 2÷, Pb2+, Sb3+ and As 3+. An optical center with a two-valence lead ion has frequently been studied [1,2]. Optical transitions of 6s-electrons happen between the fundamental 1S0 level and the excited Sp states of the 3P1 configuration. Figure 2 shows an absorption band of the transition and a fundamental absorption boundary of the sodium silicate matrix formed by an electron charge transfer from oxygen to sodium. The value of the energetic gap 1S0-3p1 for a free lead ion is 8 eV (160 rim). In the figure a shift of the absorption band is seen, which is due to the interaction of lead with oxygen ligands, partial transfer of oxygen negative charge to lead and an enhancement of 6s electron screening from the lead nucleus. This swelling, called a nepheloxic effect, makes a transition of the 6s electron to an excited level energetically easier. In the paper by Wong and Angell [3], attention is given to a non-standard cation behavior in glass-like P205, the ultraviolet transmission boundary of which lies at 5.23 eV (273 nm). Introduction of cations with high polarization ability (lithium, beryllium) shifts the boundary
When observing luminescence under monochromatic laser excitation inside an inhomogeneous band, only those centers are excited whose transition energy is equal to the energy of the exciting quanta. When there is no energy transfer among the centers (small concentration), only laser excited centers are luminescing and, instead of the wide inhomogeneously broadened contour, a narrow line in the spectrum is observed. Under energy transmission in high concentration materials, besides the narrow line, the spectrum contains a broad component emitted by centers which undergo excitations as a result of the energy transfer process. In the first work [4] accomplished in this field, the luminescence of europium glasses excited by mercury lamp lines of 576.9 and 579.1 nm, situated in the same spectral region as the reasonance line 5Do-7Fo of europium, was investigated. In this case, instead of the described band, two lines, the positions of which coincide with the excitation lines, are observed in the spectrum (fig. 3). A uniform bandwidth reflects specific features of the low-frequency vibrational spectrum of the glass matrix. With a decrease of temperature, the vibrational spectrum of glass behaves in a different way than the spectrum of crystals. One recognizes two types of relaxation of the excited centers caused by an interaction with matrix vibrations. Similar
G.T. Petrooskii / On the optical properties of glasses
242
e
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Fig. 3. Resonanceband of luminescence(1% weight concentration in glass) for various methods of excitation: 1, nonselective excitation; 2, excitation by the line of the mercury spectrum; 3, the spectrum of the mercury lamp.
tO the terms of EPR-spectroscopy, they are called longitudinal and transverse relaxations. Relaxation of the first type reveals itself in the change of populations of energy levels towards the values corresponding to the thermodynamic equilibrium. This relaxation is called 'longitudinal' and is characterized by a time Tz. Relaxation of the second type, called 'transverse' is characterized by a time T2, is the result of the destruction of wavefunction coherence.
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2, crystal of yttrium orthoaluminate.
The temperature dependence of a homogeneous bandwidth 8Phom of the luminescence line 5Do7Fo of europium in the interval 1.7-100 K has been investigated by Seizer et al. [5]. A dye laser (rodamin) with a bandwidth of only 50 MHz was the excitation source. Under such conditions, the line became 105 times narrower. Values of 8Vhom(T ) do not coincide with those for activated crystals (fig. 4). The value of 81,hom for glasses exceeded corresponding values for crystals of similar composition by almost two orders of magnitude under the same conditions. These data are evidence for the electron-vibrational interaction of rare earth ions in glass. They correspond with the anomalies of other glass properties, in which low-frequency vibrations are exhibited, namely, specific heat, IR and Raman scattering spectra, and the rate of nuclear spin relaxation. All the anomalies are assigned to peculiar vibrational states of glasses - the 'disorder modes', which complement the usual Debye spectrum because of the non-zero entropy of glass at zero temperature. This leads to the presence in glasses of a specific type of low-energy excitations being the excitations of a 'two-level system'. The existence of such systems in glasses is associated with the possibility for some particular atoms or groups of atoms to reside in a number of almost energetically equivalent states. It is worthwhile to note that the theory of two-level systems is mostly a phenomenological one. There exist about ten theoretical models providing the temperature dependence of a homogeneous bandwidth in glasses. They differ in the way in which they account for the interaction of electron states of an optical center with two-level systems and phonons. Thus, a linear dependence of the broadening on T2 or quadratic law of broadening is explained. However, there are cases of broadening proportional to T 1"3 which are badly explained. A generalized approach is suggested in which both absorbing centers and the matrix have two-well adiabatic potential. The distortion of phonons under tunneling and additional phonon broadening due to the interaction of an optical electron with tunnel degrees of freedom are incorporated. This provides one with various forms of
G.T. Petrovskii / On the optical properties of glasses
dependence of the broadening on T2 or TEE with either one or two points of inflection [6]. Some peculiarities of optical properties of glasses with rare earth ions have recently been exploited in a non-traditional way for a modification of the spectral parameters under selective light action on glass. Selected by monochromatic excitations, centers differ from the others only during a period when they are in exited electron state. After relaxation to the fundamental state the centers become indistinguishable. One can achieve long-term spectral selection by means of photoburning of spectral holes, i.e. by the fight transformations of a center, after which it does not drop down to an initial state and, thus, loses the ability to absorb light of a certain frequency. This transformation may happen according to various mechanisms - purely photochemical, non-photochemical, consisting in the change of interaction between the center and its surrounding, and purely photophysical burning of a spectral hole by means of a complete saturation of one of the electron transitions. The general principle of information recording is the correspondence of one item of information to a certain place of the bulk or the surface. Light diffraction phenomenon limits focusing of a light ray over an area smaller than 2k2. Therefore, for a spectraUy non-selective binary recording (enlightened-not enlightened) for ~ - 500 nm the limiting surface density of recording is 4 × 108 b i t / c m 2. The photobuming of spectral holes provides a new way to perform recording, when one bit of information corresponds to a hole in the inhomogeneously broadened spectrum and the density of recording is limited by the total possible number of holes in an inhomogeneous band. Approximate estimates show that a density 1014 b i t / c m 2 is theoretically accessible. A stable burning of spectral holes has been obtained for the first time in inorganic glasses by Macfarlane and Shelby [7]. The authors, for a temperature interval 1.6-22 K, obtained burnt out spectral holes in alkali-silicate glasses activated by three-valent europium (0.25%) or praseodymium (0.1%).
243
4. Effect of high energy excitation Let us consider yet another type of color center formation in glasses, when high energy radiation acts as its primary generator. Ionization of a glass matrix takes place under the influence of electromagnetic radiation with high energy photons. The released charge carriers are captured into new local states forming color centers with the properties depending on the interaction between the centers and their surroundings. The centers themselves, while interacting with surrounding atoms, change the state of the latter; in the first place, they distort the distances between the atoms near color centers. The optical properties of radiatively colored multicomponent glasses are extremely complicated due to a variety of color center types and their strong parameter dispersion inside the disordered structure. The difficulties in experimental data interpretation are strongly increased by the fact that any action on glass, as a rule, gives rise to a simultaneous response of a number of types of color centers. Their decoloration has already been attempted for the optical recording of information with the help of radiative color centers (RCC). This kind of information recording is not reversible since, in such a situation, the recombination phenomenon implies an essential decrease of RCC concentration. The anisotropy of RCC in glasses enables us to introduce a reorientation process under the influence of optical radiation related to the recapture of delocalized charge carriers into a new state. Alumosilicate glasses processed by ~,-radiation have been investigated. Under the action of linearly polarized optical radiation, the absorption of light polarized identically with the activating radiation results in a decrease of the value (a~) over all the observed spectral range. Near 2.8 eV the absorption of light polarized perpendicular to the direction of activating radiation, polarization (a~) increases. In fig. 5 the dependence of the variation of a ~_ (curve 1) and a~ (curve 2) at 2.8 eV on the duration of activating radiation are plotted. Since radiation with h~, = 2.8 eV is not sufficient to ionize the glass matrix, the growth of a± indicates processes of RCC reorienP
244
G.T. Petrovskii / On the optical properties of glasses
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Fig. 5. Photostimulatedreorientationof hole color centers in alkali-silicateglass under the action of linearlypolarized optical radiation. 1, a.L, ' • 2, all.'
tation. The research showed that at the initial stage of action by linearly polarized light, RCC concentration does not change. One should stress I that at the same time the values a_L and all undergo substantial changes. This change means that in these glasses situations are implemented when the contribution of recombination processes is negligibly small compared with the background of the reorientation processes. Due to optical radiation, reorientation of the hole RCC, responsible for absorption in the band with the maximum at 2.8 eV, takes place. (An example of a unique use of radiatively colored glass is one suggested by Czechoslovakian technologists. They proposed decorating a new building of the National Theatre in Prague by glass slabs made up of a high-silica containing glass, Simax [8]. Five thousand 60 × 80 cm2 slabs were irradiated by a gamma-ray source to produce an original smoked amber-brownish color for the facade.) Let us now consider some not so ordinary phenomena accompanying interaction of a glass with ionizing photons [9,10]. The formation of color centers is accompanied by the appearance of mechanical stresses produced by volume changes in the irradiated part of the glass. The mechanism for this phenomenon is of a non-thermal nature. Variation of glass volume is caused by deformation of the surroundings of
local centers in the process of capturing charge carriers excited by ionizing radiation. The variation in the volume of one center is 6 × 10 -24 cm3, and the corresponding strain energy of its surroundings is about 1 eV, the relative variation of the volume of the irradiated zone is 5 × 10 -6, which corresponds to a value of 8 × 104 Pa (0.8 k g / c m 2) for stresses at the boundary of the zone and to a change in refraction index of approximately one part in 106.
5. Optical breakdown of glass In the discussion above we have not associated optical properties of glasses with the intensity of light flux. One of the most significant laws of optics - the law of light absorption, as has been established by P. Buger as early as in 1729 means physically that the process of photon losses does not depend on the light beam flux; in other words, the number of absorbing centers is independent of light intensity. The change comes, however, with high intensity light beams, for instance, due to energy absorption at the front of a pulse and re-irradiation into its back, or simply due to the saturation of populations of electronic states. Non-linear photostimulated processes also incorporate such an important phenomenon as the optical breakdown of glass placed in a field of powerful monochromatic radiation. An optical breakdown of materials under the action of passing radiation presents a serious obstacle to the power enhancement of laser systems. In addition to the usual optical properties of glasses, such as refraction index, dispersion and transparency range, a requirement of high radiation strength is now necessary. All the variety of processes governing the optical breakdown of transparent dielectrics reduces to the following three groups of phenomena. First, it is an admixture breakdown originating because of the interaction between laser radiation and admixture centers. Second, non-linear phenomena, such as self-focusing multiphoton absorption and forced scattering implying sub-threshold distortions of spatial-temporal characteristics of radiation within the interaction bulk, occur. Third,
G.T. Petrovskii / On the optical properties of glasses
optical breakdown may be caused by the interaction of laser radiation and the matrix without any subthreshold distortions of spatial-temporal structure in the interaction volume. Thus, in order to observe a true optical breakdown and to study its properties, one needs to apply the focusing of a single-frequency laser radiation, with quantum energy less than the halfwidth of the gap, to the bulk of a homogeneous optical material [11]. The true optical breakdown threshold is a material constant. This parameter has been measured for quartz glass and K-8 glass. The breakdown threshold value is about 10 a3 W / c m 2. A new conjecture regarding optical breakdown is essentially as follows: under the action of laser radiation, no individual ionization of atoms occurs, but a step-wise restructuring of electron states takes place which results in the disappearance of the dielectric gap. Under such circumstances, glass is transformed to a quasi-metallic state. Radiation absorption by this quasimetallic particle leads to the fast formation of a plasma cloud and to the forthcoming destruction [12]. Note, that for the first time a laser pulse of nanosecond duration was focused on a spot with a diameter less than 1 ~tm in our research. In a number of papers other authors report spots with diameters of about 3 ~tm and thresholds lower by two orders of magnitude. In the research of Moscow Physical School, an interpretation based on the dynamics of restructuring of the electron-phonon system of glass during the pulse was given [13]. They assumed that the rate of electron transfer from the irradiated spot is higher than the rate of the avalanche ionization. In the field of the laser pulse front, heated electrons more to the periphery of the spot. A hole in the electron density is formed at its center, the avalanche has no time to develop here, while in the periphery it cannot grow because of the exponential decay of the field.
6. Non-linear optical properties A set of optical phenomena in glass becomes complicated due to the collective (cooperative)
245
interaction of a system of emitters. One of them, for instance, is a superradiation phenomenon, when a system of excited atoms emits radiation during a time period much shorter than the radiation time of an individual atom, The atoms in such a system maintain mutual influence by means of a specific type of interaction between the particles, namely, with the help of their common radiation changing, suddenly, the optical properties of the material [14]. Phase transitions in quantum electronics systems originate mainly due to the re-irradiation dipole field interaction. The intensity of an exterior light wave serves as a varying exterior parameter analogous to temperature. At a certain critical value of the intensity, an order parameter appears spontaneously in the system as, for example, a step-wise variation of difference in populations. The appearance of the order parameter, in its turn, provides the changes of the spectroscopic characteristics of the material. The earmark of these phase transitions is their pronounced non-equilibrium nature: they occur either in the system inverted by an external pumping or under the action of a light wave changing, drastically, populations of electronic transitions. As an example of the cooperative behavior of an atomic system interaction via radiation field, one can point out the bistability of optical transrnitting revealed in the existence of two steady-state values of the output for a given output intensity Pm of the light wave incident on the system. One observes optical bistability when transmitting light through a material with saturable absorption. To obtain bistability, one has to have some non-linear medium with a refraction index or transmission coefficient depending on the intensity of the incident radiation. A facility is optically bistable when its o u t p u t - i n p u t optical power dependence is hysteretic. By its manifestations (step-wise variation of the transmission, hysteresis) optical bistability is the analog of a phase transition of the first kind. A curve depicting the dependence of transmission on Pm is similar to the van der Waals isotherm for the vapor-liquid phase transition. It is of extreme interest that this, new enough, optical phenomenon may be realized in the most
G. 7". Petrovskii / On the optical properties of glasses
246
27
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Fig. 6. Dependence of the transmission boundary position of glass on the size of cadmium selenide crystals (after ref. [15]).
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[18]). simple way by using familiar glasses tinted by cadmium sulpho-selenides. Actually, only a new level of understanding of process is these systems is required. Spectral-optical properties of glasses which contain semiconductor microcrystals are determined by the character of the energy spectrum of a semiconductor particles - activators. A semiconductor microcrystal inside a transparent dielectric glass matrix forms a potential well for nonequilibrium fight-produced charge carriers. For small microcrystals, their energy is specified by a dimension quantification of translational motion of non-equifibrium electron-hole pairs coupled by Coulomb interaction. This effect introduces a strong dependence of the spectral-optical properties of such heterogeneous glasses on the size of the particle activators [15,16].
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In fig. 6, as an example, we plot the experimental dependence of the spectral position of the transmission boundary of glasses containing CdSe microcrystals against the size of the latter. It can be seen that the strong dependence of the width of the semiconductor microcrystal gap on the size, caused by the quantum-dimensional effect, allows one to control the spectral position of the transmission boundary of such glasses within the limits of practically all the visible range. The first experimental observations of non-finear optical absorption in quasi-zero-dimensional structures, being microcrystals inside a glass-fike matrix, were described in 1987 [17,18]. In fig. 7 a dependence is shown of the value of transmission at h = 676.4 nm on the density of pumping measured at T = 80 K. Evidently, a substantial enlightenment is achieved at a comparatively small density of pumping radiation, i.e. P 1 M W / c m 2. In fig. 8, extracted from the paper by Yumoto et al. [18], a phenomenon of optical bistabifity is demonstrated.
0,¢, 7. Refraction index gradients in glass 0
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So far we have discussed the optical properties of homogeneous glasses or systems with inhomogeneities of a microcrystalline nature. We now
G.T. Petrovskii / On the optical properties of glasses
discuss the optical properties of glasses with optical inhomogeneities in the form of a refraction index gradient. As early as in 1886, Exner [19] noted that a cylinder made up of a translucent material with refraction index axially symmetric and parabolically dropping towards the axis forms an image similar to a lens. A ray propagating in such a cylinder happens to be sinusoidal and the optical force is a periodic function of the length of the element. In other words, the element may produce a real, imaginary, inverse or direct image of a subject depending on its length. One of the most fundamental elements of integral optics is a planar light guide - a thin layer of material whose refractive index differs from that of a substrate sufficiently for light to propagate only inside the layer due to total internal reflection. The waveguide effect in such glass films was studied in detail for the first time in the paper of Osterberg and Smith in 1964 [20]. A thorough study performed recently by us [21] shows that the change of refractive index under low-temperature ion exchange is controlled by three mechanisms, each of them having an absolute value larger than the resulting variation. In fig. 9, the variation of refractive index, determined by the total action of all three mechanisms, is shown. One can see that a positive contribution to the variation is given only by an increase of polarizability of K by replacing Na + (fig. 9, curve 1). The variation of refractive index
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Fig. 9. Influence of an exchanged potassium ion concentration on the refractive index variation according to various mechanisms. Curve 4 shows the experimentally determined total growth (see text for description of curves 1-4).
247
due to this mechanism may be as large as 250 × 10 -4 . Real values of refractive index variation happen to be two orders of magnitude smaller (curve 4) due to a joint negative contribution of the other two mechanisms - a concentration one, that is due to a decrease of the bulk concentration of particles because of the expansion of glass (curve 2), and one due to stress (curve 3). The absolute value of the first contribution grows more than linearly with the concentration of the diffusing ion, due to the non-proportionality of the volume change of glass with the concentration of K ÷ ions. From fig. 9 it can be seen that the variation of the refractive index caused by the action of microstresses in the investigated range of K ÷ concentrations (curve 3) remains approximately constant. This may be understood if one accounts for the fact that the influence of microstresses on the refractive index is determined by the deformation of each ion present in the glass as well as by their total number. With a high initial content of N a in glasses, the structure is open. It may be assumed that the deformation of K ÷ ions in such glasses would decrease. Therefore, since the concentration of the diffusing ion grows with decreasing Na concentration, the total effect on microstresses changes only slightly.
8. Glass as a prospective material for integral optics Films of terbium-containing glasses possess magneto-optical properties. These properties were also observed in glasses with three valent cerium and with cobalt ferrite microcrystals [22]. There exists the possibility of their integration with a radiation source, since waveguides on substrates made up of laser glasses have already been developed [23]. There also exists a possibility of their integration with a receiver of radiation, because any photosensitive layer has a good contact with glass. The possibility of integration with electronic circuits is excellent as well, since an ohmic buffer between the input and the output is provided. As for manufacturing, technological difficulties are here smaller than those when dealing with monocrystals.
248
G.T. Petrovskii / On the optical properties of glasses
Thus, only for the versions for which large electro-optical effects are required are there no satisfactory glass-like structures. On the other hand, the property of photosensitivity has only been implemented in waveguides on glass substrates. For waveguides on photochromic glasses, we have demonstrated [24] the possibility of the direct control of light flux by the action of another light beam. In waveguides formed by ion exchange diffusion in substrates of photochromic glass, an increase of photosensitivity with the growth of the guiding layer depth is detected, which is related to the departure of copper from the surface layers of glass into the salt melt under diffusion processing. Waveguides in photochromic glasses demonstrate simultaneously the quantum nature of light in the processes of color center formation and the wave nature in the processes of the discrete propagation of eigenmodes. Field maxima of waveguide modes with different numbers are essentially displaced one against the other: the main maxima of modes with large numbers are situated at greater depth. Under the action of exterior activating radiation, an absorbing (negative) mask is formed in the waveguide with the absorption profile corresponding to the photosensitivity gradient. When wavegide modes of different numbers travel through such an apodization mask, the higher number modes are absorbed more strongly than the lower number ones since the maxima of their field lie in the domain of maximum absorption. Such an interaction of modes with the absorbing mask leads to the selection of lower order waveguide modes (fig. 10). One of the most important problems is the creation of optical materials for optical frequency multipliers. To have an efficient generation of the second harmonics of optical radiation it is necessary to have phase synchronism, that is, equality of the first and the second harmonic velocities in a non-linear material. Since in any material propagation velocities of the first and second harmonics differ because of the dispersion, phase synchronism is usually achieved in anisotropic crystals if the difference of the refractive indices of ordinary and extraordinary rays is larger than the disper-
k
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sion of the material. In view of this fact, the choice of materials for second harmonic generation is limited. We have demonstrated the possibility of phase synchronism under the conditions of the non-linear interaction of waves in a gradient waveguide, i.e., the achievement of the equality of refractive indices of the same waveguide [25]. The limiting condition for at least one condition of phase synchronism to exist is that the maximum refractive index in the waveguide for the first harmonic has to be greater than the refractive index of the substrate for the second harmonic. Let us consider the possibility of the practical implementation of phase synchronism of the first and the second harmonics of a neodymium laser in a planar waveguide on optical K8 glass obtained by diffusion of silver. The parameters of the waveguide are for X = 1.06 ~tm, c1 = 1.5062, B 1 = 1.5262; for X = 0.53 ~tm, c 2 = 1.5192, B E --1.5393. The dependence of the model refractive indices on the thickness of the waveguide is shown in fig. 11. One is able to see that at certain thicknesses the modal curves of the first and second harmonic have an intersection. This means that for those thicknesses the propagation velocities of the harmonics coincide, i.e. the phase synchronism condition is fulfilled. As a conclusion the author would like to express his hope that the above exposition demon-
G.T. Petrovskii / On the optical properties of glasses
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Fig. 11. Dependence of the modal refractive indices in a gradient waveguide of K8 glass on the thickness of the waveguide for h =1.06 ~m (solid lines) and h=0.53 ixm (dashed lines).
strates to some extent the unbounded possibilities associated with the study and use of the optical properties of an old and eternally young material: glass.
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[6] J. Osadko, Zh. Eksp. Teor. Fiz. 90 (1986) 1453. [7] R. Macfarlane and R. Shelby, Opt. Commun. 45 (1983) 46. [8] Z. Prasil and M. Pesek, Sklar Keram. 36 (1986) 33. [9] L. Glebov, V. Dokuchaev and G. Petrovsky, Dokl. Akad. Nauk SSSR 274 (1984) 568. [10l L. Glebov, N. Niconorov and G. Petrovsky, Fiz. Khira. Stekla 12 (1986) 345. [11] O. Efimov and L. Glebov, Trudy Opt. Inst. 69 (1988), 1. [12] L. Glebov, O. Efimov, M. Libenson and G. Petrovsky, Dokl. Akad. Nauk SSSR 287 (1986) 1114. [13] A. Kozneev, V. Osadchiev and S. Pozdnyakov, Preprint Ing. - Fiz. Inst. N 024, Moscow (1987). [14] A. Andreev, V. Emelyanov and Y. llinsky, Koop. Javi. Opt. Moscow (1988). [15] A. Ekimov, A. Efros and A. Onushchenko, Sofid State Commun. 56 (1985) 921. [16] N. Borrelli, D. Hall, H. Holland and D. Smith, J. Appl. Phys. 61 (1987) 5399. [17] Y. Vandyshev, V. Dneprovsky and A. Ekimov, Zh. Eksp. Teor. Fiz~ Pisma 46 (1987) 393. [18] J. Jimoto, S. Fukushima and K. Kubodera, opt. Lett. 12 (1987) 831. [19] S. Exner, Repet. Phys. 22 (1986) 299. [20] H. Osterberg and L. Smith, J. opt. Soc. Am. 54 (1964) 1074. [21] L. Glebov, V. Dokuchaev, S. Evstropev and G. Petrovsky, Fiz. Khim. Stelda 14 (1988) 79. [221 G. Petrovsky, T. Sarubina and J. Edelman, Opt.-mech. Prom. 11 (1987) 33. [23] M. Babikova, V. Berenberg, L. Glebov and G. Petrovsky, Kvant. Elektron. 12 (1985) 1973. [24] G. Petrovsky, K. Agafonova and N. Nikonorov, Kvant. Elektron. 8 (1981) 2266. [25] L. Glebov, J. Morozova and G. Petrovsky, Dokl. Akad. Nauk SSSR 261 (1981) 589.