Hydrogen as origin of thermal fixing in LiNbO3: Fe

Protons are identified as the mobile ions, which neutralize the electronic space charge fields during thermal fixing of holograms in LiNbO₃:Fe. The protons form OH^? centers and can be observed via vibrational absorption bands. In crystals with reduced OH^? content holograms cannot be fixed thermally. In crystals containing fixed patterns the OH^? concentration shows a spatial variation corresponding to the electric charge distribution.     

Bi-directional holographic information storage based on the optical reorientation of FA centers in KCl:Na

The dichroism of F_A centers in KCl is used to store erasable, high-resolution and high-contrast Bragg-angle holograms. The photochromic system has a bi-directional photo-response, high-writing sensitivity, practically non-destructive low-temperature read-out, and permits good recyclability. The holograms have nearly unlimited lifetime at freezer temperatures.     

New class of holographic materials based on semiconductor CdF2 crystals with bistable centers: III. Mechanisms of recording and decay of holographic gratings

The mechanisms of recording and decay of holographic gratings in CdF₂ crystals with bistable gallium or indium centers are considered. The analysis of the decay kinetics allows one to determine potentialities and conditions for using these crystals both for static recording of holograms and for holography in real time. This time scale is fairly broad and covers the time range from 1 s to 10 μs or shorter. Energy characteristics of the bistable centers, namely, the binding energies of the deep and shallow levels and heights of the barriers between them, are refined.     

A highly stable holographic medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: III. Properties of holograms

The properties and characteristics of the holograms recorded on colloidal centers in calcium fluoride crystals have been considered. The amplitude-phase character of the holograms, with a dominance of the amplitude component, is demonstrated. It is shown that the effective thickness of the holograms is somewhat smaller than the sample thickness. The diffraction efficiency and resolution of the holograms have been determined. A characteristic feature of the holograms considered is their high stability.     

Polarization sensibility of the photodichroic holographic recording

Volume holograms are stored in KCl:Na crystals using the photoinduced reorientation of F_A centers. The dependence of the diffraction efficiency on the directions of polarization of the recording and the reconstructing waves are shown. Experimental results are in good agreement with the calculated curves, predicted by the theory.     

Dynamic reflection holograms in CdF2 crystals with bistable centers

The diffraction efficiency and the recording and relaxation times of dynamic reflection holograms, recorded in CdF₂ crystals with bistable centers are studied experimentally in the temperature range 20–100°C. In the model experiments which measured the quality of the wave reflected from the hologram, the dynamic wavefront distortions are demonstrated to be efficiently compensated using a holographic corrector based on these crystals. CdF₂ crystals with bistable centers are likely to be useful in solving problems of correction of laser light wavefront and image correction in observation telescopes with nonideal primary mirrors.     

Effective room-temperature broad-band recording of 3D dynamic holograms in CdF<Subscript>2</Subscript>:Ga crystals

Photochromic CdF₂:Ga crystals with bistable impurity centers were effectively used for the dynamic recording of holograms and readout over the visible and near IR spectral regions at spatial frequencies of up to 5000 mm^?1 at room temperature. The diffraction efficiency of the dynamic holograms was as high as 60% at maximum and exceeded 1% when the beams’ intensities were in the ratio 1:100. As one goes from the low temperatures (≤200 K) to 300 K, the peak diffraction efficiency of the dynamic holograms decreases approximately by a factor of 1.5, while the speed of their response and photosensitivity in the long-wavelength spectral region increases by more than an order of magnitude. For the sake of comparison, the dynamic holograms were recorded under the same conditions as the widely used electrooptical SBN crystals. Comparative analysis ascertained a unique combination of the useful features offered by CdF₂:Ga crystals in holography.     

Postexposure sensitization of holograms on the basis of color centers in CaF2 crystals by incoherent radiation

It has been reported about recording highly stable holograms on the basis of color centers in calcium fluoride crystals at a temperature of 200°C by UV light. In this study, holograms were recorded by visible light (4880 Å). The recorded holograms have a low diffraction efficiency, which can be significantly increased using postexposure irradiation of a sample by incoherent UV light at a temperature of 200°C. This increase demonstrates an important role of the drift component in the diffusion-drift mechanism of recording holograms based on color centers in ionic crystals.     

Holograms in CaF2 crystals with high content of alkaline impurities

Similarities and dissimilarities of holograms recorded in fluorite crystals with color centers that are pure or contain relatively high concentrations of an alkaline impurity (sodium) are considered. It is shown that, in these two cases, the mechanism of recording is the same, but formation of the impurity-vacancy complexes in heavily doped crystals causes certain peculiarities in the types of color centers in the initial crystals and those with holograms and determines specific features of spatial redistribution of centers in the process of recording. These features reduce the diffraction efficiency of holograms in heavily doped crystals compared to pure and weakly activated crystals.     

Energy barrier between states of a bistable center in photochromic crystals CdF2:Ga and CdF2:In

The establishment of thermal equilibrium between photoinduced (shallow) and ground (deep) states of bistable DX centers in photochromic crystals CdF₂:In and CdF₂:Ga, which are used as real-time holographic media, is studied based on the notions of chemical kinetics. Two mechanisms of mutual transformation of shallow and deep centers—the tunnel mechanism and the mechanism with the participation of free charge carriers—are considered. Equations describing the decay of a photoinduced shallow state are obtained. These equations take into account the distribution of electrons between the photoinduced and ground states and the conduction band. Analysis of the experimental kinetic curves of the decay of photoexcited shallow centers makes it possible to determine the activation energies and barrier height for thermally activated processes of mutual transformation of shallow and deep centers. In CdF₂:In and CdF₂:Ga, this barrier, which determines the decay kinetics of holograms, lies above the bottom of the conduction band by ～10 and ～500 meV, respectively.     

Fluorescence excitation spectroscopic characterisation of F2 - colour centers in a LiF crystal

LiF crystals with F₂ ^- centers contain a whole series of F-based colour centers with spectrally overlapping absorption spectra. On the short-wavelength side the absorption spectrum of the F₂ ^- centers is overlapping with the absorption spectra of other colour centers and is therefore not directly accessible by transmission measurements. Fluorescence excitation spectroscopy is employed here to gain information on the absorption spectrum of F₂ ^- centers in spectral regions where the absorption by other centers is present. The fluorescence signal emitted beyond 1200 nm is detected as a function of the excitation wavelength in the range from 350 nm to 1100 nm. The F₂ ^- center absorption cross-section spectrum in the wavelength range from 690 nm to 1100 nm and from 350 nm to 410 nm is determined from the fluorescence excitation quantum distribution and the transmission spectrum. Influences of fluorescence re-absorption and Förster-type energy transfer on the fluorescence excitation quantum distribution and the subsequent F₂ ^- center absorption cross-section determination are discussed. PACS 61.72.Ji; 78.40.Ha; 78.55.Fv     

Optimization of the recording conditions for holograms recorded in additively colored KCl crystals

The dependences of the characteristics of volume holograms recorded in additively colored crystals of potassium chloride on the spatial frequency and the recording temperature are studied. It is found that these dependences are nonmonotonic, which indicates the presence of nonlocal mechanisms of holographic recording in the crystals. These mechanisms involve diffusion and drift processes of redistribution of color centers between the nodes and antinodes of a recorded interference pattern. The results obtained are used to determine the recording conditions providing the maximum possible diffraction efficiency of the holograms.     

A nondiffusion mechanism for writing shifted holograms in LiNbO3

This paper attempts to explain the experimental facts that the resolution of LiNbO₃ crystals in writing holograms is poor, and that large discrepancies exist between the measured characteristics of diffraction efficiency and predictions based on current and widely used models. A new model for charge transport is proposed that takes into account macro-and mesoscopic nonuniformities. It is shown that the electric fields arising from modulation of the spontaneous polarization by defects lead to asymmetric diffusion of photoexcited electrons, which makes it highly unlikely that the writing of shifted components of holograms is via a diffusion mechanism. Possible characteristics of shifted and non-shifted holograms written via photogalvanic currents are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1636–1642 (September 1997)     

Effect of high-energy electron irradiation on forsterite laser crystals

The effect of 21-MeV electron irradiation on the optical absorption characteristics of Czochralski-grown forsterite (Mg₂SiO₄) single crystals (both undoped and chromium-doped) has been investigated. The irradiation is found to induce additional optical absorption (AOA) in the crystals in the range of 225–1200 nm due to the formation of color centers based on intrinsic host point defects and the change in the oxidation state of chromium ions. The AOA spectra have been decomposed into elementary bands. The influence of the chromium concentration in crystals, the oxygen content in the growth atmosphere, and additional doping with lithium on the behavior of these bands has been analyzed. A possible structure of the color centers responsible for the AOA is discussed. It is shown that the electron irradiation somewhat decreases the intensity of the characteristic absorption bands of tri- and tetravalent chromium ions and gives rise to a new absorption band in Mg₂SiO₄:Cr and Mg₂SiO₄:Cr,Li crystals heavily doped with chromium.     

Oscillator strength of electron-type color centers in KCl single crystals irradiated with electrons and protons

The absorption spectra of KCl single crystals irradiated with electrons and protons at energies of 15 and 100 keV and a particle flux ranging from 5×10¹² to 10¹⁵ cm^?2 are investigated. The absorption bands attributed to simple (F, F _a, K) and complex (M, R ₂, R ₄, N) color centers are identified in the spectra. The correlation dependences of the absorption coefficients for M, R ₂, and R ₄ centers on the absorption coefficient of F centers and the correlation dependences of the absorption coefficients for R ₂ and R ₄ centers on the absorption coefficient of M centers are established. The oscillator strengths are calculated for M, R ₂, and R ₄ color centers.     

The diffraction efficiency of hologram grating recorded in an absorptive medium

The profile of the hologram grating is described considerating the absorption of the recording medium and its diffraction efficiency is calculated for special cases using the coupled wave theory. The maximum diffraction efficiency obtainable is not affected by the absorption of the recording medium in the case of transmission holograms, while it decreases with increasing absorption in the case of absorptive reflection holograms.     

Luminescence excitation spectra and characteristics of luminescence centers with overlapping absorption bands

For an ensemble of different types of luminescence centers with overlapping absorption bands, with no restrictions on the optical densities, we have obtained relations describing the luminescence excitation spectra for each type of center. We consider transformations of the relations in some limiting cases. We suggest a procedure for using the equations obtained to determine the characteristics of the luminescence centers. Some of these procedures have been experimentally implemented in study of intrinsic radiation color centers in lithium fluoride crystals. We have determined the ratios of the luminescence quantum yields for F₂ and F₃⁺ color centers, and we have observed that a major role is played by nonradiative transitions in deactivation of the first excited singlet state of F₃⁺ centers. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 365–371, May–June, 2008.     

Image polarisation characteristics storage in birefringent crystals

The possibility of image polarization characteristics storage on the base of Bragg condition difference for orthogonally polarized light waves in birefringent media is considered theoretically. Experimental results on holographic writing in uniaxial LiNbO₃ are in good agreement with theoretical predictions. An example of the writing and restoring of the object beam polarisation characteristics in LiNbO₃ is given. A possibility of electrically controlled volume holograms writing in the electro-optic crystals is discussed and demonstrated.     

The photoluminescence excitation spectra of multicomponent media

With no restrictions on the value of optical density, we derive an equation that describes the photoluminescence excitation spectra of separate components with overlapping absorption bands in a multicomponent medium. Based on the equation obtained, we propose methods for measuring the ratios of absorption optical densities of the components and quantum yields of their luminescence. The applicability of the proposed methods is demonstrated by measuring the characteristics of coumarin and porphyrin dissolved in different proportions in ethanol. We have also measured the quantum yield and luminescence decay time of the radiationinduced color centers F ₃ ⁺ in the lithium fluoride crystal and calculated probabilities of radiative and nonradiative transitions from the first excited singlet state of these centers.     

Recording volume holograms on color centers in a CaF2 crystal

The conditions for preparing photochromic CaF₂ crystals, recording holograms with 532-nm radiation in them, and postexposure photothermal treatment of samples with holograms that provide the formation of holographic elements of significant thickness (about 1 cm) are considered. It is shown that the diffraction efficiency of volume holograms in such crystals in the case of reading out by radiation with wave-lengths of 532 and 632.8 nm can be as high as 10–15%. The role of the recording mechanism and the Borrmann effect in the fabrication of these holograms is demonstrated.