A. E. Angervaks

Additive coloring rate and intensity for pure and doped fluorite crystals

The mechanism of the additive coloring of calcium fluoride crystals, both pure and doped with I- and III-group elements, has been investigated. The coloring rate and intensity and the dependences of these parameters on the experimental conditions, vapor mixture pressure, and temperature have been experimentally determined. It is shown that the concentration of anion vacancies/electrons that can be introduced into crystals is in the range of 1017–1018 cm−3; in crystals with a high concentration of rare earth ions, the limiting concentration can be an order of magnitude higher.

A highly stable holographic medium based on CaF2: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.

Holographic prism as a new optical element: II. Method of measurement of reproducible angles

To determine the precise values of angles stored and reproduced by a new multivalued measure of plane angles, i.e., a holographic prism, it is expedient to use a procedure for collating two circular scales, one of which is determined by the prism upon its rotation on a rotary table, while the other is the device’s own scale. The two scales are considered to be equivalent. The collation procedure is discussed taking into account target (deviations of sought values of angles of both scales from their nominal values) and a number of nuisance parameters of the procedure. The scheme of the collation device is considered. The geometric model that determines sources of errors caused by design features of signal reading and recording devices of the holographic prism is analyzed.

Holographic prism as a new optical element: III. Experimental implementation of a holographic prism of modification II and comparative characterization of the two holographic prism modifications from the application point of view

The conditions for the formation of a holographic prism of modification II (for which a beam of diffracted rays is induced by a reference laser beam in the plane oriented perpendicularly to the reference beam) are described. A sample of this prism is implemented experimentally, and its response to the signal (reference) beam is demonstrated. Two modifications of a holographic prism as a basis for solving metrological problems of storing and reproducing flat angles and transferring the angle size to gradable and/or calibrated angle-measuring tools are compared. Each modification is found to have advantages in particular fields; thus, none can be considered as unconditionally preferred. The final choice of a particular modification can be made only after detailed analysis of the devices on their basis.

Dependence of the profile of a hologram recorded on color centers in a CaF2 crystal on the type of hologram-forming centers

A hologram recorded in a CaF2 crystal by modulating the concentration of simple centers (F, M, R, and N) and highly aggregated (mainly colloidal) color centers has been exposed to incoherent 365-nm light to a dose of about 4000 J/cm2 at a temperature of 80°C. As a result of this exposure, the color centers were almost completely transformed into quasi-colloidal centers. The hologram was retained, and its diffraction efficiency increased; the intensity and number of observed diffraction orders increased as well. Analysis of the angular dependences of the diffraction response made it possible to construct a spatial profile of the hologram, which turned out to be much narrower in comparison with the initial-hologram profile; both profiles differ from a sinusoidal one. The hologram’s stability to such a large exposure to incoherent light at an elevated temperature is related to the peculiar diffusion-drift mechanism of hologram recording on color centers in ionic crystals.

Two-dimensional metal inclusions in a dielectric crystal

Highly aggregated color centers in crystals of a wide-band-gap calcium fluoride insulator heated in a reducing atmosphere of vapors of calcium metal cation (“additive coloration” of crystals) have been studied using spectroscopy and atomic-force microscopy methods. These centers are large aggregates of anion vacancies and electrons, which transform into metal structures. The evolution of these structures with increasing number of such components has been traced from individual particles with two-dimensional structure to large film fragments. It has been shown that fragments are concentrated in isolated planes {111}, which are cleavage planes of the crystal with high contents of highly aggregated centers.

Formation kinetics of volume holograms in reversible photochromic media

The specific features of recording a volume hologram in a photochromic reversible medium, which is characterized by simultaneous occurrence of hologram recording and relaxation, have been considered by the example of two ion crystals, i.e., cadmium fluoride with bistable gallium centers and calcium fluoride with color centers.

A highly stable holographic medium based on CaF2:Na crystals with colloidal color centers: II. Mechanisms of hologram recording and erasure

The mechanisms of formation and erasure of holographic gratings recorded in calcium fluoride crystals with a low sodium content have been considered. It is found that in the case of recording by UV radiation, an effective holographic grating is formed due to the spatial modulation of the colloidal centers arising under the action of the radiation at temperatures about 200°C. The decisive role of photoinduced diffusion of anion vacancies in this modulation is demonstrated.

Photothermal transformation of color centers in CaF2 crystals

Photothermal transformations of color centers in additively colored calcium fluoride crystals and in a colored crystal with a recorded hologram have been investigated. It is shown that the absorption spectrum of a colored crystal can be reconstructed within the entire transparency range of the matrix crystal by varying both factors affecting the sample—actinic radiation wavelength and temperature—as well as the duration of their effect. This possibility is important for the application of additively colored CaF2 crystals as a holographic medium.

Transformation of color centers during hologram recording in an additively colored CaF2 crystal

When holograms are recorded on color centers in calcium fluoride crystals, these centers undergo spatial redistribution in the crystal bulk, which is accompanied by their transformation. The nature of this transformation has been investigated by optical spectroscopy and confocal scanning microscopy. It is shown that, under the recording conditions we used, the degree of center aggregation increases in both minima and maxima of the interference field in which the recording performed. The enhanced aggregation in field minima is caused by the increase in the concentration color centers, while the additional aggregation in maxima is determined by the specific conditions of hologram recording: the wavelength and power density of recording radiation and the crystal temperature.