Breaking the stoichiometric composition of calcium fluoride crystals under the impact of an ionizing radiation or at heating in the reduction atmosphere of metal vapors (so called “additive coloring” of crystal) results in color centers formation. In the process of additive coloring these centers arise due to recombination of anion vacancies and electrons which are produced on the heated crystal surface and diffuse into the crystal bulk. The centers are subdivided into “simple” (F, M, R and N) centers, which are composed of 1–4 anion vacancies, respectively, with an equal numbers of electrons and highly-aggregated centers. It was initially proposed that such centers are colloidal particles of calcium, which arise as a result of transformation of large accumulation of anion vacancies and electrons into metal inclusions [1,2]. Another type of highly-aggregated centers was found later, that is “quasi-colloidal” centers (see [3]). Their structure is unknown, it is likely that they are in an intermediate position between simple and colloidal center by the number of vacancies/electrons. Color centers have characteristic absorption bands. The most intensive absorption bands of simple centers are in the wavelength range of l < 550 nm, the absorption band of colloidal center is within the wavelength range of l = 550–650 nm, the bands of quasi-colloidal centers cover the wide spectral range of ~0.6–6 mm. As a rule, additively colored CaF₂ crystals contain predominantly simple and colloidal centers. Their ratio depends on the concentration of vacancies/electrons introduced into the crystal during its coloring, i.e. on the coloring mode. The stiffer the mode (the higher the calcium vapor pressure and the sample temperature) the larger the relative content of colloidal centers [4]. It was recently found that actually these centers are two-dimensional rounded or prolate metal islands with thickness of 1.3–1.4 nm and lateral size of 30–200 nm. Such islands occur at vacancies/electrons concentration of ~10¹⁷ cm^-3[5]. When concentration increases up to ~10¹⁸ cm^-3 islands coalesce forming as it film fragments with pores of arbitrary shape and different size. Nevertheless the height of coalesced islands remains equal to 1.3–1.4 nm. Absorption spectrum of such crystals practically contains only the band of colloidal centers. That means the most of color centers are coalesced into “colloidal” particles scattered randomly over the crystal volume and only their small part form film fragments. These fragments are disposed in selected {111} planes; the crystal is cleaved exactly along these planes. Absorption spectrum of the crystals with even higher concentration of color centers changes drastically. The intensity of absorption band of “colloidal” centers decreases, and intense non-selective absorption arises typical for thin metal films. In such a crystal a majority of color centers are coalescent into fragments of such films. Under impact of the radiation resonant to absorption bands of particular color centers and the temperature the conversion of color centers occurs [6,7]. Their photochromic transformations underlie using CaF₂ crystals with color centers as highly-stable volume holographic media [8,9]. One should note that the specific diffusion-drift mechanism of hologram recording results in accumulation of color centers in the minima of fringe pattern; the larger the exposure the narrower are the regions of centers concentration [10,11]. This feature favors metal inclusion formation. Thus, recording holograms in additively colored CaF₂ crystal with sufficiently high concentration of anion vacancies/electrons can convert it into a metamaterial-like state.

1. H.V. Den Hartog, W. Tinbergen, W.G. Perdok, Phys. Status Solidi A, 2 (1970) 347-358.

2. W. Hayes, ed., Crystals with the Fluorite Structure: Electronic, Vibrational, and Defect Properties (Clarendon Press, Oxford, UK, 1974).

3. A.S. Shcheulin, Thesis (St.-Petersburg, 2008, in Russian).

4. A.S. Shcheulin, T.S. Semenova, L.F. Koryakina, M.A. Petrova, A.E. Angervaks, A.I. Ryskin, Opt. Spectrosc., 110 (2011) 617-623.

5. A.E. Angervaks, A.S. Shcheulin, A.I. Ryskin, P.P. Fedorov, R.V. Gainutdinov, Appl. Surf. Sci., 267 (2013) 112-114.

6. V.M. Orera, R. Alkala, Solid State Commun., 27 (1978) 1109-1112.

7. N.E. Korolev, I.Yu. Mokienko, A.E. Poletimov, A.S. Shcheulin, Opt. Spektrosk., 70 (1991) 1030-1034 (in Russian).

8. A.S. Shcheulin, A.K. Kupchikov, A.E. Angervaks, A.I. Ryskin, Opt. Spectrosc., 103 (2007) 651-654.

9. A.S. Shcheulin, A.V. Veniaminov, Yu.L. Korzinin, A.E. Angervaks, A.I. Ryskin, Opt. Spectrosc., 103 (2007) 655-659.

10. A.V. Veniaminov, A.S. Shcheulin, A.E. Angervaks, A.I. Ryskin, J. Opt. Soc. Am. B, 29 (2012) 335-339.

11. A.S. Shcheulin, A.E. Angervaks, A.V. Veniaminov, V.V. Zakharov, A.I. Ryskin, Opt. Spectrosc., 113 (2012) 638-642.

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