**2. Additive coloring of fluorite crystals**

As mentioned above, the color centers in fluorite crystals are anion vacancies that capture electrons. The additive coloring procedure is executed in gas‐controlled heat pipe [1, 2]. The essence of heat‐pipe method that implies the use of furnace and water‐cooled refrigerator (**Figure 1**) is in the spatial separation of a buffer inert gas (He) and metal vapor due to vertically directed metal diffusion at the temperature gradient formed by the furnace and the refrigerator. The metal vapor is condensed on the manipulator rod at a dew point temperature in a zone above the container with the crystal, drains to the hot zone and evaporates in it. As a result, the vapor‐gas mixture pressure is determined by the pressure of He that is in equilibrium with the metal vapor, thus being almost independent of the temperature of a sample under coloration.

The dynamic mode of the heat pipe—continuous circulation of metal vapor within it—is implemented at a fairly low pressure of saturating metal vapor at its freezing temperature. This condition is not satisfied for calcium, but is fulfilled for alkali metals. Therefore, a calcium‐lithium mixture (*10*) is used to implement the aforementioned mode. In this case, the dew point is determined by lithium that dominates in the mixture composition (*~*99%), and the coloring agent is calcium vapor.

Thus, the heat‐pipe method allows one to control the calcium vapor pressure, *p*, and temperature, *T*, of the colored crystal almost independently. The ranges of the parameter magnitudes are as follows: *p* = 10-4–1 Torr, *T* = 730–870°C.

Fluorite Crystals with Color Centers: A Medium for Recording Extremely Stable but Broadly Transformable Holograms http://dx.doi.org/10.5772/66114 407

widely used in the modern techniques. In this paper, a holographic medium is described that satisfies both these requirements: it is extremely stable and allows for reading out holograms

centers. Color centers in fluorite are the combination of anion vacancies and electrons trapped by the latter; the centers exhibit the absorption bands in the visible region and, thus, color the crystal. To form the centers in the crystal bulk, heating the crystal in the reducing atmosphere of calcium vapor is used (so‐called "additive coloring" of the crystal). Color centers can also be created in the crystal volume under the impact of *γ*‐radiation or high‐energy electron

The photochromism of colored crystals that allows for their use as a holographic medium is due to the transformation of color centers under the illumination of the crystal in the absorption

In this paper, the technique of additive coloring of fluorite crystals is described. The types of color centers are discussed as well as their photochromic transformations. Special attention is paid to the mechanism of hologram recording because it is this mechanism that determines the most important features of holograms recorded in this medium. The phenomenon of self‐ organization of color centers under hologram recording is considered. In conclusion, possible

As mentioned above, the color centers in fluorite crystals are anion vacancies that capture electrons. The additive coloring procedure is executed in gas‐controlled heat pipe [1, 2]. The essence of heat‐pipe method that implies the use of furnace and water‐cooled refrigerator (**Figure 1**) is in the spatial separation of a buffer inert gas (He) and metal vapor due to vertically directed metal diffusion at the temperature gradient formed by the furnace and the refrigerator. The metal vapor is condensed on the manipulator rod at a dew point temperature in a zone above the container with the crystal, drains to the hot zone and evaporates in it. As a result, the vapor‐gas mixture pressure is determined by the pressure of He that is in equilibrium with the metal vapor, thus being almost independent of the temperature of a sample under coloration. The dynamic mode of the heat pipe—continuous circulation of metal vapor within it—is implemented at a fairly low pressure of saturating metal vapor at its freezing temperature. This condition is not satisfied for calcium, but is fulfilled for alkali metals. Therefore, a calcium‐lithium mixture (*10*) is used to implement the aforementioned mode. In this case, the dew point is determined by lithium that dominates in the mixture composition (*~*99%), and

Thus, the heat‐pipe method allows one to control the calcium vapor pressure, *p*, and temperature, *T*, of the colored crystal almost independently. The ranges of the parameter magnitudes

, fluorite) with color

in the IR up to *~*10 μm. This medium is calcium fluoride crystals (CaF<sup>2</sup>

beams; however, such coloration is less stable.

406 Holographic Materials and Optical Systems

band of specific center at an elevated temperature.

applications of the medium are discussed.

the coloring agent is calcium vapor.

are as follows: *p* = 10-4–1 Torr, *T* = 730–870°C.

**2. Additive coloring of fluorite crystals**

**Figure 1.** Schematic representation of the heat‐pipe system: (*1*) helium supply, (*2*) stainless steel vacuum chamber, (*3*) vacuum valve, (*4*) manipulator for displacing the container with a sample, (*5*) line to the vacuum pump, (*6*) water‐cooled refrigerator, (*7*) furnace, (*8*) container, (*9*) sample, and (*10*) metal weight.

About several tens of coloring procedures can be implemented, with reproducible results, using the same lithium‐calcium weight.

The formation of color centers under the additive coloring is related to the deviation of the crystal from stoichiometry. The crystal surface builds up when interacting with metal vapor and using anions borrowed from the crystal bulk. In other words, the anion vacancies diffuse into the crystal simultaneously with electrons supplied by calcium to support the charge neutrality of the colored sample. One should note that, at sufficiently high coloring temperature, the surface undergoes decomposition (erosion). During this process, the metal cations pass to the gas phase at a low vapor pressure and remain on the crystal surface at pressures close to the saturation vapor pressure. Fluorine that evolved during the surface layer decomposition recombines with anion vacancies formed as a result of the interaction with the metal film on the surface. Thus, from the viewpoint of crystal stoichiometry violation, these two reactions, namely, the surface building up and decomposition, are oppositely directed. One should note that, for CaF<sup>2</sup> crystal, the process of building‐up the surface prevails over its decomposition.<sup>1</sup>

The recombination of anion vacancies and electrons diffusing into the crystal bulk produces a variety of color centers. The method used allows for the uniform coloring of CaF<sup>2</sup> crystals of a large size (**Figure 2**).

After the furnace is switched off in the end of the coloring procedure, the crystal cools down to the room temperature; a minute quantity of oxygen present in helium penetrates, because

<sup>1</sup> This is not the general case; for example, in CdF<sup>2</sup> crystal that has the fluorite structure, the rates of both processes are comparable and the surface of additively colored crystals turns out to be greatly eroded.

of absence of Ca vapor, into the surface layers of colored crystal and substitutes fluorine ions for the O<sup>2</sup> ions with electrons borrowed from the color centers. So, the surface layers of the crystal become partially discolored. For hologram recording, plates of uniformly colored inner part should be cut out and polished.

**Figure 2.** Samples of fluorite crystals 12 mm in diameter and 6 mm thick: initial (right) and additively colored (left).

To form color centers that consist of anion vacancies and electrons, it is necessary to use high‐ purity and high‐quality fluorite crystals. Luckily, fluorite is an important material of photolithography optics used together with excimer lasers for semiconductor chip production, which is why the technology of growing such crystals is well elaborated. So, such crystals are available and not expensive.
