**3. Color centers and their photothermal transformation**

Color centers in fluorite crystals may be separated into three groups. "Simple" centers are those comprising 1–4 anion vacancies and equal number of trapped electrons (*F*‐, *M*‐, *R*‐, and *N*‐centers, respectively); the structure and energy levels of simple centers in fluorite crystals are well studied [3]. "Colloidal" centers (colloids) are the giant agglomerates of vacancies and electrons converted, at the coloring temperature, into the metal calcium drops less than *~*50 nm in diameter (its value depends on the coloring mode). "Quasi‐colloidal" centers occupy, by the number of vacancies/electrons, an intermediate position between simple and colloidal centers; probably, they are more or less large agglomerates of simple centers.

All centers are characterized by the specific absorption bands. The bands of simple centers are located in the *λ* < 550 nm wavelength range. The extinction of colloidal centers is well described by Mie theory [4–6]; the visible band of these centers is located in the 500 nm < *λ* < 600 nm range depending on the coloring mode (the second band of colloidal centers is located at *~*200 nm). There is a lot of quasi‐colloidal centers, their bands covering a wide spectral range, 550 nm < *λ* < 10 μm. The bigger the quasi‐colloidal center, the closer its absorption band to the band of colloids [7].

The modification of coloring conditions (calcium‐vapor pressure and temperature) determines the composition of color centers in the colored crystal. The higher the calcium pressure the larger the amount of colloidal particles formed (**Figure 3**); their size increases with an increase in their concentration in the colored crystal. Only a minute quantity of quasi‐colloidal centers arises during the coloring process because they are less stable compared to the simple and colloidal centers and cannot exist at the coloring temperature.

of absence of Ca vapor, into the surface layers of colored crystal and substitutes fluorine ions

crystal become partially discolored. For hologram recording, plates of uniformly colored

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

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

Color centers in fluorite crystals may be separated into three groups. "Simple" centers are those comprising 1–4 anion vacancies and equal number of trapped electrons (*F*‐, *M*‐, *R*‐, and *N*‐centers, respectively); the structure and energy levels of simple centers in fluorite crystals are well studied [3]. "Colloidal" centers (colloids) are the giant agglomerates of vacancies and electrons converted, at the coloring temperature, into the metal calcium drops less than *~*50 nm in diameter (its value depends on the coloring mode). "Quasi‐colloidal" centers occupy, by the number of vacancies/electrons, an intermediate position between simple and colloidal

All centers are characterized by the specific absorption bands. The bands of simple centers are located in the *λ* < 550 nm wavelength range. The extinction of colloidal centers is well described by Mie theory [4–6]; the visible band of these centers is located in the 500 nm < *λ* < 600 nm range depending on the coloring mode (the second band of colloidal centers is located at *~*200 nm). There is a lot of quasi‐colloidal centers, their bands covering a wide spectral range, 550 nm < *λ* < 10 μm. The bigger the quasi‐colloidal center, the closer its absorption band

The modification of coloring conditions (calcium‐vapor pressure and temperature) determines the composition of color centers in the colored crystal. The higher the calcium pressure the larger the amount of colloidal particles formed (**Figure 3**); their size increases with an increase

**3. Color centers and their photothermal transformation**

centers; probably, they are more or less large agglomerates of simple centers.

ions with electrons borrowed from the color centers. So, the surface layers of the

for the O<sup>2</sup>


408 Holographic Materials and Optical Systems

available and not expensive.

to the band of colloids [7].

inner part should be cut out and polished.

**Figure 3.** Absorption spectra of 2.4 mm‐thick CaF<sup>2</sup> crystals colored at temperature *T* = 830°C and pressure *p* equal to (i) 3 × 10-4 Torr ("weakly colored," solid line) and (ii) 8 × 10-3 Torr ("strongly colored," dotted line).

Temperature determines the coloring time, but it also plays an important role in determining the composition of color centers present in the crystal in another aspect. The illumination of crystal by radiation resonant to the absorption band of a specific center at elevated temperature results in the destruction of this center and formation of another type of centers. This type crucially depends on temperature. *T* > 300°C is favorable for the simple center formation because of high entropy of these centers. At *T* = 150–200°C, the colloidal centers arise. The 70–150°C temperature range is favorable for the quasi‐colloidal center formation. The lower temperatures of this range correspond to the formation of long‐wavelength quasi‐colloids; at higher temperatures, the short‐wavelength quasi‐colloids arise (**Figure 4**).

Photochromism of color centers in additively colored CaF<sup>2</sup> crystals allows for hologram recording on the crystals.

**Figure 4.** Absorption spectra of samples additively colored (*p* = 3 × 10-4 Torr and *T* = 830°C) and irradiated for 30 hours with the high‐pressure mercury lamp (*λ* = 365 nm) at *T* = 70, 85, 125, and 160°C (solid, dotted, dashed‐dotted, and dashed lines, respectively).
