**6. Hologram convertibility under the photothermal treatment using the incoherent radiation**

The small amount of color centers (electron traps) between the holographic planes is a premise for the hologram stability with respect to the optical radiation and temperature. When the crystal with hologram is illuminated by incoherent radiation, most of the photoionized electrons arising in the holographic planes cannot be captured by these centers and be localized between the holographic planes. They return to the planes under the effect of electric field generated by their removal from centers subjected to the ionization. However, if the optical radiation is resonant with respect to the color centers dominating in the planes, the recombination of returning electrons with photoionized centers results in the formation of other centers, the type of these centers depending on the crystal temperature. This process opens up a unique possibility for the hologram reconstruction with the incoherent radiation [16, 17].

In **Figure 10**, the absorption spectra of additively colored CaF<sup>2</sup> crystal and sample cut of this crystal with hologram (Hologram *1*) are shown. One may see that hologram recording leads to increased absorption of colloidal centers (the shoulder at *~*600 nm) at the expense of simple centers. This sample was subjected to the series of successive photothermal transformations that converted Hologram *1* to Holograms *2*–*5* (**Table 2**). The spectra of the samples with these holograms are shown in **Figure 11**. All holograms were read out using the DPSS laser (532 nm), and Holograms *2*–*5* were also read out with diode Thorlabs S3FC1550 laser (1.55 μm). The types of holograms and their DE values are shown in **Table 2**.

As seen (**Figure 12**), noticeable narrowing of the profile of the treated holograms and an accompanying increase in the intensity of the higher diffraction orders occur.

These facts can be explained by spatial redistribution of various center types in Hologram *1*. The highly aggregated color centers are located predominantly in the immediate vicinity of the

Fluorite Crystals with Color Centers: A Medium for Recording Extremely Stable but Broadly Transformable Holograms http://dx.doi.org/10.5772/66114 419

The hologram profile determined from the luminescence measurements (i.e., the spatial distribution of luminescent color centers) is adequately described by the sum of three harmonic components (amplitude ratio 100:50:19, **Figure 8**). This ratio does not differ strongly from the harmonics ratio for the absorption profile 100:58:22 that follows from the angular dependences of diffraction efficiency and represents the spatial distribution of all color centers forming the grating. This confirms that both the spatial profiles reconstructed from holographic and microscopic measurements are determined by the same spatial distribution

The modulation amplitude of absorption coefficient found from the analysis of angular dependences shows that the concentration of colored centers between the holographic planes is small as compared to the average absorption of the crystal with the hologram. At the saturation value of DE, this concentration does not exceed several percents of the total amount of the centers. The overwhelming majority of color centers present in the crystal with hologram are

**6. Hologram convertibility under the photothermal treatment using the** 

The small amount of color centers (electron traps) between the holographic planes is a premise for the hologram stability with respect to the optical radiation and temperature. When the crystal with hologram is illuminated by incoherent radiation, most of the photoionized electrons arising in the holographic planes cannot be captured by these centers and be localized between the holographic planes. They return to the planes under the effect of electric field generated by their removal from centers subjected to the ionization. However, if the optical radiation is resonant with respect to the color centers dominating in the planes, the recombination of returning electrons with photoionized centers results in the formation of other centers, the type of these centers depending on the crystal temperature. This process opens up a unique possibility for the hologram reconstruction with the incoherent radiation [16, 17].

crystal with hologram (Hologram *1*) are shown. One may see that hologram recording leads to increased absorption of colloidal centers (the shoulder at *~*600 nm) at the expense of simple centers. This sample was subjected to the series of successive photothermal transformations that converted Hologram *1* to Holograms *2*–*5* (**Table 2**). The spectra of the samples with these holograms are shown in **Figure 11**. All holograms were read out using the DPSS laser (532 nm), and Holograms *2*–*5* were also read out with diode Thorlabs S3FC1550 laser (1.55 μm). The

As seen (**Figure 12**), noticeable narrowing of the profile of the treated holograms and an

These facts can be explained by spatial redistribution of various center types in Hologram *1*. The highly aggregated color centers are located predominantly in the immediate vicinity of the

accompanying increase in the intensity of the higher diffraction orders occur.

crystal and sample cut of this

In **Figure 10**, the absorption spectra of additively colored CaF<sup>2</sup>

types of holograms and their DE values are shown in **Table 2**.

of color centers.

418 Holographic Materials and Optical Systems

located in the holographic planes.

**incoherent radiation**

**Figure 10.** Absorption spectra of CaF<sup>2</sup> sample with color centers before (solid line) and after (dotted line) recording Hologram *1*.


**Table 2.** Sample treatment parameters and hologram characteristics.

**Figure 11.** Absorption spectra of samples with Holograms *2*–*5* (a) and Hologram *2* in the extended wavelength range (b).

**Figure 12.** Refractive index profiles of Hologram *1* (dotted line), Hologram *2* (solid line), and Hologram *3* (dashed line) as reconstructed from the angular dependences of the diffraction response at 532 nm. The half‐widths of the profiles are 1.00 μm (Hologram *1*), 0.65 μm (Hologram *2*), and 0.70 μm (Hologram *3*).

fringe pattern minima, where the density of vacancies/electrons is relatively high, whereas the peripheral areas of these minima contain mainly simple centers. During Stage (1), the 365 nm radiation effectively destroys the simple centers because all of them have the absorption bands located near this wavelength. However, this radiation just weakly affects the highly aggregated centers. Electrons arising under the photoionization of simple centers move towards the central areas of holographic planes, where the concentration of color centers (traps) is higher than that at the peripheral areas and localize there generating an electric field that attracts vacancies. This increases somewhat the center concentration in the central areas and forms the quasi‐colloidal centers stable at temperature of about 80°C. This results in narrowing the Hologram *2* profile that becomes more meander‐like. As shown in **Figure 11**, the transformation of the long‐wavelength quasi‐colloidal centers into the predominantly short‐wavelength quasi‐colloidal and colloidal ones does not result in a noticeable change of the profile. It should be noted that, at elevated temperatures, the color centers of various types are in equilibrium with each other. The equilibrium state can shift toward a certain type of color centers depending on temperature and, on illuminating the sample, on the wavelength and intensity of the light. At Stages (1) and (2), both heating and the illumination play the same role in facilitating the concentration of the centers and their transforming into the long‐wavelength quasi‐ colloidal centers during Stage (1) and into the short‐wavelength and colloidal centers during Stage (2); this is the reason for narrowing the profiles of Holograms *2* and *3*. During Stage (3), in contrast, these factors act in the opposite directions. The temperature of 186°C favors the colloidal center formation; however, the 578 nm radiation destroys them, as well as the short‐ wavelength quasi‐colloidal centers present in Hologram *3*, thus hindering the accumulation of color centers, which results in some broadening the Hologram *4* profile.

**Figure 11.** Absorption spectra of samples with Holograms *2*–*5* (a) and Hologram *2* in the extended wavelength range (b).

**Figure 12.** Refractive index profiles of Hologram *1* (dotted line), Hologram *2* (solid line), and Hologram *3* (dashed line) as reconstructed from the angular dependences of the diffraction response at 532 nm. The half‐widths of the profiles are

1.00 μm (Hologram *1*), 0.65 μm (Hologram *2*), and 0.70 μm (Hologram *3*).

420 Holographic Materials and Optical Systems

The sample with the hologram under above transformations was maintained at 80–190°C for more than 8 days and *~*2/3 of this period the sample was under the impact of actinic radiation with the total exposure of 22 kJ cm-2. Such treatment does not result in the hologram erasure. It should be noted that (i) the absorption spectrum of the sample with Hologram *4* thus treated practically coincides with the spectrum of the initial sample and (ii) DE of the hologram is reduced only by *~*1.3% as compared to that of Hologram *1* (it should be taken into account that these two holograms are similar but not wholly identical). As seen, one can state an extremely high stability of holograms in this medium with respect to the optical radiation and temperature.

The read out of Hologram *5* with 532 nm laser shows the equality of intensities of diffracted and transmitted radiation (Borrmann effect) due to a large absorption at the readout wavelength.

The above considerations allow for managing the hologram type (amplitude‐phase, mostly amplitude, or mostly phase one), characteristics, and diffraction efficiency. Such changes can be implemented throughout the visible and IR spectral ranges up to the CaF<sup>2</sup> transparency limit (10 μm). It should be noted that **Figure 11** shows only the examples of center‐type transformation. Actually, one can perform the finer "tuning" of the center type by modifying the wavelength of incoherent radiation and temperature. The other ruling parameter is the concentration of color centers in the crystal (the additive coloring mode).
