# Chalcogenide Thin Films--Holographic Media for Augmented Reality Devices
## 4 Experimental Results and Discussion
### Chalcogenide Glasses with Optimized Compositions
#### 4.2.2 The Second Series of ChGs Samples  
In order to make ChGs samples more transparent in the blue spectral region and taking into account the recording results obtained in Section 4.2.1, three new Ge-S compositions of chalcogenide glasses were synthesized (Ge\({}_{18}\)S\({}_{82}\), Ge\({}_{25}\)S\({}_{75}\), and Ge\({}_{30}\)S\({}_{70}\)). With previously used Ge\({}_{24}\)S\({}_{76}\) and Ge\({}_{20}\)S\({}_{80}\), they formed the second series of samples. These samples were  
Figure 11: Green-blue FOV transmitted through the periscope system recorded in Ge-S composition.  
Figure 10: UV-VIS spectra of the Ge-S samples before and after illumination.  
characterized by a shift of fundamental absorption to the ultraviolet region of spectrum (Figure 12). The thickness of all deposited ChGs films was about 1 \(\upmu\)m, and the refractive index \(>\) 2.2.  
Figure 12 shows possibility of synthesis a chalcogenide composites transparent in the visible range of the spectrum (red spectral range is not shown, but ChGs are highly transparent there).  
However, the issue of holographic recording on the obtained samples was topical. To carry out holographic recording on these compositions, it was required to get into the absorption region of the material. In this regard, we used an ultraviolet (UV) 325-nm He-Cd laser for the recording. Verification of the possibility of recording volume holograms on chalcogenide composites started from registration of kinetic dependences of the holograms diffraction efficiency on the recording dose for all available samples. Kinetics of the hologram recording was studied in the Lloyd interferometer recording setup (Figure 13).  
We chose the Bragg angle of 10\({}^{\circ}\); thus, the period of the recorded volume gratings was 935 nm. \(s\)-Polarized recording beams were used, which did not affect the contrast of the interference pattern. Power density of the recording radiation was 230 \(\upmu\)W/cm\({}^{2}\); thus, the exposure of 1 J/cm\({}^{2}\) could be achieved within just 137 s. Measurement of the kinetic dependences were carried out by 396-\(\upmu\)W probing green light (\(s\)-polarized 532-nm laser diode) during the recording process. Diffraction angle was 16.5\({}^{\circ}\) according to the Bragg law. Power measurements of transmitted and diffracted beams intensities were both done behind the sample. Recording kinetics for studied samples are presented in Figure 14. It is to be noted that Ge\({}_{18}\)S\({}_{82}\) sample was used as test one to choose the recording power density; that is why its DE was very low and is not shown in Figure 14.  
From Figure 14, it is seen that the recording dynamics for Ge\({}_{25}\)S\({}_{75}\) and Ge\({}_{30}\)S\({}_{70}\) samples are much slower than for all others; however, these samples showed the highest diffraction efficiency. According to the kinetics trend, maximum efficiency can be expected in the range of 80-90 J/cm\({}^{2}\). Peak diffraction efficiency was not achieved due to extremely long exposure times. However, DE of 0.18% was measured at about 70 J/cm\({}^{2}\) exposure. In terms of recording time we see that sample Ge\({}_{25}\)S\({}_{75}\) is the most sensitive to recording light among this set of samples.  
Figure 12: Absorption spectra of the second series of Ge-S type ChGs compositions in UV–green spectral range. Vertical line indicates the hologram recording wavelength, 325 nm (see the text).  
It is also seen that the slower kinetics the higher peak diffraction efficiency, and the shift of the chalcogenide composite absorption edge to the ultraviolet wavelength region leads to a deterioration in the maximum diffraction efficiency of the recorded volume holograms (as it is seen from DE comparison of these samples and the ChGs from the first optimized compositions group, Section 4.2.1).