# Chalcogenide Thin Films--Holographic Media for Augmented Reality Devices
## 1 Introduction  
Augmented reality (AR) is a promising technology that has generated a great deal of interest in the last decade. It is expected to become key to the next generation of mobile consumer electronics devices [1]. AR-technology is based on the superimposition of a virtual image on the image of the real world and can be used for entertainment, education [2], medicine [3], defense technologies [4], and in other spheres of life [5]. Personal wearable AR-devices, among which AR-glasses are the most demanded, have the most stringent technical requirements [6]: for comfortable use by a mass consumer, AR-glasses must have compact dimensions, a wide field of view (FOV), small weight and low cost, as well as high resolution and image quality [7]. To date, the best performance in accordance with the above requirements is achieved through the use of AR displays on planar waveguides with integrated holographic or diffractive optical elements (HOEs/DOEs) [8]. The principle of operation of such displays is as follows: light from an image generation system (for example, a micropoplector) falls on the input DOE, diffracts on it into the volume of the planar waveguide, and the diffracted light propagates inside the planar waveguide under the conditions of total internal reflection (TIR); then, the light is alternately diffracted by at least two other DOEs, due to which it multiplies in two dimensions and is removed from the volume of thewaveguide into the user's eye. The models of the largest companies on the AR market work according to this principle, including Digilens [9], Microsoft [10], WaveOptics [11], and others. The main disadvantage of such systems is associated with a fundamental limitation on the maximum FOV provided: since light propagates in the volume of the waveguide under TIR conditions, these conditions limit the angular size of the transmitted image [12; 13]. This limitation depends on the refractive index (\(n\)) of the optical system, which is typically 1.5. One of the most promising ways for increasing the field of view (FOV) of waveguide-based AR displays is to increase the refractive index of both the waveguide and the holographic material. Modeling comparison between refractive indices of 1.5 and 1.9 reveals that the red-green-blue (RGB) FOV reached with \(n=1.5\) by three waveguides can be reached with \(n=1.9\) by only one waveguide (Figure 1) [14]. Using high refractive index systems allows decreasing the thickness and weight as well as increasing the transparency and resolution of the AR display. While there are some existing glasses with refractive index of 1.9 for waveguides production, there are no holographic materials with such high refractive index. One of the potential candidates for holographic material with high refractive index is chalcogenide glasses (ChGs) thin films, which are evaporated on the glass substrate. Chalcogenide glasses are semiconductor-based materials having photosensitive properties with refractive index modulation (\(\Delta n\)) up to 0.12. They can also be used for relief recording [15; 16; 17; 18]. However, most of the ChGs compositions have high absorption (poor transparency) in blue wavelength range, which dramatically affects the resulted picture (Figure 2). It is possible to increase their transparency in the blue wavelength range through two ways. The first one is an optimization of the material composition. The other way of decreasing the losses is to decrease the ChG film thickness. The latter approach, however, has a drawback that is revealed in the efficiency loss. In the presented paper, the modeling of a single- and two-exposure HOEs diffraction efficiency depending on the ChG film thickness was carried out using rigorous coupled-wave analysis (RCWA) method.