THREE-DIMENSIONAL HOLOGRAPHIC OPTICAL ELEMENTS BASED ON NEW MICROSYSTEMS
Authors: Tyurin A.V. , Zhukov S.A. , Akhmerov A. Yu. 
Published: February 2024

The origination and improvement of holographic methods, as well as technical equipment for their implementation [1–3] revived interest in light diffraction in three-dimensional periodic structures [4]. This is due to the fact that holographic methods allow to create a relatively simple and affordable technology for the manufacture of three-dimensional diffraction structures for both transmitted and reflected electromagnetic radiation of the visible range of the spectrum. Previously, light diffraction was used only in two-dimensional periodic diffraction structures, the manufacture of which was possible by other methods (chemical, photographic, mechanical, etc.) [5]. Diffraction in three-dimensional periodic structures for transmitted radiation has become widespread only for X-rays, for which a crystal lattice of various substances could be used as a three-dimensional periodic structure [6]. The use of diffraction of electromagnetic radiation of the visible spectrum on holographic three-dimensional structures (holograms) for practical purposes allows to create optical elements and optoelectronic devices of a fundamentally new class based on them, which have the widest range of applications [7–13]. For the first time the basic principles of obtaining three-dimensional diffraction structures for both transmitted and reflected electromagnetic radiation of the visible range of the spectrum by holographic optics (transmitting and reflecting holograms) were formulated by Denisyuk Yu.N. in 1962 [14]. The basis of this technology was a three-dimensional light-sensitive environment that provides registration (recording) of the interference pattern in its entirety. In order for the three-dimensional properties of diffraction (reading) on such a hologram to be most pronounced, the thickness of the hologram should be ≈100 μm or more [15], and diffraction should be carried out not only by changing the absorption coefficient of light-sensitive layer, as in traditional silver containing photoemulsions (amplitude hologram), but also as a result of changes in the refractive index of the layer (phase hologram). In the case of pure phase hologram light losses at diffraction should be minimal and diffraction effectiveness may reach 100% [16]. In the development of light-sensitive carriers, there are two approaches to three-dimensional holograms, which provide diffraction when reading in transmitted light, as well as preservation at room temperature and diffraction in the absence of recording light. The first of them is a two-stage process [17–20]. In the first stage – exposure at room temperature – the recording medium plays a passive role, memorizing the distribution of intensities of beams passing through it, in the second stage, using various chemical and photographic treatments, also at room temperature, this distribution is amplified and fixed. The use of silver halide compounds [21] provides a two-step process, both of which are realized at room temperature, an important advantage, such as high (boundary) sensitivity to hologram recording. But dividing possibility of such holograms with high diffractive effectiveness did not exceed 1000 lines/mm [22]. The second way is to move to non-silver environments [23–26]. The most promising from this point of view are photochromic systems based on colored alkaline halide crystals (AHS) and chalcogenide glassy semiconductors (CGS) 27–32. These materials do not require any intermediate work and change their optical characteristics directly under the action of radiation, forming in the volume of the medium at elevated temperatures amplitude-phase hologram, which provides diffraction in light, as modulation of the absorption coefficient and refractive index. When cooled to room temperature, they are resistant to reading with high diffraction efficiency and angular selectivity [8, 31]. For such holograms, the stages of formation (at elevated temperatures) and fixation (by cooling to room temperature) are inextricably linked and occur simultaneously, and the process of recording-fixation can be considered as one-stage. The main disadvantage of such environments is the need for elevated temperatures and low sensitivity in rather narrow range (400650 nm) of optical radiation, under the action of which a three-dimensional diffractive structure is formed. In this paper, for the registration of three-dimensional transmitting holograms at room temperature, we proposed an emulsion containing a heterophase microsystem "core CaF_2 – shell AgBr", which provides recording of holograms with high resolution and diffraction efficiency; high (boundary) sensitivity and wide spectral range (4001000 nm) optical radiation, under the action of which a three-dimensional hologram is formed. We also consider our proposed applications of holographic optical elements based on three-dimensional transmitting diffraction structures to solve some practical problems. Photochemical transformations in monolithic CGSs of As-S composition corresponding to holographic recording are considered. When using photochromic systems based on colored alkali-halide crystals and chalcogenide glassy semiconductors for the registration of three-dimensional transmitting holograms at elevated temperatures, we proposed spatial stabilization of the recording interference pattern, which achieves optimal characteristics of the recorded holograms. We also consider our proposed applications of optical elements based on three-dimensional transmitting diffraction structures to solve some practical problems.