**1. Introduction**

70 Holograms – Recording Materials and Applications

Volume recording media and methodology of their investigation, presented in the paper, contribute in the authors' unpresuming opinion to the solution of the problems in question.

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**8. References** 

After the invention of holography and development of hologram micro-embossing, mass replication of hologram became possible. Use of a hologram as a security option for valuable documents and products was soon realized and won a large-scale application, which did not hinder development of other security features though. A great variety of other diffractive microstructures commonly known as diffractive optically variable image devices (DOVID) has appeared. Alongside the development of computer technology, digital holographic recording in applied holography has acquired a wide range of application. Nowadays, various types of digital holograms are used for protection and identification of industrial products and documents against counterfeiting, as well as for packing and decorating.

Now two optical technologies for digital recording of protection holograms and DOVID, having similar principle of image formation, are widely used: dot-matrix and image-matrix holographic recordings (Pizzanelly, 2004). A dot-matrix hologram consists of an array of fine diffractive dots holding an image together (**Fig. 1a**). Each dot in such a hologram consists of a uniform diffraction pattern in which the grating pitch and grating orientation

Fig. 1. Photos of microstructure of dot-matrix (**a**) and image-matrix (**b**) holograms recorded on As40S20Se40 photoresist.

Digital Holographic Recording in Amorphous Chalcogenide Films 73

DDM devices in which a laser with an intensity modulator and an optical scheme are separate parts, our device has a laser and an optical scheme integrated into one unit. This approach simplifies the overall system construction and adjustment, reduces number of optical components in design and increases reliability. This allows for recording a twodimensional array of diffracting pixels with preset parameters on the surface of a photosensitive media thus making it possible to produce a holographic image using pixels

The fundamental concept of dot-matrix holograms lies in writing the image pixel-by-pixel varying the grating orientation, grating pitch and, if possible, pixel size. Many other approaches to the creation of a similar dot matrix hologram are known. One of them consists in applying laser beam or e-beam lithography to create each grating pixel. Dotmatrix holograms generated by the mentioned type of lithography are commonly referred to as e-beam holograms. They generally achieve an even better resolution and brightness than the mentioned dot-matrix holograms*.* However, the use of an e-beam machine is extremely expensive. Since the cost of dot matrix holography is much lower than that of e-beam or laser beam holography, it is expected that the former will gain a broad range of application. A dot matrix hologram comprises a two-dimensional array of micron-sized diffractive elements or pixels. Each element contains a diffraction grating formed by two-plane coherent wave interference. The period of interference fringes is determined by the angle

The recording is made by converging two focused laser beams to a point of a preset size on the photoresist surface. Thousands of such closely spaced pixels form a dot matrix

Fig. 2. SEM images of dot-matrix hologram recorded on As40S20Se40 photoresist at different

On illumination, each pixel of the hologram diffracts light at a specific angle chosen in the process of making. The angle and the direction in which the diffraction grating of a pixel



magnification. **a**) Structure of a dot-matrix hologram. **b**) Image of one pixel.

reflects light are determined by the following two factors:

laser beams before pixel recording.

with parameters that are computer-controlled during the recording.

between the interfering laser beams and their wavelength.

hologram.

**c**) Diffraction grating in the pixel.

between two laser beams.

may vary from dot to dot. In contrast, an image-oriented hologram is composed of an array of microscopic images (**Fig. 1b**). Positive organic photoresists are conventionally used in both technologies for recording relief-phase holograms.

In its turn**,** formation of surface relief in the process of selective dissolution of amorphous chalcodenide films is well known. The phenomenon is based on difference in dissolution rate of the exposed and unexposed areas of chalcogenide film surface in alkali developers (Teteris, 2002). This phenomenon of photo-induced changes in the dissolution rate of a large group of amorphous chalcogenide semiconductor films was the basis for an extensive development of a new class of inorganic resists.

Holographic recording in amorphous chalcogenide films is the subject of investigation for many scientists. In 1970, the first articles about the recording of holographic diffraction grating in the amorphous films As2S3 were published. Due to essential photoinduced changes of optical properties, thin films of amorphous chalcogenide semiconductors are very promising media for phase hologram recording. It is worth mentioning that diffraction efficiency of phase hologram in chalcogenide media reached up to 80%, and optical resolution exceeds 5000 mm-1.

Organic photoresists are mainly used for recording relief-phase holograms; they are sensitive enough only in the ultraviolet part of spectra λ < 480 nm. For lasers with the wavelength λ=400-650 nm, use of photoresists based on chalcogenide films is very promising. Amorphous chalcogenide films have been recently used as a material for holography in visible spectrum (λ ≤ 650 nm) with high resolution (> 5000 lines/mm) and light sensitivity in the range 1-10 J/cm2.

Successful results of recording holograms on As-S-Se films have long been known. Photoresists based on amorphous chalcogenide films are effectively employed by some holographic companies in the production of rainbow holograms and diffraction gratings. But use of these films for recording dot-matrix and image-matrix holograms has not yet been studied. There seem to be two main reasons for this. First, As-S-Se films have a low sensitivity (~1-20 J/cm2) in comparison with organic photoresists (~1-100 mJ/cm2). Second, image-matrix technologies of hologram recording result in great losses of laser radiation energy (up to 99 %) at optical and Fourier filtrations.

To overcome the above-specified problems in recording dot-matrix and image-matrix holograms, a device specially adapted for the use of As-S-Se films has been developed. Optimum modes of optical recording are investigated, and relief-phase holograms with high diffraction efficiency (DE) were recorded by means of the created device. Another positive result was that the time for recording holograms on the As-S-Se based photoresist was comparable to the time for recording them on organic materials.

The present article contains data of an experimental recording of relief-phase digital holograms and DOVID on chalcogenide thin films. The obtained holograms can be duplicated by standard modern technologies.
