**7. Conclusion**

68 Holograms – Recording Materials and Applications

Fig. 13. Distribution of radiation intensity in diffracted (a) and zero (b) radiation beams, recorded on a camera matrix (bottom); processing result for experimental data (top).

When measuring the spectral selectivity contour of a volume hologram according to schematic diagram, shown on Fig. 12b, the hologram is to be scanned with a collimated radiation beam of a wavelength, varying within the spectral range of the contour. At different times, this was carried out in different ways. In work (Denisyuk et al., 1970), the measurements involved a monochromator, with a collimated beam of radiation with a wide spectral range being formed in front of its entrance slit. In work (Sukhanov et al., 1984), the reading radiation wavelength was changed with the help of frequency-tuned dye laser with excimer pump: in this case, the laser radiation divergence was less than 0.5 mrad and the spectral width of scanning radiations was 0.01 nm. The halfwidth of spectral selectivity contour, measured in the present work for a reflection hologram, was Δλ = 0.16 nm (at DE = 80%). Such measurements, naturally, require special equipment and cannot be accomplished using standard techniques for measuring

Applying the collimated laser radiation with wide spectral interval of wavelengths to measurement of the spectral selectivity contour of volume holograms was proposed by the authors and was implemented with the use radiation of femtosecond laser and semiconductor laser. The schematic diagram of measurements is given on Fig. 12d. A collimated beam of laser radiation with a wide spectrum illuminates hologram, which can be placed in positions "outside Bragg conditions" (Id = 0) and "under Bragg conditions" (Id is at the maximum for wavelength λBr within the reading radiation wavelength range). A radiation beam, having passed a hologram with the direction unchanged (I0), arrives at the entrance slit of spectral instrument (3 on Fig. 12d), the spatial pattern of wavelength spectrum expansion being recorded behind it on a CMOS-matrix as a spectrogram. With the hologram placed "outside Bragg conditions", reading radiation spectrum is recorded (curves 1 on Fig. 14); with the hologram "under Bragg conditions", spectrum of zero diffraction beam (curves 2 on Fig. 14) is recorded, in similarity to the distribution, given on Fig. 13c. Spectral selectivity contour of a hologram represents in this case a differential

contour (curves 3 on Fig. 14), resulting from comparison of two spectrograms.

**6.3 Spectral selectivity contour** 

spectral characteristics of optical elements.

One of the cardinal problems of 3D holography is provision of research in the field with recording materials (Denisyuk, 1980). Volume recording media for holography are at present manufactured in laboratory conditions in the form of isolated specimens or small batches. Obtaining samples with stable and reproducible performance is, as the authors' experience shows, still possible even in such conditions.

The current studies reveal those properties of devised materials, which open up new application opportunities far beyond narrow professional use of recording media for holography. A number of special features of recording media, considered in the paper, can be quite in demand to accomplish unconventional tasks in various fields of science and engineering.

AgHal-PG-media exhibit a set of parameters, pertaining to commonly used traditional AgHalmedia: possibility to achieve high sensitivity, the width of spectral sensitization, the variety of techniques of post-exposure treatment etc. The list of the most important parameters of silverhalide media is supplemented by AgHal-PG-media with new opportunities: obtaining samples with thickness of several millimeters; shrinkproof; limitation of the maximum particle size in the light-sensitive agent and post-treatment products.

Polymeric medium with diffusion enhancement has a modulation transfer function, which is untypical for traditional light-sensitive materials and allows excluding the region of low spatial frequencies during the information recording. A no less important and rather unique property is the possibility to obtain the structure of high-efficiency hologram as a latent image at the recording stage and thus achieve a distortionless recorded interference structure in a wide dynamic range after post-treatment. It should be also noted that enhancement and fixation of holograms recorded on such a medium require no treatment in water solutions.

Advancement of volume holography and provision of this line of research with experimental base for comprehensive studies makes it necessary to investigate the processes taking place in the bulk of recording media during hologram construction, which, in its turn, calls for improvement of research techniques and methods to control the parameters of target processes.

**4** 

*Latvia* 

Andrejs Bulanovs

**Digital Holographic Recording in** 

**Amorphous Chalcogenide Films** 

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

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.

**1. Introduction** 

decorating.

*Innovative Microscopy Center, Daugavpils University* 

**a b** 

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.

### **8. References**

Ashley, J. et al. (2000). Holographic data storage, *IBM J. Res. Dev.* Vol. 44(3), pp. 341-368


Denisyuk, Yu.N. (1980). Holography and its prospects, *J. Appl. Spectrosc*, Vol. 33, pp. 397-414


Kreibig, U. et al. (1981*). Surf. Sci.,* Vol. 106, pp. 308-312


Sukhanov, V.I. (1991). *Proceedings of SPIE*, Vol. 1238, pp. 226-230

