Home – Home – Jornals – Avtometriya 2020 number 2

An overview of the current developments and prospects for the use of metasurfaces in optics, particularly holography, is presented. Particular attention is paid to the physics of controlling the phase delay of radiation scattered by metaatoms and the transformation of an incident wave front by a metasurface. A number of examples demonstrate the effectiveness of focusing radiation and forming a holographic image by metasurfaces of various types.

A method and a device are developed for measuring the basic parameter of synthesized security holograms with the use of capabilities of microscopy with micro-size resolution and diffractometry with a multiangular system of illumination. Based on consecutive illumination of the hologram surface by collimated monochromatic light sources at different angles in the field of vision of a special digital microscope and subsequent processing of the resultant images, an information map of the surface is formed, which contains information on the basic parameters of "holopixels" forming the structure of the security hologram (pitch of diffraction gratings, their angular orientation, and normalized intensity of reflected light), as well as the image of the hologram surface in scattered light matched with this map. By moving the hologram under consideration in the focal plane of the digital microscope to a distance equal to the field of vision of the microscope with subsequent matching of the data obtained, it is possible to construct the information map of the entire hologram surface with micron resolution.

A possibility of using a micromirror modulator for optical encoding with time integration in quasi-monochromatic spatially incoherent light is considered. An experimental setup for optical recording of encoded optical images by a digital camera is presented. Images containing text and graphical information are encoded; these images are successfully decoded by the method of inverse filtration. The use of time integration of optically encoded images allows reduction of noise induced by binarization of holograms containing encoding keys subjected to modulation and enhancement of the diffraction efficiency of the encoding process.

Factors responsible for parasitic distortions of interferograms in control systems based on reference single-component high-aperture computer-synthesized holograms (CSHs), which are written on flat substrates and have a binary phase structure, are analyzed as applied to the problem of interferometric inspection of spherical and aspherical surfaces with high angular apertures. To improve the quality of such CSHs, methods, tools, and recording materials (titanium and chromium films) are used, which are suitable for the technology of direct laser recording of their diffraction structure to ensure the formation of interferograms with the minimum parasitic distortions. In particular, to reduce the variations of the interferogram contrast, it is proposed to perform modulation of the local transmission coefficient of the CSH structure by means of modulation of the duty ratio of diffraction zones. Possible applications of the thermochemical laser technology of recording on titanium and chromium films for fabrication of high-aperture holograms with a normalized orifice of f/1 and more are discussed.

Different versions of one of the types of zero-order holograms, namely trackograms for the formation of security features for nonexpert verification of their authenticity, are considered. A volumetric image in the trackograms is formed by glare on the walls of toroidal track surfaces, which are relief bands on the recording material. Varying the track shape, the track length, and the number of sections of different tracks at one point in an image can help one obtain security features convenient for nonexpert hologram identification. The dimensions and depths of the tracks are compatible with conventional embossed hologram technology. The photographs of the holograms forming images in white light are given.

A method and an optical circuit for recording digital hyperspectral holograms in the light of an incoherent source are considered. The optical scheme comprises an interferometer with a scanning mirror for generating reference waves. A set of complex amplitudes of an object field is calculated using the Fourier transform of interferograms in each pixel of the recording matrix. The experimental results of holographic images of microobjects obtained on various optical circuits (transmission and reflection) are presented. The circuit a shared path interferometer, in which the reference wave is part of the object field is considered

It is proposed to use spectral diffractive lenses with the focus of the same position of several specified wavelengths as the basis for a point light source with several specified wavelengths. A spectral diffractive lens designed to focus three wavelengths: 462, 608, and 952 nm (excitation wavelengths for tubulin fluorescence β) is considered. The spectral lenses were fabricated by direct laser writing in photoresist with iterative correction of recording parameters, which ensured an error of less than 30 nm in the formation of the diffractive microrelief height. An experimental determination of the wavelengths focused by the obtained lens using a point aperture located at the focus of a spectral lens is described.

This paper presents a brief review of theoretical and experimental studies of the structural and polarization conversion of laser beams in anisotropic crystals. The feasibility of creating laser beams with complex polarization-phase state is discussed. It is shown that adaptive tuning of structural and polarization-phase states can be carried out by relatively slow changes in the parameters of the optical system and fast high-speed electro-optical modulation methods.

A multifactor model of a system of optical encoding with spatially incoherent illumination is proposed. The model takes into account the influence of photosensor noise, changes in the image rasters when passing through the optical system, spurious illumination, and noise of the synthesis of the encoding diffraction optical element on the decoded image quality. It is demonstrated that the proposed model predictions agree well with the optical experiment (the decoding error is within 6%).

In this paper, we consider two types of alternative treatment of gelatin in layers dichromated gelatin (DCG) layers: structuring using selective light tanning by He-Cd laser light (λ = 440 nm) and destruction by irradiating the layers with short-wave UV radiation (λ < 270 nm). It is shown that UV irradiation of samples plays a decisive role in the formation of high-frequency relief-phase gratings with a diffraction efficiency of more than 50% on DCG layers less than 2 mkm thick. It is noted that the use of traditional methods of treating DCG layers leads to smoothing of the relief due to surface tension forces at high spatial frequencies. In addition, a significant expansion of the region of recorded spatial frequencies (up to 1500 mm^-1) for recording relief holographic gratings was achieved by using an advanced method of processing gelatin based on a reduction of water treatment time, and using isopropanol baths for rapid dehydration of the layer

This paper presents the results of experimental studies and digital methods of recording holograms using spatial light modulators. The use of holographic interference methods to measure nanoscale irregularity is described for the case of a nanostep (70 nm high and 30 mkm wide). Examples of using the method to increase sensitivity are given to measure the nanostep and the hook method in a digital way. The probability of implementing the method for correcting internal system distortions in the interferometer is proved. It is shown that the use of holographic interference methods with digital processing of holograms provides an increase in the sensitivity of interferograms without losing basic information on the object and without increasing measurement accuracy

This paper presents the results of studying the influence of the structure of an object and the form of its representation on the quality, resolution, and depth of images restored using binary synthesized projection holograms. The quality criterion is taken to be the degree of correspondence between the restored image and the object, estimated as the range of threshold gradations during threshold processing of the restored image, in which its structure completely corresponds to the structure of the original object. A method of projection holographic photolithography was developed based on mathematical modeling of real physical processes of synthesis and restoration of holograms of two-dimensional binary transparencies

Binarization of digital holograms is used for storage and on-line transmission of recorded information, as well as for optical image recovery via binary micromirror light modulators. The possibilities of binarization of optically recorded holograms by iterative methods using the error diffusion operation at each step of changing the binarization threshold or only at the final iteration are considered. The parameter variation of the methods is analyzed, and the quality of the images recovered from binarized holograms is compared. It is determined that the method using the error diffusion operation only at the final iteration improves the image recovery quality by 12% as compared to single-step methods