Browsing by Author "Stoykova E."
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Item Basic holographic characteristics of a panchromatic light sensitive material for reflective autostereoscopic 3D display(2009-02-16) Sainov V.; Petrova T.; Ivanov B.; Zdravkov K.; Nazarova D.; Stoykova E.; Minchev G.Basic holographic characteristics of a newly developed panchromatic ultrafine grain silver halide light sensitive material for RGB recording of reflective holographic screen for autostereoscopic 3D display are presented. The average grain size is less than 10nm which ensures high resolution, diffraction efficiency, and signal-to-noise ratio (more than 100:1) in a large dynamic range for RGB reflective holographic recording. The decrease of the diffraction efficiency in recording of scattering objects is less than 30 from the maximal values for specular reflection. The analysis of color recording of the reflective holographic screen with one viewing zone is presented on the basis of the so-called sandwich structure built of two layers for multiple holographic recording in blue, green, and red spectral regions.Item COMPOSITE THIN FILMS OF AZOPOLYMER AND EMBEDDED GOLD NANOSIZED PARTICLES: EVALUATION OF THE EFFECTIVE COMPLEX REFRACTIVE INDEX(2022-01-01) Berberova-Buhova N.; Sharlandjiev P.; Mateev G.; Nedelchev L.; Blagoeva B.; Stoykova E.; Nazarova D.Azobenzene and azobenzene-containing polymers are intensively studied because of their high potential for many novel applications in polarization holography, design of optical polarization-sensitive elements, optical storage, all-optical switching, diffractive optical elements, etc. We study in the paper a thin hybrid layer composed of a polymer matrix and embedded gold nanoparticles. For a polymer matrix we used polymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethan-ediyl, sodium salt], denoted shortly as PAZO. Film reflectance and transmittance strongly depend on the morphology of nanoparticles and the composite film filling factor. Herein, we present essential details concerning the estimation of the film complex refractive index. The embedded particles are considered as ensemble of non-aggregated spheres with no multiple scattering by individual particles. The complex refractive index of the composite film is evaluated from the volume scattering efficiency of the Au particles, the PAZO matrix complex refractive index and the filling factor. Embedded Au spheres with ensemble mean diameters of 20, 60 and 100 nm are considered as well as filling factors of 0.008, 0.010 and 0.030Item Diffraction efficiency calculations of polarization diffraction gratings with surface relief(2018-04-05) Nazarova D.; Sharlandjiev P.; Berberova N.; Blagoeva B.; Stoykova E.; Nedelchev L.In this paper, we evaluate the optical response of a stack of two diffraction gratings of equal one-dimensional periodicity. The first one is a surface-relief grating structure; the second, a volume polarization grating. This model is based on our experimental results from polarization holographic recordings in azopolymer films. We used films of commercially available azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly denoted as PAZO. During the recording process, a polarization grating in the volume of the material and a relief grating on the film surface are formed simultaneously. In order to evaluate numerically the optical response of this ``hybrid`` diffraction structure, we used the rigorous coupled-wave approach (RCWA). It yields stable numerical solutions of Maxwell's vector equations using the algebraic eigenvalue method.Item Diffraction efficiency of polarization holographic gratings recorded in azopolymer thin films coated using different solvents(2020-01-01) Blagoeva B.; Nedelchev L.; Mateev G.; Stoykova E.; Nazarova D.We present a study of the diffraction efficiencies of polarization holographic gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]). Two series of layers have been prepared using two different solvents - distilled water and methanol. The gratings are inscribed by two plain waves with orthogonal circular polarizations (left and right circular) from a He-Cd gas laser (442 nm) at recording angle 20°, corresponding to grating period 1.3 μm. Higher diffraction efficiency is obtained for the thin film samples spin-coated from the methanol solution for thicknesses below 600 nm. Diffraction efficiency higher than 27% was achieved, as well as surface relief height more than 500 nm.Item Dynamic speckle analysis with smoothed intensity-based activity maps(2017-06-01) Stoykova E.; Berberova N.; Kim Y.; Nazarova D.; Ivanov B.; Gotchev A.; Hong J.; Kang H.Pointwise intensity-based algorithms are the most popular algorithms in dynamic laser speckle measurement of physical or biological activity. The output of this measurement is a two-dimensional map which qualitatively separates regions of higher or lower activity. In the paper, we have proposed filtering of activity maps to enhance visualization and to enable quantitative determination of activity time scales. As a first step, we have proved that the severe spatial fluctuations within the map resemble a signal-dependent noise. As a second step, we have illustrated implementation of the proposed idea by applying filters to non-normalized and normalized activity estimates derived from synthetic and experimental data. Statistical behavior of the estimates has been analyzed to choose the filter parameters, and substantial narrowing of the probability density functions of the estimates has been achieved after the filtering. The filtered maps exhibit an improved contrast and allowed for quantitative description of activity.Item Dynamic speckle imaging with SVD compression(2022-01-01) Stoykova E.; Levchenko M.; Ivanov B.; Madjarova V.; Nazarova D.; Nedelchev L.; Machikhin A.; Park J.Dynamic speckle imaging (DSI) of areas with different speed of processes ongoing in industrial or biological objects relies on statistical processing of a large number of images of the speckle patterns formed on the objects surface under laser illumination. The DSI visualizes the speed spatial distribution as an activity map. We propose compression of the raw DSI data by applying singular value decomposition (SVD). A specific feature of speckle images for DSI is lack of a structure with areas of close intensity values. The gain from the direct SVD application may be modest in cases when a great number of non-zero singular values is needed to build an activity map comparable in quality to the ground truth map from bitmap images. For higher compression, we propose SVD to be applied to the 2D arrays containing the differences between the successive images. The SVD compression has been verified by using synthetic and experimental data.Item Improvement of the photoinduced birefringence in PAZO azopolymer doped with ZnO via electrospray deposition(2020-06-02) Mateev G.; Marinov G.; Nedelchev L.; Nazarova D.; Stoykova E.; Babeva T.We present a study of the birefringence photoinduced in thin nanocomposite films of an azopolymer (PAZO) doped with different concentrations of nanoparticles (NPs) of ZnO with spherical shape and size in the range 40 - 100 nm. The nanocomposite thin films were prepared by electrospray deposition of ZnO over a thin azopolymer film with a thickness below 200 nm. The different NPs concentrations were obtained through different electrospray deposition times varied from 0 to 10 minutes. Birefringence was induced by a He-Cd laser at a wavelength of 442 nm and measured by a probe diode pumped solid state (DPSS) laser at 635 nm. The birefringence dependence on the ZnO NP deposition time reached its maximum for a deposition time of 90 s.Item Intensity-based dynamic speckle method for analysis of variable-rate dynamic events(2023-01-01) Stoykova E.; Nedelchev L.; Blagoeva B.; Ivanov B.; Levchenko M.; Berberova-Buhova N.; Nazarova D.We study efficiency of intensity-based dynamic speckle method for characterization of dynamic events which occur at variable rate in time within the temporal averaging interval. We checked the ability of the method to describe the speed of evolution by i) numerical simulation at variable speed, ii) processing of speckle patterns obtained from phase distributions fed to a SLM at controllable change of the temporal correlation radius of speckle intensity fluctuations and iii) conducting experiments with a polymer solution drying by using a hot-stage. The numerical and SLM simulation experiments allowed for modification of the used estimates in order to obtain relevant information.Item Intensity-based dynamic speckle method using JPEG and JPEG2000 compression(2022-02-10) Stoykova E.; Blagoeva B.; Berberova-Buhova N.; Levchenko M.; Nazarova D.; Nedelchev L.; Park J.Statistical processing of speckle data enables observation of the speed of processes. In intensity-based pointwise dynamic speckle analysis, a map related to speed’s spatial distribution is extracted from a sequence of speckle patterns formed on an object under coherent light. Monitoring of time evolution of a process needs storage, transfer, and processing of a large number of images. We have proposed lossy compression of these images using JPEG and JPEG2000 formats. We have compared the maps computed from noncompressed and decompressed synthetic and experimental images, and we have proven that both compression formats can be applied in the dynamic speckle analysis.Item Modelling of the diffraction efficiency of surface relief gratings with varying spatial frequency, height and shape of the relief(2021-04-09) Blagoeva B.; Berberova N.; Nazarova D.; Nedelchev L.; Mateev G.; Stoykova E.; Otsetova-Dudin E.; Sharlandjiev P.An important goal in the development of diffractive optical elements is to achieve high diffraction efficiency. One class of these elements are the surface relief gratings recorded by polarization holography. For recording media, we used azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly denoted as PAZO. The aim of the present work is to simulate a highly efficient surface relief grating formed in the azopolymer PAZO. The model is based on experimental results already obtained by our group. The tasks of the research are related to the analysis of the diffraction efficiency in the spectral range from 300 nm to 800 nm and for: 1) range of grating spatial frequencies from 1200 ln/mm to 1800 ln/mm, 2) surface relief height varying from 100 nm to 400 nm and 3) different shapes of the formed relief.Item Monte-Carlo simulation of the birefringence kinetics in azobenzene-containing materials(2018-04-05) Stoykova E.; Mateev G.; Blagoeva B.; Nedelchev L.; Nazarova D.The photoinduced optical anisotropy (birefringence and dichroism) in azobenzene-containing materials gives rise to a variety of applications in photonics by way of controlling the anisotropic properties at a nanoscale level. These applications require modeling the photoisomerization process kinetics. This is a complex and stochastic task that depends on many parameters. The paper presents a computationally effective Monte-Carlo simulation of recording, relaxation and erasure of the photoinduced anisotropy for a large number of 3D-oriented molecules at small angular increments between the neighboring orientations. The modeling is in good agreement with the experiment for measuring the birefringence in the PAZO polymer.Item Noise analysis in outdoor dynamic speckle measurement(2023-04-01) Levchenko M.; Stoykova E.; Ivanov B.; Nedelchev L.; Nazarova D.; Choi K.; Park J.The dynamic speckle method (DSM) is an effective tool for the estimation of speed of processes. The speed distribution is encoded in a map built by statistical pointwise processing of time-correlated speckle patterns. For industrial inspection, the outdoor noisy measurement is required. The paper analyzes the efficiency of the DSM in the presence of environmental noise as phase fluctuations due to the lack of vibration isolation and shot noise due to ambient light. The usage of normalized estimates for the case of non-uniform laser illumination is studied. The feasibility of the outdoor measurement has been proven by numerical simulations of noisy image capture and real experiments with test objects. Good agreement has been demonstrated in both the simulation and experiment between the ground truth map and the maps extracted from noisy data.Item Normalization in dynamic speckle analysis for non-destructive monitoring of speed of processes(2021-12-17) Stoykova E.; Nazarova D.; Nedelchev L.; Levchenko M.; Berberova-Buhova N.; Ivanov B.The paper is dedicated to analysis of normalized intensity-based pointwise algorithms for processing dynamic speckle images with spatially varying speckle statistics in non-destructive visualization of regions of faster or slower changes across an object. Both existing and newly proposed algorithms are analyzed. Extraction of speed of changes is done by acquiring correlated in time speckle images formed on the object surface under laser illumination. The studied algorithms have been applied to simulated low and high contrast speckle data. Their performance has been compared to processing of binary patterns as another approach for dealing with varying speckle statistics in the acquired images. The efficiency of the algorithms have been checked on the experimental data, including data in a compressed format. We have proven that the algorithms with normalization at successive instants by a sum of two intensities or a single intensity outperform as a whole the algorithms which apply the time-averaged estimates of the mean value and the variance of speckle intensity.Item OPTICAL RESPONSE EVALUATION OF AZOPOLYMER THIN SOLID FILMS DOPED WITH GOLD NANOPARTICLES WITH DIFFERENT SIZES(2022-01-01) Berberova-Buhova N.; Nedelchev L.; Stoykova E.; Nazarova D.Azobenzene and azobenzene-containing polymers are intensively studied in the last decades because of their high potential for various applications: polarization holography, holographic optical polarization-sensitive elements, high-density storage by multiplexing, ptychography, etc. In this article are reported studies based on the optical response of azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethan-ediyl, sodium salt], (PAZO) thin films with embedded Au spherical nanoparticles with different sizes. We found that effective medium approximations (Maxwell-Garnett and Bruggeman) are not satisfactory in evaluation of films optical response. The complex refractive index of the composite film is evaluated as a dependence on three factors: 1) the PAZO matrix complex refractive index; 2) volume scattering efficiency of the Au particle and 3) the filling factor of the composite film. The scattering of a single particle in the azopolymer matrix is calculated using the exact vector Maxwell equations. The particles with mean radius 10, 30 and 50 nm are treated as ensemble of non-aggregated spheres. Multiple scattering by individual particles was ignored. Three specific filling factors of 0.008, 0.010 and 0.030 are considered. The optical transmittance and absorption of a 440 nm thick composite film were evaluated. The algorithm is efficient and the results are robust. The photoinduced spectral response of the composite layers is under study.Item Performance of intensity-based non-normalized pointwise algorithms in dynamic speckle analysis(2015-09-17) Stoykova E.; Nazarova D.; Berberova N.; Gotchev A.Intensity-based pointwise non-normalized algorithms for 2D evaluation of activity in optical metrology with dynamic speckle analysis are studied and compared. They are applied to a temporal sequence of correlated speckle patterns formed at laser illumination of the object surface. Performance of each algorithm is assessed through the histogram of estimates it produces. A new algorithm is proposed that provides the same quality of the 2D activity map for less computational effort. The algorithms are applied both to synthetic and experimental data.Item Photoinduced birefringence in azopolymer doped with Au nanoparticles(2019-10-08) Nazarova D.; Nedelchev L.; Stoykova E.; Blagoeva B.; Mateev G.; Karashanova D.; Georgieva B.; Kostadinova D.A study of the photoinduced birefringence in nanocomposite thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with gold (Au) nanoparticles (NPs) is presented. The NPs are with a spherical shape and mean size 10 nm. Transmission electron microscopy (TEM) and Dynamic Light Scattering (DLS) analyses of the particles have also been made. Samples were prepared with five different concentrations of the NPs in the azopolymer films varying from 0 (non-doped azopolymer film) to 4 a.u. (1 a.u. = 0.015 wt %). Birefringence was induced by a He-Cd laser with a wavelength 442 nm and measured with probe Diode Pumped Solid State (DPSS) laser at 635 nm. An increase of the photoinduced birefringence in doped azopolymer nanocomposite films compared to samples of undoped azopolymers was observed. The maximal enhancement was achieved for films with 2 a.u. of Au NPs.Item Photoinduced chirality in azopolymer-based nanocomposites with different TiO2nanoparticles concentrations(2021-04-09) Mateev G.; Nedelchev L.; Nazarova D.; Blagoeva B.; Berberova-Buhova N.; Stoykova E.After its discovery, photoinduced chirality in azopolymer materials has been studied with growing interest because of its potential applications. According to our previous investigations, it is possible to induce chirality in an amorphous azopolymer with a high linear photoanisotropy by illumination with elliptically polarized light. In the present work, we studied the photoinduced chirality in thin nanocomposite films based on the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt], shortly PAZO, doped with TiO2 nanoparticles with a size of about 20 nm. The thickness of the prepared films was approximately 550 nm and the concentration of the nanoparticles varied from 0 to 3 wt. %. A He-Cd laser beam (? = 442 nm) was used, with the input ellipticity varying from-1 (left circularly polarized) to +1 (right circularly polarized). The ellipticity was set using a quarter-wave and a half-wave plates. We studied the dependences of the output ellipticity and the angle of self-rotation of the polarization azimuth on the input ellipticity. Based on the experimental data obtained, we could analyze the dependence of the self-rotation of the polarization azimuth on the concentration of the nanoparticles. The results show that small concentrations of nanoparticles lead to an enhancement of the self-rotation, hence, the chirality.Item Photoinduced variation of the Stokes parameters of light passing through thin films of azopolymer-based hybrid organic/inorganic materials(2016-04-07) Berberova N.; Nazarova D.; Nedelchev L.; Blagoeva B.; Kostadinova D.; Marinova V.; Stoykova E.Hybrid materials synthesized by combining polymers and inorganic nanoparticles (NPs) have received considerable attention recently due to their advantageous electrical, optical, or mechanical properties. Here we present a polarimetric study of thin films prepared from photoanisotropic hybrid materials: azopolymer (poly [1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with different concentrations of ZnO NPs with sizes below 50 nm. To obtain full information about the polarization response of these thin films, we determined the kinetics of the Stokes parameters of light passing through the films on illumination by linearly polarized light within the absorption band of the material, namely 473 nm. Using the Stokes parameters, the birefringence values were calculated as a function of time and compared for the films with different NP concentrations.Item Polarization Diffraction Gratings in PAZO Polymer Thin Films Recorded with Digital Polarization Holography: Polarization Properties and Surface Relief Formation(2024-05-01) Berberova-Buhova N.; Nedelchev L.; Mateev G.; Nikolova L.; Stoykova E.; Ivanov B.; Strijkova V.; Hong K.; Nazarova D.In this work, we study the polarization properties of diffraction gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt]) using digital polarization holography. Using two quarter-wave plates, the phase retardation of each pixel of the SLM is converted into the azimuth rotation of linearly polarized light. When recording from the azopolymer side of the sample, significant surface relief amplitude is observed with atomic force microscopy. In contrast, recording from the substrate side of the sample allows the reduction of the surface relief modulation and the obtaining of polarization gratings with characteristics close to an ideal grating, recorded with two orthogonal circular polarizations. This can be achieved even with a four-pixel period of grating, as demonstrated by our results.Item Polarization properties of two-dimensional polarization holographic gratings inscribed in azopolymer thin films(2023-01-01) Mateev G.; Nedelchev L.; Nazarova D.; Nikolova L.; Ivanov B.; Strijkova V.; Stoykova E.Azopolymers are efficient photoanisotropic materials and for this reason are commonly used for inscription of polarization-selective diffraction gratings by polarization holography. More often however, the characteristics of one-dimensional (1D) gratings are studied. Here, we present real time diffraction efficiency kinetics and polarization properties of two-dimensional (2D) polarization holographic gratings, each of them obtained by overlapping two perpendicular 1D gratings. The gratings are consecutively inscribed using as recording medium the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt], or PAZO. To record the gratings, He-Cd laser (λ = 442 nm) was used with left and right circular polarizations (LCP and RCP) of the recording beams. Our results show that the volume polarization grating has lower influence on the polarization properties of the 2D grating than the surface relief scalar grating.