Browsing by Author "Berberova-Buhova N."
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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 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 Nanocomposite Photoanisotropic Materials for Applications in Polarization Holography and Photonics(2023-11-01) Nazarova D.; Nedelchev L.; Berberova-Buhova N.; Mateev G.Photoanisotropic materials, in particular azodyes and azopolymers, have attracted significant research interest in the last decades. This is due to their applications in polarization holography and 4G optics, enabling polarization-selective diffractive optical elements with unique properties, including circular polarization beam-splitters, polarization-selective bifocal lenses, and many others. Numerous methods have been applied to increase the photoinduced birefringence of these materials, and as a result, to obtain polarization holographic elements with a high diffraction efficiency. Recently, a new approach has emerged that has been extensively studied by many research groups, namely doping azobenzene-containing materials with nanoparticles with various compositions, sizes, and morphologies. The resulting nanocomposites have shown significant enhancement in their photoanisotropic response, including increased photoinduced birefringence, leading to a higher diffraction efficiency and a larger surface relief modulation in the case of polarization holographic recordings. This review aims to cover the most important achievements in this new but fast-growing field of research and to present an extensive comparative analysis of the result, reported by many research groups during the last two decades. Different hypotheses to explain the mechanism of photoanisotropy enhancement in these nanocomposites are also discussed. Finally, we present our vision for the future development of this scientific field and outline its potential applications in advanced photonics technologies.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 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 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 Tunable polarization and surface relief holographic gratings in azopolymer nanocomposites with incorporated goethite (α-feooh) nanorods(2021-08-01) Nedelchev L.; Mateev G.; Strijkova V.; Salgueiriño V.; Schmool D.S.; Berberova-Buhova N.; Stoykova E.; Nazarova D.We employ two approaches to tune the properties of concurrently inscribed volume polarization and surface relief gratings in nanocomposite thin films containing the azopolymer PAZO (poly[1-4-(3-carboxy-4-hydrophenylazo)benzensulfonamido]-1,2-ethanediyl, sodium salt]) and goethite (α-FeOOH) nanorods. The first one is applied on the stage of sample preparation by varying the concentration of the goethite nanorods from 0% to 15%. Then, different angles between the recording beams are set in the holographic scheme, which allow us to obtain gratings with spatial periods in the range from 0.86 to 2.51 µm. Surface relief modulation close to 300 nm is achieved as well as total diffraction efficiency in the ±1 diffracted orders of more than 50%. The influence of the incorporated goethite nanorods on the properties of both volume birefringence and the surface relief grating are discussed.Item Visualization of pathologic changes in liver tissue via polarized light(2022-01-01) Stoilova A.; Blagoeva B.; Nazarova D.; Stoykova E.; Berberova-Buhova N.; Nedelchev L.; Machikhin A.In this work, we applied two polarized light based approaches to visualize histological patterns of liver pathologies. The first one involves acquisition of two images through a polarizing microscope, one image (Ppar) acquired with the analyzer oriented parallel to the polarization of illumination and the other (Pper) acquired with the analyzer oriented perpendicular to the illumination. The final image is based on the polarization ratio, Preconstructed = (Ppar - Pper)/(Ppar + Pper). Using the second technique, the histological specimens were illuminated with a polarized laser beam with wavelength of 635 nm. Polarimetric parameters as azimuth, angle of ellipticity, degree of polarization and reflected power have been measured to quantify the change in the polarization state of the incident light after interaction with the sample of the healthy tissue and of the tissue with abnormal morphological changes.