Browsing by Author "Nedelchev L."
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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 Density Functional Theory Prediction of Laser Dyes–Cucurbit[7]uril Binding Affinities(2024-09-01) Petkova V.; Dobrev S.; Kircheva N.; Nazarova D.; Nedelchev L.; Nikolova V.; Dudev T.; Angelova S.Among a variety of diverse host molecules distinguished by specific characteristics, the cucurbit[n]uril (CB) family stands out, being widely known for the attractive properties of its representatives along with their increasingly expanding area of applications. The presented herewith density functional theory (DFT)-based study is inspired by some recent studies exploring CBs as a key component in multifunctional hydrogels with applications in materials science, thus considering CB-assisted supramolecular polymeric hydrogels (CB-SPHs), a new class of 3D cross-linked polymer materials. The research systematically investigates the inclusion process between the most applied representative of the cavitand family CB[7] and a series of laser dye molecules as guests, as well as the possible encapsulation of a model side chain from the photoanisotropic polymer PAZO and its sodium-containing salt. The obtained results shed light on the most significant factors that play a key role in the recognition process, such as binding mode, charge, and dielectric constant of the solvent. The observed findings provide valuable insights at a molecular level for the design of dye–CB[7] systems in various environments, with potential applications in intriguing and prosperous fields like photonics and material science.Item 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 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 Effect of InP/ZnS quantum dots aggregation on the kinetics of birefringence recorded in thin azopolymer composite films(2024-09-01) Stoilova A.; Dimov D.; Trifonova Y.; Mateev G.; Lilova V.; Nazarova D.; Nedelchev L.The article describes the preparation of new thin film composite materials based on the azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with three different concentration of InP/ZnS quantum dots with size of 7.5 nm. Clusters of aggregated InP/ZnS were microscopically observed in the fabricated composite samples. Birefringence (Δn) was induced in the films at two different wavelengths of the pump laser (355 nm and 444 nm) and the obtained higher values of Δnmax for the samples doped with quantum dots up to 2 wt% in comparison to the non-doped film and the sample with the highest nanocrystals concentration are discussed, based on the measured fluorescence spectra, in terms of possible local energy transfer between the azobenzene chromophores and the InP/ZnS quantum dots.Item Enhanced photoanisotropic response in azopolymer doped with elongated goethite nanoparticles(2016-04-07) Nedelchev L.; Nazarova D.; Berberova N.; Mateev G.; Kostadinova D.; Mariño-Fernández R.; Salgueiriño V.; Schmool D.We present a study of the photoinduced birefringence in nanocomposite films of an azopolymer (PAZO) doped with goethite (a-FeOOH, a characteristic antiferromagnetic material) nanoparticles (NPs). The NPs had an elongated shape with a size 15×150 nm, i.e., a ratio of 1:10. Samples were prepared with different concentrations of the NPs in the azopolymer varying from 0% (undoped azopolymer film) to 15 wt %. An unusual dependence of the birefringence on the concentration was observed - two peaks of enhancement at 1 % and at 10 % concentration. Our previous studies on ZnO and SiO2 NP have indicated only one peak of increase at low concentrations - 0.5 wt % and 2 wt %, respectively. This effect could be related to the elongated shape of the nanoparticles and the presence of two characteristic NPs sizes - 15 and 150 nm. Moreover, the birefringence increase for the samples with 10 wt % NPs concentration (compared with the non-doped samples) was rather significant - nearly 70%.Item Hybrid Dispersion Model Characterization of PAZO Azopolymer Thin Films over the Entire Transmittance Spectrum Measured in the UV/VIS/NIR Spectral Region(2022-12-01) Minkov D.; Nedelchev L.; Angelov G.; Marquez E.; Blagoeva B.; Mateev G.; Nazarova D.Notwithstanding the significant optical applicability of PAZO polymer films, there are no accurate data about their optical characteristics. To remedy this shortcoming, in this study three PAZO polymer thin films are characterized, with dissimilar thicknesses, on glass substrates using only one UV/VIS/NIR transmittance spectrum T(λ) per sample and an original hybrid dispersion model (HDM). HDM is based on the Tauc–Lorentz model, the new amorphous dispersion formula, the Tauc–Lorentz–Urbach model of Foldyna and the Tauc–Lorentz–Urbach model of Rodriguez. HDM with two oscillators is employed in characterizations of the PAZO polymer films in the range [300, 2500] nm, whereby the root-mean-square deviation (RMSD) of the fitted transmittance spectrum with respect to T(λ) does not exceed 1.6 × 10−3. Decreasing RMSD by 2.3% to 94.4% is demonstrated by employing HDM compared with the above mentioned four popular dispersion models, for each one of the studied films. HDM is applicable to amorphous films independent of their thickness as well as to cases of non-transparent substrate.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 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 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 constants of azopolymer PAZO thin films in the spectral range 320 - 800 nm(2018-04-05) Berberova N.; Sharlandjiev P.; Stoilova A.; Nedelchev L.; Nazarova D.; Blagoeva B.Azobenzene containing polymers (azopolymers) have been investigated for a number of applications - holographic data storage, polarization optical elements, diffractive gratings, etc. Herein we report on the optical characteristics of films obtained from the commercially available water soluble azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt). Thin films were deposited by spin coating and their thickness was determined by a Talystep precision profilometer. The FTIR spectra of PAZO were measured between 3000 cm-1 and 500 cm-1. For the first time, to the best of our knowledge, we evaluated the PAZO complex refractive index continuously in a broad spectral range (320 - 800 nm) based on transmission and reflection spectrophotometric data. The wavelength dependence of the complex refractive index of organic dyes layers cannot be readily described with dispersion models used for crystals, metals or semiconductors. This is why we used a derivative approach on a wavelength-by-wavelength basis to solve a set of two nonlinear equations with two unknowns - the real and imaginary parts of the refractive index. The determination of the optical constants of PAZO films is effective and robust.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 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 birefringence in pazo polymer nanocomposite films with embedded particles of biologically active metal complexes(2019-01-01) Mateev G.; Stoilova A.; Nazarova D.; Nedelchev L.; Todorov P.; Georgieva S.; Trifonova Y.; Lilova V.The composite materials based on azo polymers doped with metal particles attract considerable attention due to their advantageous electrical and optical properties. The present communication reports results from measuring the photoinduced birefringence in composite films of the azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly PAZO, doped with metal complexes of Cu(II) 3-amino-5,5'-dimethylhydantoin (CLP) and Ni(II) 3-amino-5,5'-dimethylhydantoin (NLP) of different concentrations. The thin film materials are prepared via spin coating. The thickness of the samples is in the range between 500 nm and 800 nm. The photoinduced birefringence is measured by a classical polarimetric setup, where a vertically polarized He-Cd laser emitting at ? = 442 nm is used as a pump laser, while a DPSS laser emitting at = 635 nm is used as a probe laser. The influence of the dopants concentration on the maximal induced birefringence and the time of response are discussed.