Browsing by Author "Berberova N."
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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 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 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 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 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 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 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 holographic gratings in PAZO azopolymer recorded with different recording-beams polarizations(2018-04-05) Mateev G.; Nedelchev L.; Ivanov D.; Tomova R.; Petrova P.; Strijkova V.; Berberova N.; Nazarova D.Polarization holographic gratings (PHG) were recorded using a laser emitting a wavelength of 491 nm in thin films of the (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) azopolymer, shortly denoted as PAZO. Thin azopolymer films with various thicknesses were spin-coated on plastic and glass substrates. Four different polarization states of the recording beams were used, and the results compared: a) two vertical linear polarizations, b) horizontal and vertical linear polarizations, c) linear polarizations at +45° and -45° relative to the recording plane, and d) two orthogonal circular polarizations - left- and right-handed (LCP and RCP). The diffraction efficiency in the +1 diffraction order was monitored in real time by a probing laser beam at the wavelength of 635 nm. The results indicate that the highest diffraction efficiency is achieved when recording with orthogonal polarizations - linear at ±45° or left and right circular. This is explained by the ability of the azopolymer material to record the variations in the polarization state of light better than the variations in its intensity. The holographic gratings obtained can be used to enhance the light-extraction efficiency of an OLED device.