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Dive into the research topics where R Todorov is active.

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Featured researches published by R Todorov.


Journal of Non-crystalline Solids | 2003

Light-induced changes in the optical properties of thin films of Ge–S–Bi(Tl, In) chalcogenides

R Todorov; Tz. Iliev; Kiril Petkov

Abstract In this paper, we report some results from the study of changes in transmittance, reflectance and optical band gap of thin Ge–S–Bi(Tl, In) thin layers depending on the composition and the conditions of evaporation and illumination. Using two triple, TR f R m and TR b R m methods, the optical constants ( n and k ), and the thickness ( d ) of very thin layers from the Ge–S system have been determined to an accuracy of ±2 nm. R f , R b and R m is the reflection from the film side, from the glass substrate and that of 100 nm thick film deposited on Si substrate, respectively. The calculated values of the optical constants of the ternary chalcogenide films were compared with those obtained from the ellipsometric measurements and some conclusions for their practical applications were done.


Journal of Optics | 2009

Optical properties of silica MFI doped acrylamide-based photopolymer

Tzvetanka Babeva; R Todorov; Svetlana Mintova; T. Yovcheva; Izabela Naydenova; Vincent Toal

The optical properties of acrylamide-based photopolymer doped with pure silica MFI-type zeolites are studied by refractometric and spectrophotometric means. Dynamic light scattering and transmission electron microscopy are used for zeolite characterization, and laser refractometry and white light interferometric profilometry are used for surface characterization of the composites. The refractive indices and absorption coefficients of the composites are determined from their transmittance and reflectance spectra. The calculated dispersion curves are further used for deriving the zeolites refractive index and porosity, and the porosity is compared to the values of total pore volume obtained from N2-sorption measurements. The impact of the doping level on the composites optical properties both on the surface and in the volume are discussed.


Vacuum | 2000

X-ray microanalysis and optical properties of thin As–S–Bi (Tl) films

Kiril Petkov; Tz. Iliev; R Todorov; D Tzvetkov

Abstract Chalcogenide glasses are an important class of materials because of their wide range of applications in infrared optics, infrared detectors, electronic and optical switching devices and phase optical recording. In this paper, we discuss the results obtained studying the influence of the conditions of deposition (temperature and rate of evaporation), thin film concentration and light exposure on the optical properties of thin As-containing chalcogenide films. A technique named “phase contribution method” (PCM) is used for quantitative elemental analysis of thin layers from the system As–S–Bi (Tl) deposited on soda-lime glass substrates. The method has been tested with thin chalcogenide films with different thicknesses deposited on glass and graphite substrates. The relationship between the changes in the composition and optical properties has been derived. The changes allow the practical application of thin-layered information recording media and high-resolution inorganic photoresists.


Advances in Condensed Matter Physics | 2013

Ellipsometric Characterization of Thin Films from Multicomponent Chalcogenide Glasses for Application in Modern Optical Devices

R Todorov; J. Tasseva; V. Lozanova; A. Lalova; Tz. Iliev; A. Paneva

A review is given on the application of the reflectance ellipsometry for optical characterization of bulk materials and thin films with thickness between /20 and 2 (at  nm). The knowledge of the optical constants (refractive index, , and extinction coefficient, ) of thin films is of a great importance from the point of view of modelling and controlling the manufacture of various optical elements, such as waveguides, diffraction gratings, and microlenses. The presented results concern the optical properties of thin films from multicomponent chalcogenide glasses on the base of As2S3 and GeS2 determined by multiple-angle-of-incidence ellipsometry and regarded as a function of the composition and thickness. The homogeneity of the films is verified by applying single-angle calculations at different angles. Due to decomposition of the bulk glass during thermal evaporation, an optical inhomogeneity of the thin As (Ge)-S-Bi(Tl) films is observed. The profile of in depth of thin As-S-Tl (Bi) films was investigated by evaporation of discrete layers. It is demonstrated that homogenous layers from the previous compounds with controlled composition can be deposited by coevaporation of As2S3 and metals or their compounds (Bi, Tl, In2S3).


Journal of Optics | 2013

Holographic investigation of silver electromigration in nano-sized As2S3 films

S. Sainov; R Todorov; I Bodurov; T. Yovcheva

Holographic gratings with a diffraction efficiency (DE) greater than 8% and a spatial resolution of 2237 mm−1 are recorded in very thin As2S3 films with a thickness of 100 nm. Silver photo-diffusion is observed during the holographic recording process while applying a corona discharge. We use the method of holographic grating relaxation spectroscopy (forced Rayleigh scattering) based on the evanescent waves to determine that the silver diffusion coefficient in the thin As2S3 film is in the range of (0.9–10.3) × 10−13 cm2 s−1 depending on the corona charge polarity.


Journal of Physics: Conference Series | 2012

Diffraction efficiency growth of nano-scale holographic recording produced in a corona discharge

I Bodurov; T. Yovcheva; I. Vlaeva; A. Viraneva; R Todorov; G Spassov; S. Sainov

The nano-scale holographic gratings ware recorded in 29 nm and 56 nm thick As2S3 films. The chalcogenide layers were deposited on a transparent chromium electrode with thickness 10 nm, produced on a glass substrate. Both chromium and chalcogenide films were deposited in one vacuum cycle by e-beam and thermal evaporation, respectively. The diode 532 nm diode laser was used as a light source in the present holographic experiments. The total internal reflection arrangement (Stetson-Nassenstein) was used in holographic recordings. The reference beam was totally reflected from the air-As2S3 boundary surface by an input glass prism. The object beam was normally incident on the recording medium. The corona charging was performed by a needle fixed at the distance of 1 cm from the holographic recording medium by applying a – 5 kV voltage. The diffraction efficiency increased from 9 to 30 times when the corona discharge was applied during the holographic recording, in comparison to the uncharged recording. The possible reason of the observed effect is discussed on the basis of the Franz-Keldysh effect and Moss rule.


Journal of Physics: Conference Series | 2012

One dimensional photonic crystals from As2S3 and PMMA films for photonic and sensor applications

Tz. Babeva; G Marinov; J Tasseva; A Lalova; R Todorov

One dimensional photonic crystal (1D-PhC) in form of 19 layered stack was prepared through layer-by-layer deposition of spin coated Poly (methyl methacrylate) (n = 1.49 at 1.55 ?m) and vacuum deposited As2S3 (n = 2.27 at 1.55 ?m) as low and high refractive index materials, respectively. The optical properties of the single layers and the multilayered stack are studied at normal and oblique incidence of linearly polarized light. The possibility of using the 1D-PhC as an omnidirectional reflector was exploited theoretically and confirmed experimentally. Besides, the prospect of using the studied structure for chemical sensing with optical read-out is demonstrated.


Journal of Optics | 2010

Electric charging influence in holograms of total internal reflection, recorded in a very thin chalcogenide film

I. Vlaeva; K Petkov; J Tasseva; R Todorov; T. Yovcheva; S. Sainov

We report the results of electric field influence on holographic recording in very thin chalcogenide glass films. The total internal reflection prism recording technique (Stetson’s scheme) is applied for holographic recording. The main advantage of this scheme is the possibility of holographic recording in micro- and nanometer thick photosensitive materials. In the present work, 30 nm, 50 nm and 1.0 μm thick films are used. In the 1.0 μ mt hick fi lm two slanted gratings are simultaneously recorded. In this recording geometry only one reconstructed beam is observed. The corona charging influence on the diffraction efficiency of the recorded gratings is investigated. A negative voltage of 5 kV is applied to the corona electrode (needle) prior to the holographic recording. The observed diffraction efficiency of charged samples is always higher in comparison with uncharged samples. The reconstructed beam intensity is monitored with a red (635 nm) semiconductor laser. The possible reason is an additional refractive index modulation due to the increase in polarization, caused by the electric charging.


Journal of Non-crystalline Solids | 2016

Thermoanalytical properties and structure of (As2Se3)100 − x(Sb2Se3)x glasses by Raman and 77Se MAS NMR using a multivariate curve resolution approach

E. Černošková; Jana Holubová; Bruno Bureau; Claire Roiland; Virginie Nazabal; R Todorov; Zdeněk Černošek


Thin Solid Films | 2017

Microstructure and ellipsometric modelling of the optical properties of very thin silver films for application in plasmonics

R Todorov; V. Lozanova; Petr Knotek; E. Černošková; Milan Vlček

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S. Sainov

Bulgarian Academy of Sciences

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I. Vlaeva

Bulgarian Academy of Sciences

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Kiril Petkov

Bulgarian Academy of Sciences

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Tz. Iliev

Bulgarian Academy of Sciences

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J Tasseva

Bulgarian Academy of Sciences

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V. Lozanova

Bulgarian Academy of Sciences

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Izabela Naydenova

Dublin Institute of Technology

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Vincent Toal

Dublin Institute of Technology

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Petr Knotek

University of Pardubice

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