Browsing by Author "Marinova, V."

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Atomic layer deposition prepared Al-doped ZnO for liquid crystal displays applications
(2018-05-01)
Su, Y. C.
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Chiou, C. C.
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Marinova, V.
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Lin, S. H.
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Bozhinov, N.
Aluminum-doped zinc oxide (AZO) layers are prepared by using atomic layer deposition technique. The obtained layers possess high optical transmittance at visible and near-infrared spectral range. By varying the Al content, optimal growth compositions of AZO are established where the electrical conductivity of the film is highest. Based on performed optical and electrical characteristics measurements, selected AZO films with optimal conductivity are implemented as transparent electrodes in liquid crystal display devices. The electro-optical modulation characteristics of these devices are found to be comparable to those of devices using commercial ITO electrodes.
center dot-ray induced effects in Sm-doped strontium borate glasses
(2013)
Marinova, V.
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Tomov, V.
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Chuang, C. I.
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Lin, Y. C.
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Lin, S. H.
Strontium borate glasses SrB4O7 doped with Sm ions are prepared by conventional melting-quenching method. The effect of center dot-ray irradiation on the glass samples is studied by measuring the absorption spectra and the photoluminescence. It is found that the center dot -ray irradiation caused partial reduction of Sm3+ to Sm2+ at room temperature. Borate framework is supposed to be responsible for the Sm3+ to Sm2+ transition.
Characterization of doped Bi4Ge3O12 single crystals by light-induced absorption, electrical and photoelectrical measurements
(2007-02-01)
Petrova, D.
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Marinova, V.
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Liu, R. C.
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Lin, S. H.
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Hsu, K. Y.
Measurement of light-induced absorption changes, dark- and photoconductivity have been performed on Bi4Ge3O12 crystals doped with Fe, Rh and Fe+Rh. All samples exhibited a strong photochromic effect at room temperature, which was fully reversible after thermal annealing. It was found that Rh-doped Bi4Ge3O12 crystals possess a lower dark conductivity (sigma(d) = 5.2 x 10(-14) (Omega.cm)(-1)) at room temperature in comparison with other investigated samples. New centers in the Bi4Ge3O12 inter-band structure, with thermal activation energies between 0.25 and 0.37 eV, have been created by doping with Fe and Rh. Fe-doped and Fe+Rh co-doped samples showed a non-linear photoconductivity dependence on light intensity, under illumination with a 514-nm laser.
Chemical vapour deposition growth and Raman characterization of graphene layers and carbon nanotubes
(2016-01-01)
Lai, Y-C
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Rafailov, P. M.
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Vlaikova, E.
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Marinova, V.
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Lin, S. H.
Single-layer graphene films were grown by chemical vapour deposition (CVD) on Cu foil. The CVD process was complemented by plasma enhancement to grow also vertically aligned multiwalled carbon nanotubes using Ni nanoparticles as catalyst. The obtained samples were characterized by Raman spectroscopy analysis. Nature of defects in the samples and optimal growth conditions leading to achieve high quality of graphene and carbon nanotubes are discussed.
Chemical vapour deposition growth of graphene layers on metal substrates
(2014-01-01)
Lai, Y-C
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Yu, S-C
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Rafailov, P. M.
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Vlaikova, E.
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Valkov, S.
Graphene layers were grown by chemical vapour deposition (CVD) on Si wafers covered by a SiO2 substrate layer and a Ni interlayer, and on copper and nickel foil. The obtained graphene layers were characterized by Raman spectroscopy. The films grown on SiO2/Ni substrate and Ni foil comprise mainly multilayer defect-rich graphene, while those on Cu foil exhibit the spectroscopic fingerprint of relatively defect-free single-layer graphene due to the low carbon solubility in copper and the suitably chosen substrate position in a quasi-closed volume. Optimal growth conditions and the nature of defects in the layers are discussed.
Light-induced and holographic properties of ruthenium and manganese co-doped Bi(4)Ge(3)O(12) crystals
(2010-07-01)
Lin, S. H.
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Marinova, V.
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Lin, Y. C.
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Petrova, D.
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Chou, W. C.
The effect of Ru + Mn co-doping on the light-induced properties of Bi(4)Ge(3)O(12) (BGO) crystals is studied and compared with single Ru- and Mn-doped crystals. Optical absorption spectra of Ru + Mn co-doped crystal demonstrates a photosensitivity shift to the visible spectral range. In addition, a strong photochromic effect, being fully reversible by thermal annealing is observed. It is verified by photoluminescence emission that charge traps could be manipulated under ultraviolet (UV) excitation, as the photoluminescence intensity decreases revealing the existence of the state filling effect. Ru + Mn doping leads to a faster response time during holographic recording at 633 nm and a prolonged read-out process, especially after a preliminary exposure to UV light. The ability to modify the defect structure and corresponding absorption bands suggests the future potential for optical fixing using a two-step sensitization method.
Light-induced and holographic properties of ruthenium and manganese co-doped Bi4Ge3O12 crystals
(2010-07-01)
Lin, S. H.
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Marinova, V.
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Lin, Y. C.
;
Petrova, D.
;
Chou, W. C.
The effect of Ru + Mn co-doping on the light-induced properties of Bi4Ge3O12 (BGO) crystals is studied and compared with single Ru- and Mn-doped crystals. Optical absorption spectra of Ru + Mn co-doped crystal demonstrates a photosensitivity shift to the visible spectral range. In addition, a strong photochromic effect, being fully reversible by thermal annealing is observed. It is verified by photoluminescence emission that charge traps could be manipulated under ultraviolet (UV) excitation, as the photoluminescence intensity decreases revealing the existence of the state filling effect. Ru + Mn doping leads to a faster response time during holographic recording at 633 nm and a prolonged read-out process, especially after a preliminary exposure to UV light. The ability to modify the defect structure and corresponding absorption bands suggests the future potential for optical fixing using a two-step sensitization method.
Liquid crystal cell with graphene electrodes
(2017-01-01)
Marinova, V.
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Tong, Z. F.
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Petrov, S.
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Lin, S. H.
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Chen, M. S.
We demonstrate a graphene-based nematic liquid crystal cells fabricated by photo alignment technique. The modulation characteristics have been measured revealing that photo alignment method shows great potential for protecting the graphene layer on glass substrate during LC device fabrication.
Liquid crystal electro-optic modulator based on transparent conductive TiO2/Ag/TiO2 multilayers
(2018-06-01)
Su, Y. C.
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Chiou, C. C.
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Marinova, V.
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Lin, S. H.
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Dikov, Hr.
In this paper, TiO2/Ag/TiO2 multilayers, deposited by magnetron sputtering technique at room temperature are used as transparent conductive layers for Liquid Crystal (LC) devices fabrication. The measured electro-optical modulation characteristics and phase retardation of LC cell based on TiO2/Ag/TiO2 electrode show improved electro-optic performances in comparison with LC reference cell using commercial Indium Tin Oxide (ITO). Low sheet resistance and high transparency of TiO2/Ag/TiO2 multilayers open a great potential for next generation ITO-free liquid crystals electro-optical modulators.
Liquid crystal light valve operating at near infrared spectral range
(2016-04)
Marinova, V.
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Chi, C. H.
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Tong, Z. F.
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Liu, R. C.
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Berberova, N.
A near infrared sensitive liquid crystal light valve, based on Ru-doped Bi12TiO20 (BTO:Ru) inorganic crystal and liquid crystal is proposed. The two-beam mixing experiments at Raman-Nath regime show promising gain value at 1064 nm. An image evolution through the proposed device is demonstrated supporting all optical switching property.
Polarization holographic recording in thin films of pure azopolymer and azopolymer based hybrid materials
(2017-02-01)
Berberova, N.
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Daskalova, D.
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Strijkova, V.
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Kostadinova, D.
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Nazarova, D.
Recently, a birefringence enhancement effect was observed in azopolymers doped with various nano particles. The paper presents comparison between the parameters of polarization holographic gratings recorded in a pure azopolymer PAZO (Poly[1-[4(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]1,2-ethanediyl, sodium salt]) and in a hybrid PAZO-based organic/inorganic material with incorporated ZnO nanoparticles of size less than 50 nm. Laser emitting at 491 nm is used for the holographic recording. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The time dependence of the diffraction efficiency is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that both the diffraction efficiency and the height of the surface relief for the hybrid samples are enhanced with respect to the pure azopolymer films. (C) 2016 Elsevier B.V. All rights reserved.
Structural and optical properties of LuVO4 single crystals
(2017-01-01)
Dimitrov, D.
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Rafailov, P.
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Marinova, V.
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Babeva, T.
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Goovaerts, E.
The synthesis of large single crystals with good optical quality which is a preliminary condition for the practical applications of these materials frequently is complicated. It is found that large LuVO4 single crystals with high optical quality are possible to be prepared using high temperature solution growth method. It is obtained by X-ray crystallographic analysis that the grown crystals possess centrosymmetric tetragonal structure with the point group symmetry D-4h and space group I41/amd (zircon-type structure). The unit cell parameters of a = 7.0236 angstrom, b = 7.0236 angstrom, c = 6.2293 angstrom, volume = 307.30(3) angstrom(3) are measured. The crystals composition as well as vanadium oxidation state were measured in order to confirm that the crystal phase is mainly LuVO4. Optical transmission and Raman Spectroscopy are further performed on LuVO4 single crystal to reveal the optical quality and structure details
The influence of Rh doping on the light-induced properties of Bi(4)Ge(3)O(12) single crystals
(2009-01-01)
Marinova, V.
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Petrova, D.
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Lin, S. H.
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Hsieh, M. L.
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Hsu, K. Y.
The influence of different doping concentrations of rhodium on the light-induced and holographic properties of BGO crystal is investigated. It is found that Rh addition generates sufficient density of traps for photoinduced charge carriers such that holographic gratings are successfully recorded in the blue-green spectral range. The measured steady state diffraction efficiency at 514 nm is 0.14%, which corresponds to a refractive index modulation of Delta n similar to 1.5 x 10(-6) for higher Rh concentration. A photochromic effect and nonlinear dependence of photoconductivity on light intensity are observed indicating deep level and shallow level contributions to the charge transport mechanism of Rh-doped BGO.
The influence of Rh doping on the light-induced properties of Bi4Ge3O12 single crystals
(2009-01-01)
Marinova, V.
;
Petrova, D.
;
Lin, S. H.
;
Hsieh, M. L.
;
Hsu, K. Y.
The influence of different doping concentrations of rhodium on the light-induced and holographic properties of BGO crystal is investigated. It is found that Rh addition generates sufficient density of traps for photoinduced charge carriers such that holographic gratings are successfully recorded in the blue-green spectral range. The measured steady state diffraction efficiency at 514 nm is 0.14%, which corresponds to a refractive index modulation of Delta n similar to 1.5 x 10(-6) for higher Rh concentration. A photochromic effect and nonlinear dependence of photoconductivity on light intensity are observed indicating deep level and shallow level contributions to the charge transport mechanism of Rh-doped BGO.