| 1. |
- Valyukh, Iryna, 1971-, et al.
(author)
-
Characteristic functions for uniformly twisted birefringent media
- 2007
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102:6, s. 063110-1-063110-7
-
Journal article (peer-reviewed)abstract
- We present two functions for characterization of birefringent media with application to twisted nematic liquid crystal (LC) cells in the field off state. The first function is for reflective LC cells and the second is for transmissive LC cells. Based on the Stockes formalism, it is shown that these functions describe the ability of a layer of the twisted birefringent medium to change polarization of the output light and are invariant to the layer orientation. The characteristic functions are found in simple procedures and can be used for simultaneous determination of retardation, its wavelength dispersion, and twist angle, as well as for solving optimization problems.
|
|
| 2. |
- Valyukh, Iryna, 1972-, et al.
(author)
-
Characterization of the Photo-Alignment Material SD-1/SDA-2 with Spectroscopic Ellipsometry
- 2007
-
In: Proceedings of the IDW'07, Japan. ; , s. 391-394, s. 391-394, s. 391-394
-
Conference paper (peer-reviewed)abstract
- Variable angle spectroscopic ellipsometry (VASE) is applied for investigation of the azo-dye SD-1/SD-2 which are used for photoalignment of liquid crystals. The azo-dye was spin coated on silicon substrates. Samples as prepared as well as samples illuminated with polarized UV light were studied. The measured optical constants were modelled with an ensample of the Lorentz oscillators. The results confirm the reorientation mechanism of the photo-induced order formation in the azo-dye films under the action of polarized light, and also show that there exist some photochemical processes.
|
|
| 3. |
- Valyukh, Iryna, et al.
(author)
-
Monochromatic bistable reflective and transfiective ferroelectric LCDs
- 2005
-
In: Proceedings of the 25th International Display Research Conference, Eurodisplay 2005. ; , s. 305-308
-
Conference paper (peer-reviewed)abstract
- We study single and double polarizer monochromatic bistable reflective and transfective direct view ferroelectric liquid crystal displays (FLCDs). Three types of monochromatic displays are considered Black & White, Black & Yellow, Black & Green. Theoretical approach for finding optimal parameters of an FLCD is presented.
|
|
| 4. |
|
|
| 5. |
- Valyukh, Iryna, et al.
(author)
-
Optical properties of thin films of mixed Ni-W oxide made by reactive DC magnetron sputtering
- 2011
-
In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 519:9, s. 2914-2918
-
Journal article (peer-reviewed)abstract
- Thin films of NixW1-x oxides with x = 0.05, 0.19, 0.43 and 0.90 were studied. Films with thicknesses in the range 125-250 nm were deposited on silicon wafers at room temperature by reactive DC magnetron co-sputtering from targets of Ni and W. The films were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and spectroscopic ellipsometry (SE). XRD spectra and SEM micrographs showed that all films were amorphous and possessed a columnar structure. The ellipsometric angles psi and Delta of as-deposited films were measured by a rotating analyzer ellipsometer in the UV-visible-near infrared range (0.63-6.18 eV) and by an infrared Fourier transform rotating compensator ellipsometer in the 500-5200 cm(-1) wavenumber range. SE measurements were performed at angles of incidence of from 50 degrees to 70 degrees. Parametric models were used to extract thicknesses of the thin films and overlayers of NixW1-x oxide at different compositions, band gaps and optical constants. Features in the optical spectra of the NixW1-x oxides were compared with previous data on tungsten oxide, nickel oxide and nickel tungstate.
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Valyukh, Iryna, 1972-, et al.
(author)
-
Theoretical study of the optical properties of low voltage stacked cholesteric liquid-crystal displays
- 2004
-
In: Asia Display/IMID'04, South Korea. ; , s. 202-204
-
Conference paper (peer-reviewed)abstract
- We study theoretically optical properties of thin layered stacked monochrome cholesteric liquid-crystal displays. Thin thickness of the layers (~1 µm) allows us appreciably to reduce driving voltage and use such displays in smart cards. Good selective reflection is achieved due to stacked structure. Dependence of the reflectivity of this type of displays on the quality of the layers, their thickness, and liquid crystal birefringence is investigated.
|
|
| 9. |
|
|
| 10. |
- Valyukh, Iryna, et al.
(author)
-
UV-induced in-plane anisotropy in layers of mixture of the azo-dyes SD-1/SDA-2 characterized by spectroscopic ellipsometry
- 2008
-
In: Physica Status Solidi. C, Current topics in solid state physics. - Weinheim, Germany : Wiley-VCH Verlagsgesellschaft. ; , s. 1274-1277, s. 1274-1277
-
Conference paper (peer-reviewed)abstract
- The optical properties of the azo-dyes SD-1/SDA-2, which are used for photoaligning of liquid crystals (LCs), are investigated with Variable Angle Spectroscopic Ellipsometry (VASE). Films of mixture of SD-1/SDA-2 are deposited by spin coating on silicon wafers. The estimated thickness is approximately 10 nm. To achieve photo-induced anisotropy, one of the samples is illuminated during 15 minutes with linearly polarized UV light followed by thermal stabilization during 1 hour at 150 °C. VASE measurements are performed in the wavelength range 200–1350 nm at several angles of incidence and at different sample orientations. Dielectric functions of azo-dye films without/with polarized UV light illumination were modelled using an ensemble of Lorentz oscillators. The results confirm the diffusion model proposed recently for explanation of the formation of the photo-induced order in azo-dye films under the action of polarized light. Refractive indices, their wavelength dispersion and thicknesses of films of SD-1/SDA-2 are reported here.
|
|
| 11. |
|
|
| 12. |
|
|
| 13. |
|
|
| 14. |
- Valyukh, Sergiy, et al.
(author)
-
Liquid-crystal based light steering optical elements
- 2011
-
In: Photonics Letters of Poland. - : Photonics Society of Poland. - 2080-2242. ; 3:2, s. 88-90
-
Journal article (peer-reviewed)abstract
- Potential abilities of active optical elements based onliquid crystals are investigated. We consider liquid crystal cells in whicha desired distribution of the liquid crystal director is achieved by a non-uniform electric field and by inhomogeneous boundary conditions. Theoptical elements built on principles of diffractive optics are discussed.
|
|
| 15. |
- Valyukh, Sergiy, et al.
(author)
-
Liquid crystal light deflecting devices based on nonuniform anchoring
- 2010
-
In: APPLIED PHYSICS LETTERS. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 97:23, s. 231120-
-
Journal article (peer-reviewed)abstract
- Tunable liquid crystal light deflecting devices based on nonuniform anchoring energy are proposed. These devices have uniform thicknesses of the layers they are composed of, and beam deviation is controlled with a uniform electrical field. Potential applicability of such an approach in beam deflectors and active lenses is investigated. It is shown that the approach is a competitive alternative to liquid crystal light deflecting devices, in which the needed spatial distribution of liquid crystal molecules is achieved either due to nonuniform thickness or due to generation of nonuniform electrical field.
|
|
| 16. |
|
|
| 17. |
- Valyukh, Sergiy, et al.
(författare)
-
Spectrophotometric determination of reflective liquid crystal cell parameters
- 2006
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550 .- 8750-7587 .- 1522-1601. ; 99, s. 053102-1-053102-6
-
Tidskriftsartikel (refereegranskat)abstract
- We introduce a method for determination of the optical retardation, its wavelength dispersion, the cell twist angle, and the orientation of the input director in a reflective liquid crystal (LC) cell. These parameters are found from the extremes of a characteristic function defined as a sum of two spectral reflectivities of the LC cell placed between a pair of linear polarizers. The reflectivities are measured for two cell orientations, one of which is arbitrary and the other one is turned through 45°. Both theoretical analysis and experimental procedures are presented. Excellent agreement between the experiment and our theory has been found. The proposed method can be applied to the measurements of reflective LC cells with small and large cell gaps, as well as with small and large twist angles.
|
|
| 18. |
- Valyukh, Sergiy, et al.
(författare)
-
Study on Birefringent Color Generation for a Reflective Ferroelectric Liquid Crystal Display
- 2006
-
Ingår i: Japanese Journal of Applied Physics. - 0021-4922 .- 1347-4065. ; 45:10A, s. 7819-7826
-
Tidskriftsartikel (refereegranskat)abstract
- We study the possibility of a layer of a surface stabilized ferroelectric liquid crystal coupled with several retardation plates for birefringent color generation. Double and single polarizer reflective bistable dichromatic ferroelectric liquid crystal displays are considered. We demonstrate that one or two retardation plates are sufficient for a display having good color characteristics and high brightness. Optimal parameters for green/red and blue/yellow ferroelectric liquid crystal displays are found.
|
|
| 19. |
- Johansson, Malin B., et al.
(författare)
-
Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations
- 2013
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:20, s. 205502-
-
Tidskriftsartikel (refereegranskat)abstract
- The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (P-tot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low P-tot show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies E-g approximate to 3.1 eV, which increase with increasing P-tot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic delta-WO3, and monoclinic gamma- and epsilon-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The delta-WO3 and gamma-WO3 phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (>3 eV) and allowed transitions (>3.8 eV). The calculations show that E-g in epsilon-WO3 is higher than in the delta-WO3 and gamma-WO3 phases, which provides an explanation for the P-tot dependence of the optical data.
|
|
| 20. |
- Lansåker, Pia, et al.
(författare)
-
Spectral density analysis of thin gold films : Thickness and structure dependence of the optical properties
- 2013
-
Ingår i: Proceedings. - Stockholm : The Electromagnetics Academy. - 0351-6067. ; , s. 443-447, s. 443-447
-
Recension (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we study the feasibility of representing the optical properties of ultrathin gold films by effective medium theories. Gold films with mass thicknesses in the range of 1.4 to 9.2 nm were deposited by DC magnetron sputtering onto non-heated glass substrates. Optical measurements in the range 0.25 to 2 µm were carried out by spectroscopic ellipsometry, and the effective complex dielectric function of each film was determined. The gold films were modelled as a mixture of gold and air, and a general effective medium description using the spectral density function (SDF) was used to describe their optical properties. Numerical inversion of the experimental dielectric function gave a broad and rather featureless SDF with a few superimposed peak structures, both for island structures and percolating films. The broad background is qualitatively similar to the predictions of the Bruggeman [Ann. Phys. (Leipzig), 5th series, 24 (1935) 636-679] model.
|
|
| 21. |
- Valyukh, Iryna, et al.
(författare)
-
Ellipsometrically determined optical properties of nickel-containing tungsten oxide thin films : Nanostructure inferred from effective medium theory
- 2012
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:4, s. 044308-
-
Tidskriftsartikel (refereegranskat)abstract
- Films of NixW1−x oxide with 0.05 ≤ x ≤ 0.53 were produced by reactive dc magnetron co-sputtering onto Si. Such films have documented electrochromism. Spectroscopic ellipsometry was used to extract accurate data on the dielectric function in the photon range 0.062 to 5.62 eV. The results for 0.62 to 5.62 eV were compared with computations from the Bruggeman effective medium theory applied to two nanostructural models: one representing a random mixture of structural entities characterized by the dielectric functions of WO3 and NiWO4 and the other describing a random mixture of WO3 and NiO. Unambiguous evidence was found in favor of the former model, and hence the films are composed of nanosized tungsten oxide and nickel tungstate. This agrees excellently with an earlier investigation of ours on NixW1−x oxide films, where nanostructure was inferred from Raman spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction.
|
|
| 22. |
- Valyukh, Iryna, 1972-
(författare)
-
Optical Characterization and Optimization of Display Components : Some Applications to Liquid-Crystal-Based and Electrochromics-Based Devices
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation is focused on theoretical and experimental studies of optical properties of materials and multilayer structures composing liquid crystal displays (LCDs) and electrochromic (EC) devices. By applying spectroscopic ellipsometry, we have determined the optical constants of thin films of electrochromic tungsten oxide (WOx) and nickel oxide (NiOy), the films’ thickness and roughness. These films, which were obtained at spattering conditions possess high transmittance that is important for achieving good visibility and high contrast in an EC device. Another application of the general spectroscopic ellipsometry relates to the study of a photo-alignment layer of a mixture of azo-dyes SD-1 and SDA-2. We have found the optical constants of this mixture before and after illuminating it by polarized UV light. The results obtained confirm the diffusion model to explain the formation of the photo-induced order in azo-dye films. We have developed new techniques for fast characterization of twisted nematic LC cells in transmissive and reflective modes. Our techniques are based on the characteristics functions that we have introduced for determination of parameters of non-uniform birefringent media. These characteristic functions are found by simple procedures and can be utilised for simultaneous determination of retardation, its wavelength dispersion, and twist angle, as well as for solving associated optimization problems. Cholesteric LCD that possesses some unique properties, such as bistability and good selective scattering, however, has a disadvantage – relatively high driving voltage (tens of volts). The way we propose to reduce the driving voltage consists of applying a stack of thin (~1µm) LC layers. We have studied the ability of a layer of a surface stabilized ferroelectric liquid crystal coupled with several retardation plates for birefringent color generation. We have demonstrated that in order to accomplish good color characteristics and high brightness of the display, one or two retardation plates are sufficient.
|
|
| 23. |
- Valyukh, Iryna, et al.
(författare)
-
Optical Properties of Hydrated Tungsten Trioxide 3WO3ˑH2O
- 2014
-
Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 571, s. 644-647
-
Tidskriftsartikel (refereegranskat)abstract
- Spectroscopic ellipsometry was used to determine the optical properties of plate-like hydrated tungsten trioxide (3WO3·H2O) films in the energy range 300–4000 cm− 1. Films with different thicknesses were deposited on glass substrates pre-coated with fluorine-doped tin oxide via an efficient and simple hydrothermal method. Parametric models were used to extract thicknesses and optical constants of the thin films. The WO3 was found to be more hydrated for thicker films. Moreover, the nano-plates are larger in thicker films, which leads to a decrease of the transmission due to an increase of the scattering. Features in the obtained dielectric functions of the 3WO3·H2O thin films were compared with the earlier published optical spectra of WO3 and its hydrates ½WO3·H2O and WO3·H2O.
|
|
| 24. |
- Valyukh, Iryna, 1972-, et al.
(författare)
-
Spectroscopic ellipsometry characterization of electrochromic tungsten oxide and nickel oxide thin films made by sputter deposition
- 2010
-
Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 94:5, s. 724-732
-
Tidskriftsartikel (refereegranskat)abstract
- Electrochromic films of tungsten oxide and nickel oxide were made by reactive dc magnetron sputtering and were characterized by X-ray diffraction, Rutherford backscattering spectrometry, scanning electron microscopy, and atomic force microscopy. The optical properties were investigated in detail by spectroscopic ellipsometry and spectrophotometry, using a multiple-sample approach. The W oxide film was modeled as a homogeneous isotropic layer, whereas the Ni oxide film was modeled as an anisotropic layer with the optical axis perpendicular to the surface. Parametric models of the two layers were then used to derive complex refractive index in the 300-1700-nm-range, film thickness, and surface roughness. A band gap of 3.15 eV was found for the W oxide film, using a Tauc-Lorentz parameterization. For the Ni oxide film, taken to have direct optical transitions, band gaps along the optical axis, perpendicular to it, and in an isotropic intermediate layer at the bottom of the film were found to be 3.95, 3.97, and 3.63 eV, respectively. Parameterization for the Ni oxide was made by use of the Lorentz model.
|
|
| 25. |
- Valyukh, Sergiy, et al.
(författare)
-
Characterization of flexible reflective liquid crystal cells
- 2005
-
Ingår i: Journal of the Society for Information Display. - : Wiley. - 1071-0922 .- 1938-3657. ; 13:6, s. 501-506
-
Tidskriftsartikel (refereegranskat)abstract
- We consider peculiarities in testing flexible reflective liquid-crystal (LC) cells. Several new methods for measuring optical retardation of filled reflective LC cells on plastic substrates are proposed. Cases when the plastic is anisotropic and the LC cell consists of either one or two internal polarizers are studied. The majority of proposed methods can be applied for transmissive LC cells as well as for measuring twist angle.
|
|
