Waveguide properties of optical structures fabricated by oxidation of porous silicon

Results of an investigation of the optical properties of channel waveguides fabricated by oxidation of porous silicon are described. The waveguide parameters are estimated and the existence of optical anisotropy is established. The effective refractive index of the dominant quasi-TM waveguide mode is measured. The results suggest that a buffer layer exists between the waveguide and the silicon substrate. It is hypothesized that a second refractive index peak exists within this layer. Pis’ma Zh. Tekh. Fiz. 23, 86–89 (May 26, 1997)     

Refractive index of polymethylmethacrylate oriented by fluid temperature under electrical field

We prepared micron and submicron polymethylmethacrylate (PMMA) layers by the spin-coating method. We investigated the possibility to orientate polymer dipoles in electric field in the glass transition area (T _g) and the fluid temperature of PMMA with the aim to increase its refractive index (n) after the layer is cooled below T _g. We have studied the effect of electric field (up to 12 kV cm⁻¹) on change of surface morphology of the layer, dependence of n and contact angle (surface wettability) on the field and dependence of layers orientation on orientation of electric field. The surface morphology was examined using atomic force microscopy (AFM), contact angles were measured by goniometer, film thickness was measured by profilometer, refractive index of films was determined using refractometer. The change of refractive index as dependent on the PMMA layer orientation in electric field depends on temperature and electric field. The highest change in n was found for electric field 11 kV cm⁻¹. The change in contact angle (wettability) on surface of an orientated PMMA layer confirms the dipoles orientation in electric field unambiguously. The orientation of layers causes a “slight” change in their morphology and a “slight” increase of surface roughness only for one direction of field effect. Change in colour for oriented layers does not depend on orientation of electric field.     

Optical properties of samarium doped zinc-tellurite glasses

Glasses with the composition, (Sm₂ O ₃) _x (ZnO)_(40-x)(TeO ₂)₍₆₀₎, were prepared by conventional melt quenching method. The density, molar volume, and optical energy band gap of these glasses have been measured. The refractive index, molar refraction and polarizability of oxide ion have been calculated by using Lorentz-Lorentz relations. Optical absorption spectra of these glasses were recorded in the range 300–700 nm at room temperature. The oxide ion polarizabilities deduced from two different quantities, viz. refractive index and optical energy band gap, agree well compared with other glasses. The nonlinear variation of the above optical parameters with respect to samarium dopant has been explained.     

Refractive index of Al<Subscript>3</Subscript>C<Subscript>2</Subscript>B<Subscript>48</Subscript> aluminum borocarbide crystals

Aluminum borocarbide single crystals have been grown from an Al-based solution melt. The crystal lattice parameters have been determined, the dispersion of the refractive index in a 0.55–1.3 μm wavelength interval has been studied, and the temperature coefficient of the refractive index in a 300–600 K range has been measured. The crystals are characterized by a high refractive index in the visible spectral range in combination with at a high hardness, which makes them of interest for jewelry, as well as for both traditional and X-ray optics.     

Thickness dependence of refractive index and optical gap of PMMA layers prepared under electrical field

Optical parameters (refractive index, dispersion energy, optical gap) of polymethylmethacrylate (PMMA) layers prepared by spin coating and modified by electric field have been studied. Refractive index was measured using a refractometer, internal structure was investigated as a structural parameter (E _d) within the One Oscillator Model. Optical gap width (E _g^opt) was assessed using Tauc Approximation from UV-Vis spectra. Surface morphology and roughness was investigated using an AFM. The electric field imposed during preparation of layers increases their refractive index. The highest increase in n (Δn = 0.042) was found for the thinnest PMMA layer (70 nm). Oriented layers have produced higher E _g^opt than non-oriented ones for all studied values of thickness. The electrical field applied at preparation of the oriented layer will not change its surface morphology and roughness.     

Perturbation theory for the refractive index mismatch between the inclusion and the surrounding tissues

Abstract

Tissue refractive index is one optical contrast mechanism with diagnostic potential, it is very important to investigate the effect of the refractive index mismatch on light propagation through diffusive regions. Here, we present a new analytical solution of perturbation theory for the refractive index mismatch between the small spherical inclusion and the surrounding tissues. The solution has been used to implement fitting procedures in order to obtain the optical properties of a heterogeneous sphere in semi-infinite medium from measurements of diffuse reflectance. Finally, perturbation theory has been validated by comparisons with the results of Monte Carlo simulation. The new perturbation theory would provide a basis for allowing early disease diagnosis and automatic screening.     

Monitoring the deposition of an interference film by differential reflection of light

The change in the reflection (differential reflection) of light from an interference film as a result of the deposition of an ultrathin layer on it is investigated. Formulas describing the differential reflection around the reflectivity minima and maxima of the film are obtained by a perturbation method. It is shown that these formulas and the corresponding differential measurements can be used for an easy and unambiguous determination of the thickness and refractive index not only of ultrathin surface layers but also of the interference films themselves. The proposed method is especially convenient for monitoring the deposition of thin-film structures. Pis’ma Zh. Tekh. Fiz. 24, 78–86 (October 26, 1998)     

Room-temperature lasing in structures with CdSe quantum islands in a ZnMgSSe matrix without external optical confinement

It has been shown that lasing may be achieved in structures with submonolayer CdSe inclusions in a ZnMgSSe matrix at above-room temperatures without additional optical confinement of the active region by thick layers of lower refractive index. The temperature dependence of the excitation density at the lasing threshold is typical of structures with three-dimensional carrier localization. Pis’ma Zh. Tekh. Fiz. 23, 26–30 (April 26, 1997)     

3-D mapping with ellipsometrically determined physical thickness/refractive index of spin coated sol-gel silica layer

Precursor sol for sol-gel silica layer was prepared from the starting material, tetraethylorthosilicate (TEOS). The sol was deposited onto borosilicate crown (BSC) glass by the spinning technique (rpm 2500). The gel layer thus formed transformed to oxide layer on heating to 450°C for ∼ 30 min. The physical thickness and the refractive index of the layer were measured ellipsometrically (Rudolph Auto EL II) at 632.8 nm. About 10 × 10 mm surface area of the silica layer was chosen for evaluation of thickness and refractive index values at different points (121 nos.) with 1 mm gap between two points. Those data were utilized in the Autolisp programme for 3-D mapping. Radial distribution of the evaluated values was also displayed.     

A possible method to measure accurately the refractive index of a transparent liquid

An analysis is made of some characteristics of the propagation of a light wave through a semiconductor-gap-semiconductor structure, where the thickness of the gap between the parallel planes of the crystals is of the order of a micron. It is shown that a combination of multiple reflection and interference processes produces an exponential dependence of the transmitted radiation intensity on the refractive index of the material filling the gap. This circumstance can be used to set up a simple method for accurate determination of the refractive index of a transparent liquid. Pis’ma Zh. Tekh. Fiz. 23, 9–13 (May 12, 1997)     

Optical properties of lead-bismuth cuprous glasses

The optical transmission and absorption spectra in UV- VIS were recorded in the wavelength range 350–800 nm for different glass compositions in the system (CuO) _x (PbO) ₅₀−x(Bi₂O₃)₅₀ (x = 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 20.0). Absorption coefficient (α), optical energy gap (E_opt), refractive index (n_D), optical dielectric constant (ε′_∞), measure of extent of band tailing (ΔE), constant (β) and ratio of carrier concentration to the effective mass (N/m*) have been reported. The effects of compositions of glasses on these parameters have been discussed. It has been indicated that a small compositional modification of the glasses lead to an important change in all the optical properties including non-linear behaviour. The optical parameters were found to be almost the same for different glasses in the same family.     

Refractive indices and thicknesses of optical waveguides fabricated by silicon ion implantation into silica glass

Planar optical waveguides formed by Si ion implantation into PECVD SiO₂ have been characterized by the dark mode spectroscopy method at a wavelength of 0.6328 μm. The measured effective index values of the guided modes have been used to investigate the optical properties of the core layers of the waveguides after different pre-implantation treatments. It was found that annealing the specimens before implantation, affected both the refractive index and thickness of the core layers. In the annealed specimens a thicker core layer and a larger relative refractive index difference between the core and the buffer layer resulted.     

A liquid crystal phase grating for instrumentation applications

Abstract

A liquid crystal phase grating, suitable for instrumentation applications has been developed that utilizes refractive index matching to enable it to be switched from a diffracting to a non-diffracting state by application of an electric field. The effect of the non-uniform molecular alignment during switching is considered and compared with measured data for such a device. The observed switching potentially offers a device in which the mark/space ratio may be varied with the applied electric field.     

Nonlinear optical properties of mercury dithizonate in a polymer film

Abstract

The absorption spectra of trans and cis isomers of mercury dithizonate in a doped poly(methyl methacrylate) film are measured, and the optical properties of the two isomers are analysed. The properties indicate that different isomers of mercury dithizonate can convert into each other when irradiated by light of appropriate wavelength. With the Z-scan method, the nonlinear optical properties of mercury dithizonate are investigated, and experimental data on the nonlinear refractive index and the change in refractive index are obtained. The changes in refractive index as a function of wavelength are obtained by the Kramers-Kronig transformation.     

Optical properties of the polarized Dirac vacuum

An investigation is made of the propagation of laser radiation through the Dirac vacuum polarized by a strong electric field. Calculations are made of the refractive index of the vacuum and the angle of rotation of the plane of polarization of the radiation. The possibility of measuring strong electric fields is assessed. Pis’ma Zh. Tekh. Fiz. 24, 45–47 (July 26, 1998)     

Spectroscopic ellipsometry of TiO2 layers prepared by ion-assisted electron-beam evaporation

Single layer TiO₂ films deposited on BK7 glass substrates by the ion-assisted electron-beam evaporation technique have been characterized by phase modulated spectroscopic ellipsometry. The ellipsometry spectra were recorded in the wavelength range of 300–1000 nm. The measured spectra were then fitted with theoretically simulated curves generated assuming different model sample structures. Analyses were carried out separately in three different wavelength regimes, the transparent regime with no dispersion of refractive index (650–1000 nm), the transparent regime with dispersion of refractive index (400–650 nm) and the absorbing regime (300–450 nm). Modeling has been attempted with both homogeneous and inhomogeneous sample structures. For the inhomogeneous structure, both linear and non-linear variation of refractive index along the depth of the sample were used. Refractive indices of the samples were determined separately from the best-fit ellipsometric data in the above three wavelength regimes. Finally, variation of refractive index with the variation in ion beam currents has been studied.     

Asymmetry in the Diffraction Spectrum of a Reflection Hologram Grating

Abstract

Reflection holograms were recorded in dichromated gelatin and dichromated poly(vinyl alcohol) using Lippmann and symmetric configurations. In their diffraction spectra, asymmetry has been observed and a series of secondary peaks appeared on only one side of the main peak. A detailed theoretical analysis has been made for both recording geometries. The relationship between the appearance of secondary peaks with the refractive index was given. The number and the amplitude of the secondary peaks are related to the thickness of the recording medium as well as the recording geometry.     

Preparation of thin films of new ZnGa2S4 wide-gap semiconductors

An investigation was made of a possible method of obtaining thin films of new ZnGa₂S₄ semiconductors by pulsed laser deposition and quasi-equilibrium deposition from a Knudsen cell. It was established that these films are transparent in the range 0.3–26 μm, their band gap is 3.79 eV, and the refractive index in the region of transparency is 2.19. Pis’ma Zh. Tekh. Fiz. 24, 85–87 (March 26, 1998)     

Influence of an external electric field on the recovery of stabilized zirconium dioxide under helium ion bombardment

It is shown that the application of an external electric field during helium ion bombardment of yttrium-stabilized zirconium dioxide single crystals may have a substantial influence on the processes accompanying the radiation-stimulated recovery of yttrium-stabilized zirconium dioxide: its n-type conductivity and refractive index may vary, and metallic nanosize inclusions may appear in the yttrium-stabilized zirconium dioxide matrix. Pis’ma Zh. Tekh. Fiz. 25, 72–75 (July 26, 1999)     

Theoretical study of Ti:LiNbO3 strip waveguides in the Mid-infrared

Abstract

The transverse magnetic and transverse electric mode size and effective refractive index as functions of the Ti-strip initial width W, diffusion temperature T and Ti-strip initial thickness H in c-cut Ti-diffused LiNbO₃ waveguides have been theoretically studied at wavelengths of 3.0 and 3.39 μm on the basis of modified expressions for the waveguide surface index increment. The single-mode, multiple-mode and cut-off conditions have been identified from those effective refractive index plots. These mid-infrared numerical results are discussed comparatively with near-infrared data reported earlier.