Refractive indices of Zn-doped lithium niobate

We measured the index of refraction n(λ) of Zn-doped LiNbO₃ grown from a congruent melt as a function of the Zn concentration in the crystal. The Zn concentration c_Zn in a sample was determined by electron microprobe technique. The refractive index n(λ, c_Zn) is described by a generalized Sellmeier equation for 400 < λ < 1200 nm and c_Zn up to 8 mol% ZnO. Calculated phase-matching conditions of nonlinear effects like second-harmonic generation are in excellent agreement with experimental results obtained with the same set of crystals.     

Properties of SnO2 films fabricated using a rectangular filtered vacuum arc plasma source

Transparent and conducting SnO₂ films of 57–200nm thickness were deposited on microscope glass slide substrates, using a rectangular filtered vacuum arc deposition system. The 40 glass slides were equally distributed on a 400 × 420mm substrate carriage, and were exposed to a Sn plasma beam, produced by a rectangular vacuum arc plasma gun with a Sn cathode, and passed through a rectangular magnetic macroparticle filter towards the substrates. The carriage with the substrates was transported past the 94 × 494mm filter outlet. The SnO₂ films were fabricated on the glass substrates at room temperature by maintaining the chamber oxygen background pressure at 0.52Pa. The film composition, and electrical and optical properties were studied as a function of the film thickness. The films were stored under ambient air conditions, and their electrical resistance was measured as a function of storage time over a period of several months.

The average resistivity of films was 10–17mΩ cm for films with thickness (t) less than 100nm, but that of t > 100nm it was 5–9mΩ cm. The resistivity of the films with t > 100nm did not change significantly after 8months of storage in ambient air. The optical transmittance of the films in the visible spectrum was in the range of 75–90%. The optical constants, i.e., the refractive index and the extinction coefficient of the films at wavelength λ = 550nm were in the range of 2.02–2.09 and 0.013–0.023, respectively, and the optical band gap energy was 4.15–4.21eV. Unlike the electrical resistivity, the optical parameters weakly depended on t.     

Refractive index measurements on bismuth tellurium oxide (Bi2TeO5) single crystal

The refractive indices of the biaxial Bi₂TeO₅ single crystal have been measured for the first time at seven wavelengths in the range of 457.9–632.8 nm at room temperature with an error of about 0.0003 (at λ=632.8 nm, n₁=2.3203, n₂=2.3678 and n₃=2.4022). The optical plane was found to be the (010) plane and the crystal is shown to be optically negative. The crossing angle of the optical axes was found to be between 79° and 86°. The parameters of Sellmeier's one- and two-term dispersion equations have been determined by the least squares method. Resonance wavelengths λ₁=219 nm for the first and λ₁=182 nm and λ₂=318 nm for the second one were found. The conditions of phase-matched SHG are discussed.     

Intrinsic optical and structural properties of SrS thin films

SrS thin films were deposited by electron beam evaporation on heated silica substrates. The optical properties of the layers – complex refractive index and optical band gap –were derived from optical transmission spectra, measured by means of UV-VIS-NIR spectrophotometry. The influence of post-deposition annealing by rapid thermal processing (RTP) was studied. X-ray powder diffraction (XRD) was used to study the film crystal structure and preferential orientation.     

Optical transitions in laser-evaporated thin CuInSe2 films

The optical absorption coefficients of highly oriented laser-evaporated thin films were determined from the measured reflectance R(λ) and transmittance T(λ) in the wavelength range 400–1700 nm. The optical absorption spectrum of CuInSe₂ thin films shows three energy gaps, which are associated with the fundamental edge and valence band splitting by the tetragonal crystal-field and spin-orbit effects, and four optical transitions from the copper d levels to the conduction bands.     

Photoinduced changes in the optical constants of Ge–Se–AgI thin films

The photoinduced changes in the complex refractive index n=n − ik of thin films from the Ge–Se–AgI system with constant ratio Ge/Se=1/4 and concentrations of AgI of 0, 5 and 10 mol% are studied by real time measurements of reflectance (R) and transmittance (T) of the films. The phase delay (δ) between the components of the transmitted wave, which is proportional to the birefringence of the film, is measured in real time as well. The changes in the average value of the refractive index (Δn), and in the average value of the absorption index (Δk) in the imaginary part of n as well as the induced optical anisotropy are estimated by solving the inverse optical problem. It is found that involving small quantities of AgI into a Ge–Se matrix increases the sensitivity of the films, but the anisotropic effects are comparatively weakly in them. The maximum values of changes in the average refractive index (Δn=0.025) and in the average absorption index (Δk=−0.03) are obtained in the films, containing 5 and 10 mol% AgI, respectively. Considerable values of the photoinduced anisotropy – birefringence (Δn_a≈−0.002) and dichroism (ΔD≈0.05, which corresponds to Δk_a≈0.035) are observed in films without addition of AgI (“pure” Ge–Se film).     

Measurement of the physical properties of cyclohexane using a laser interferometric technique. II

The refractive index (n) and thermal coefficient of the refractive index (dn/dT) are measured at two laser wavelengths for the nonpolar laser dye solvent cyclohexane. The measurements are carried out using the modified laser Mach–Zehnder interferometric technique. The optical permittivity (ε) and its variation with temperature are calculated. Applying the Cauchy equation, the following refractive properties are obtained: the optical dispersion dn/dλ, the dielectric dispersion dε/dλ, the variation of −dn/dT, dε/dT as a function of wavelength (λ), and Cauchy's constants against temperature. Additionally, molar refractivity and thermal volume expansion coefficient versus temperature and wavelength are determined.     

Optical characterization of thin chalcogenide films by multiple-angle-of-incidence ellipsometry

Optical properties of thin chalcogenide films from the systems As-S(Se) and As-S-Se were investigated as a function of the film composition, film thickness and conditions of illumination by light using multiple-angle-of-incidence ellipsometry. Thin films were deposited by thermal evaporation and exposed to white light (halogen lamp) and to monochromatic light from Ar⁺ — (λ = 488, 514 nm) and He-Ne- (λ = 632.8 nm) lasers. The ellipsometric measurements were carried out at three different angles of light incidence in the interval 45-55°, at λ = 632.8 nm. An isotropic absorbing layer model was applied for calculation of the optical constants (refractive index, n and extinction coefficient, k) and film thickness, d. The homogeneity of the films was checked and verified by applying single-angle calculations at different angles. It was shown that the refractive index, n of As-S-Se films is independent of film thickness in the range of 50 to 1000 nm and its values varied from 2.45 to 3.05 for thin layers with composition As₂S₃ and As₂Se₃, respectively. The effect of increasing in the refractive index was observed after exposure to light which is related to the process of photodarkening in arsenic containing layers. The viability of the method for determining the optical constants of very thin chalcogenide films with a high accuracy was confirmed.     

Properties of tantalum oxide thin films grown by atomic layer deposition

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl₅ and H₂O at temperatures in the range 80–500 °C. The films deposited at temperatures below 300 °C were predominantly amorphous, whereas those grown at higher temperatures were polycrystalline containing the phases TaO₂ and Ta₂O₅. The oxygen to tantalum mass concentration ratio corresponded to that of TaO₂ at all growth temperatures. The optical band gap was close to 4.2 eV for amorphous films and ranged from 3.9 to 4.5 eV for polycrystalline films. The refractive index measured at λ = 550 nm increased from 1.97 to 2.20 with an increase in growth temperature from 80 to 300 °C. The films deposited at 80 °C showed low absorption with absorption coefficients of less than 100 cm⁻¹ in the visible region.     

Direct current reactive magnetron sputtered zinc oxide thin films —the effect of the sputtering pressure

ZnO thin films were deposited onto glass substrates by d.c. reactive magnetron sputtering from a metallic zinc target. A systematic study has been made on the influence of sputtering pressure in the range from 0.2 Pa to 3 Pa on the film structural and optical properties. At low sputtering pressure (0.2–0.4 Pa), the film was inhomogeneous, non-stoichiometric and had low refractive index and an almost amorphous structure. At high sputtering pressure (0.6–0.8 Pa), the film was homogeneous, stoichiometric and had high refractive index and the crystallinity was improved. As the sputtering pressure was further increased (1–3 Pa), the homogeneity and the refractive index of the film had no clear variation, but the crystallinity of the film went down. As the sputtering pressure was increased from 0.2 Pa to 3 Pa, the transmittance of the film increased and the deposition rate of the film decreased.     

Nonlinear optical properties of two oxazine dyes in aqueous solution and polyacrylamide hydrogel using single beam Z-scan

The nonlinear optical properties of the two oxazine dyes, i.e. oxazine 720 (OX720) and oxazine 750 (OX750), in aqueous solution and in polyacrylamide hydrogel (PAA) matrix were studied by Z-scan technique using CW He–Ne laser at 632.8 nm. All the polymeric and aqueous samples showed a negative nonlinearity and large nonlinear refractive index of the order of 10⁻⁸ cm²/W. The concentration-dependent nonlinear refractive index was also investigated. In the case of PAA–dye systems, two types of transit (fast) and steady state (slow) nonlinear effects were observed. It is believed that these effects are due to the thermo-optical induced refractive index gradient and a concentration gradient induced by a temperature gradient present in a medium.     

Influence of substrate temperature on atomic layer growth and properties of HfO2 thin films

Atomic layer growth of hafnium dioxide from HfCl₄ and H₂O has been studied at substrate temperatures ranging from 180–600°C. A quartz crystal microbalance was used for the real-time investigation of deposition kinetics and processes affecting the growth rate. It was shown that the layer-by-layer growth was self-limited at temperatures above 180°C. The data of ex situ measurements revealed that the structure, density and optical properties of the films depended on the growth temperature. The absorption coefficient of amorphous films grown at 225°C was below 40 mm⁻¹ in the spectral range of 260–850 nm. The refractive index of the films grown at 225°C was 2.2 and 2.0 at 260 and 580 nm, respectively. The polycrystalline films with monoclinic structure grown at 500°C had about 5% higher refractive index but more than an order of magnitude higher optical losses caused by light absorption and/or scattering.     

Optical properties of Tl2InGaS4 layered single crystal

The temperature dependence of the optical band gap of Tl₂InGaS₄ single crystal in the temperature region of 300–500 K and the room temperature refractive index, n(λ), have been investigated. The absorption coefficient, which was calculated from the transmittance and reflectance spectra in the incident photon energy range of 2.28–2.48 eV, increased with increasing temperature. Consistently, the absorption edge shifts to lower energy values as temperature increases. The fundamental absorption edge corresponds to an indirect allowed transitions energy gap (2.35 eV) that exhibits a temperature coefficient of −4.03 × 10⁻⁴ eV/K. The room temperature n(λ), calculated from the reflectance and transmittance data, allowed the identification of the oscillator strength and energy, static and lattice dielectric constants, and static refractive index as 16.78 eV and 3.38 eV, 5.96 and 11.77, and 2.43, respectively.     

Extraction of optical constants of zinc oxide thin films by ellipsometry with various models

Spectroscopic ellipsometry was used to extract the optical constants of zinc oxide (ZnO) thin films deposited on (100) silicon substrate by filtered cathodic vacuum arc technique. Three dispersion models, namely, Sellmeier dispersion model, Cauchy model and Forouhi–Bloomer model, were evaluated for determining the optical constants of ZnO thin films below the energy band gap. The study shows that the Cauchy model provides the best spectral fittings among these three models. Above the energy band gap, two ellipsometric models, namely, two-phase model and three-phase point-by-point fit, were used. This study reveals that the initial values used in the point-by-point fitting play a critical role. It also shows that the refractive index and the extinction coefficient calculated with the two-phase model can be used as the initial values for the point-by-point fitting. The spectral dependence of the refractive index and extinction coefficient obtained in this work is comparable with the data reported in the literature. In sum, a reliable methodology for determining the optical constants of ZnO thin films in the ultraviolet-visible-near infrared range (2501100 nm) has been developed.     

Spectroscopic ellipsometry studies on various zinc oxide films deposited by ion beam sputtering at room temperature

Various zinc oxide films were deposited by ion-beam sputter deposition (IBSD) under different oxygen partial pressures (P(O2)) at room temperature. The as-deposited ZnO films fabricated at P(O2)>1.0×10(-4) Torr had poly-crystalline structures to absorb water on the surface at ambient condition. Simultaneously, the film surfaces were covered and smoothed by the surface layers formed with the water, hydroxyl (OH(-)) groups, and ZnO materials investigated by X-ray photoelectron spectroscopy (XPS). When the compositions of the surface layers were used in a multilayer fitting model of spectroscopic ellipsometry, the actual optical refractive index of the ZnO film deposited at P(O2)=1.2×10(-4) Torr was found to be about 1.9618 at λ=550 nm.     

Optical waveguiding in epitaxial PbTiO(3) thin films

Epitaxial lead titanate (PbTiO(3)) thin films on SrTiO(3) (100) substrate were grown in situ by radio-frequency sputtering for optical waveguiding applications. The crystalline quality of the PbTiO(3) films deposited at 550 degrees C has been investigated through x-ray diffraction analysis. It indicates that thin films are completely c-axis oriented (rocking curve FWHM of 0.2 degrees for the 001 reflection). The transmission spectrum method has been used to measure the dispersion of the refractive index. At 632.8 nm, the PbTiO(3) film with an (001) orientation exhibits a refractive index of 2.61, which represents 98% of the bulk material. The prism-coupling technique has been also employed to determine the optical attenuation in the planar waveguide. In this study, we report a low propagation loss of 2.2 ? 0.2 dB/cm obtained in a PbTiO(3) optical waveguide.     

SiC nanocrystals embedded in oligoetheracrylate photopolymer matrices; new promising nonlinear optical materials

Photoinduced optical phenomena in SiC nanocrystallites embedded within the photo-polymer oligoetheracrylate matrices have been studied using experimental nonlinear optics, particularly photoinduced optical second harmonic generation (SHG). The YAG-Nd-laser (λ=1.06 μm; W=30 MW; pulse duration within the 30–50 ps) was used as a source of pumping light and the nitrogen laser (λ=337 nm) has been applied as a source of the photoinducing light. With increasing intensity of the photoinducing beam, the SHG (λ=0.53 μm) signal increased and achieved a maximum (χ₂₂₂=10.1 ± 0.13 pm/V) at a photon flux of about 1.61 GW/cm². With decreasing temperature, the SHG signal strongly increases within the temperature range 25–30 K. Time-dependent probe–pump measurements indicate an existence of the SHG maximum for a pump–probe time delay of about 20 ps. The SiC hexagonal structural components play a key role in the observed photoinduced nonlinear optical effects. Large values of the nonlinear optical constants as well the good technological parameters open a possibility to enhance the nonlinear optical susceptibilities.     

Preparation and characterization of chemically deposited PbSnS3 thin films

High-quality and well-reproducible PbSnS₃ thin films have been prepared by a simple and inexpensive chemical-bath deposition method from an aqueous medium, using thioacetamide as a sulphide ion source. X-ray diffraction analysis of the deposited films revealed that the as-deposited films were amorphous, however, an amorphous-to-crystalline phase transition was observed as the result of thermal annealing at 425 K for 1 h. The X-ray structure analysis of the collected powder from the bath annealed at 425 K for 1.5 h revealed an orthorhombic phase.

Analysis of the optical absorption data of crystalline PbSnS₃ films revealed that both direct and indirect optical transitions exist in the photon energy range 1.24–2.48 eV with optical band gaps of 1.68 and 1.42 eV, respectively. However, a forbidden direct optical transition with a band gap value of 1.038 eV dominates at low energy (<1.24 eV). The refractive index changes from 3.38 to 2.16 in the range 500–1300 nm. The high frequency dielectric constant and the carrier concentration to the effective mass ratio calculated from the refractive index analysis were found to be 4.79 and 2.3×10²⁰ cm⁻³, respectively. The temperature dependence of the electrical resistivity of the deposited films follows the semiconductor behaviour with extrinsic and intrinsic conduction. The determined activation energies range are 0.35–0.42 and 0.76–85 eV, respectively.     

Design of ZnO/Ag/ZnO multilayer transparent conductive films

We have studied the properties of ZnO/Ag/ZnO multilayers prepared on glass substrates by simultaneous RF magnetron sputtering of ZnO and dc magnetron sputtering of Ag. The electrical and optical performance of Ag and ZnO single layer films was also investigated. Different optimization procedures were used for good transparent conductive film. Several analytical tools such as spectrophotometer, scanning electron microscope (SEM), four-point probes were used to explore the causes of the changes in electrical and optical properties. Low sheet resistance of 3 Ω/sq. and transmittance over 90% at 580 nm was achieved. The results of optimization condition of both oxide layers and metallic Ag layers were illustrated.     

Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique

Polycrystalline BaTiO₃ thin films having the perovskite structure were successfully produced on platinum coated silicon, bare silicon, and fused quartz substrate by the combination of the metallo-organic solution deposition technique and post-deposition rapid thermal annealing treatment. The films exhibited good structural, electrical, and optical properties. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 255 and 0.025, respectively, and the remanent polarization and coercive field were 2.2 μC cm⁻² and 25 kV cm⁻¹, respectively. The resistivity was found to be in the range 10¹⁰–10¹² Ω·cm, up to an applied electric field of 100 kV cm⁻¹, for films annealed in the temperature range 550–700 °C. The films deposited on bare silicon substrates exhibited good film/substrate interface characteristics. The films deposited on fused quartz were highly transparent. An optical band gap of 3.5 eV and a refractive index of 2.05 (measured at 550 nm) was obtained for polycrystalline BaTiO₃ thin film on fused quartz substrate. The optical dispersion behavior of BaTiO₃ thin films was found to fit the Sellmeir dispersion formula well.