Epitaxial growth of ferroelectric PLZT thin film and their optical properties

A series of experiments for growing epitaxial PLZT thin films has been made with rf sputtering. X-ray and electron diffraction analyses confirm that the fabricated films grow epitaxially on SrTiO₃ and MgO crystals. A good transparency in the region above 0.4 μm to infrared with good ferroelectric properties is obtained. Propagation loss of He−Ne laser light is less than 6 dB/cm, and the PLZT thin film is a promising candidate for the optical modulator and other functional elements for integrated optics.     

Magnetooptical studies on thin iron films

Magnetooptical measurements on Fe-films in the layer system ZnS-Fe-MgF₂-Ag showed a decrease of the Kerr-intensity values with Fe-film thicknesses below 15nm, not predicted by theoretical calculations. Magnetization curves obtained magnetooptically indicate a significant influence of the layer system on the magnetic properties of Fe-films.     

Spectral excess attenuation of monomode optical fibers with sinusoidally perturbed core radius

The spectral excess attenuation caused by coupling between the guided HE₁₁ mode and radiation modes is calculated with the aid of the formula derived by Marcuse for the radiation loss of a monomode optical fiber with step-index profile and sinusoidally perturbed core radius. The pronounced minima and maxima of the spectral excess attenuation curve converge as the spatial frequency of the perturbation of the core radius increases. Even perturbation amplitudes as small as 0.01 μm may lead to excess attenuations of the order of 10 dB/km if the radiation wavelength does not coincide with an attenuation minimum. The spectral behavior of the excess attenuation can be utilized to design fibers which act as optical filters.     

Temperature and pressure dependences of Zn-doped GaN

The electroluminescence spectrum of Zn-doped GaN is independent of uniaxial pressure up to kbars and of temperature in the range 100–300 K. This work was done during a one month visit by Dr. Pankove at UNICAMP, made possible through a grant from PNUD. Financial support from TELEBRAS and FAPESP is also acknowledged.     

Equalization of reflectance of parallel-polarized electromagnetic plane waves at normal and oblique incidence of interfaces between transparent media and its use for measurement of the dielectric constant

A new angle of incidence of significance, when considering the reflection of electromagnetic waves at interfaces between transparent media, is defined. At this angle, denoted by φ_e, the reflection coefficient of parallel-polarized radiation is equal in magnitude and opposite in sign to the reflection coefficient at normal incidence. No similar angle exists for the perpendicular polarization. If ε is the relative dielectric constant, i.e., the ratio of the dielectric constant of the medium of refraction to that of the medium of incidence, we find that tan φ_e=(ε²+ε)^1/2. Measurement of φ_e, by equalization of the absolute (intensity) reflectances at normal and oblique incidence, allows ε to be determined using the inverse relation ε=(tan²φ_e+1/4)^1/2−1/2.     

Cavity-type surfaces for solar collectors

The optical shape of the heater's surface of solar energy concentrating collectors is here examined. Multi reflection effect, through a macrocavity analysis is introduced. The optical design of the cavity is accomplished and the cavity effect determined. Thermotechnical analysis of the system, in order to determine the optimal conditions for the heat transfer and for the maximization of the energy radiative balance, is introduced.     

In situ etch rate measurements of thin film combinatorial libraries

We demonstrate the use of optical reflection mapping as an in situ characterization tool to evaluate the corrosion rate of compositionally graded thin film combinatorial libraries coated with a commercial glass etching paste. A multi-channel fiber-optically coupled CCD-array-based spectrometer was used to collect a series of reflectance maps from 300 to 1000 nm versus time. The thin film interference oscillations in the measured reflection spectra have been fitted to determine the film thickness as a function of time and thereby the etch rate. Application of this technique to an In-Mo-O composition spread library is presented as an example.     

Low-loss rotator and application for isolator design

The Faraday rotation associated with the Zeeman effect in gases is used to rotate the plane or polarization of plane polarized light with low loss of intensity. Advantage is taken of the rapid decrease of absorption relative to Faraday rotation between the split Zeeman components. Calculations of this Faraday rotation are applied to obtain design parameters for an isolator for 5.26 μm which show a significant improvement over conventional designs.     

Determination of refractive index profiles in diffused optical waveguides

An improved method for the reconstruction of diffused waveguide index profiles from experimental mode spectra is proposed. The method uses a parabolic approximation between successive points. It is tested on the example of an exponential profile, and applied to the reconstruction of index profiles in Ag⁺ ion-exchanged waveguides from measured mode spectra. This method appears to work well also in the cases of very few mode wave-guides.     

Space-time resolved reflectivity measurements of picosecond laser-pulse induced phase transitions in (111) silicon surface layers

The changes in reflectivity of a silicon surface, irradiated by a green picosecond pulse, are probed during and following that pulse with a spatial resolution of 10 m. The data indicate the development of a liquid phase, and a resolidification either into a single crystal or an amorphous phase. The latter has a characteristic ring-type pattern, and occurs only at locations where the incident picosecond laser fluence lies between 0.2 and 0.26 J/cm². The reflectivity data appear to be in good quantitative agreement with a simple heating model, in which the electrons and phonons maintain a local thermodynamic equilibrium on a picosecond time scale.On leave from Philips Research Laboratories, D-2000 Hamburg 54, F. R. Germany     

Photoconducting tellurium for submillimeterwave detectors

Undoped tellurium single crystals show at low temperatures (2.2 K) strong photoeffect under submillimeter irradiation. Therefore, tellurium was used as a FIR-detector. For a comparison with other detectors signal-to-noise ratio, response time, and noise equivalent power were determined. Tests were carried out with HCN-laser radiation ( = 337 m, = 30 cm- t). The sensitivity of the Tc-photoconductor is comparable to that of a TGS-bolometer but lower than that of a Ge-bolometer. The Te-detector is much faster than the bolometers.     

Self-supporting thin-film beam splitter for far-infrared interferometers

Commercially available plastic films have been examined, in the spectral range up to 1200 cm⁻¹, for use as a beam splitter in far-infrared Michelson interferometers. The materials reported include polyethylene terephthalate, polycarbonate, polyvinylchloride, polypropylene, cellulose and polyethylene.     

Radiation sensitivity of thick core all-glass fibers in reactor radiation fields

Radiation induced losses and the recovery behavior of several optical fibers have been determined by in situ steady state and by pulse measurements in a nuclear reactor. Agreement with the literature was found for Suprasil and Fibropsil core fibers in ionizing radiation, but our results on Suprasil-W core fibers show an appreciably higher radiation hardness of this material than reported elsewhere. Additionally, increased long term losses were observed, when the mixed radiation flux contained a noticeable content of neutrons.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be −2×10⁻⁴ eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm⁻³ when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Crossed gratings: A theory and its applications

We introduce a rigorous formulation for the problem of the diffraction by a finitely-conducting, bi-periodic surface. The formationon is based on a coordinate transformation which maps the grating surface onto a plane. The transformed Maxwell equations are resolved using an iterative technique. The numerical implementation is tested against a number of criteria, including a comparison with two recently developed formalisms. As well, it is shown to give results in agreement with the Reciprocity Theorem, which is derived for efficiencies of crossed gratings in unpolarized light. The method has been shown to work throughout the whole range of values of surface conductivity. We investigate the effect of a surface-corrugation on the solar absorptance of a metallic mirror. We discuss a useful equivalence property linking the behaviour of crossed gratings with that of classical gratings. We exhibit for the first time a grating in a highly-reflecting metal capable of totally absorbing unpolarized incident light. On leave from the Department of Theoretical Physics, School of Physics, University of Sydney, Sydney, N.S.W., 2006, Australia     

Dynamics of birefringence properties in 17/30/70 PLZT ceramics

Birefringent properties of PLZT ceramics (Pb₈₃La₁₇)(Zr₃₀Ti₇₀)O₃ were investigated by measuring the transverse electrooptic effect in a 0.2 mm-thick wafer at various speeds of external voltage changes. At least two types of internal polarized microregions of different physical nature in charge of the electrooptical properties are observed.     

The influence of intrisic defects on the degradation and luminescence of GaAs and other III–V compounds

A significant contribution to the degradation of GaAs-based lasers and light-emitting diodes arises from the formation of so-called dark line defects. It is shown that these defects are accumulations of non-radiative recombination centres around dislocations. The centres are identified as As vacancies, which are emitted by climbing dislocations, concomitantly with the absorption of Ga interstitials. From scanning deep-level transient spectroscopy observations it is concluded that the so-called DX centres are Ga interstitials. The driving force for dislocation climb and thus for dark-line-defect formation is a supersaturation of Ga interstitials originating from the growth of the GaAs crystals under Ga-rich conditions as a consequence of the high volatility of As. Phenomena in other III–V compound semiconductors related to the formation of dark line defects in GaAs are also discussed.     

Pulsed laser deposition and characterization of multilayer metal-carbon thin films

Cobalt-DLC multilayer films were deposited with increasing content of cobalt, keeping carbon content constant by pulsed laser deposition technique. A cobalt free carbon film was also deposited for comparison. Excimer laser was employed to ablate the materials onto silicon substrate, kept at 250 °C, while post-deposition annealing at 400 °C was also performed in situ. The formation of cobalt grains within the carbon matrix in Co-DLC films can be seen through scanning electron and atomic force micrographs while no grains on the surface of the cobalt-free DLC film were observed. Raman spectra of all the films show D- and G-bands, which is a confirmation that the films are DLC in nature. According to Vibrating sample magnetometer (VSM) measurements, the DLC films with cobalt revealed ferromagnetic behaviour whereas the cobalt free DLC film exhibited diamagnetic behaviour. The pure DLC film also shows ferromagnetic nature when diamagnetic background is subtracted. Spectroscopic Ellipsometry (SE) analysis showed that the optical band gaps, refractive indices and extinction coefficients of Co-DLC films can be effectively tuned with increasing content of cobalt.     

Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.