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.     

Structural and dielectric properties of Bi modified PLZT ceramics

Polycrystalline samples of bismuth (Bi) doped lead lanthanum zirconate titanate (PLZT) with the general formula Pb_0.9(La_1−zBi_z)_0.1(Zr_0.65Ti_0.35)_0.975O₃, where [z=0.0, 0.3, 0.5, 0.7] near the morphotropic phase boundary (MPB) has been synthesized by a solid solution mixing technique. Some aspect of crystal structure of the compound at room temperature was studied using X-ray diffraction (XRD) technique. Detailed studies of dielectric constant (ε) and dielectric loss () obtained both as a function of frequency (100 Hz-100 kHz) at room temperature (RT) and temperature (RT-600 K) at 10 kHz suggest that compounds undergo ferroelectric-paraelectric phase transition of diffuse type. Both ac and dc conductivity have been studied over a wide range of temperature. The activation energy (E_a) of the samples was calculated from the plot of ac conductivity vs. inverse of absolute temperature. The temperature variation of resistivity shows that the compounds have negative temperature coefficient of resistance (NTCR).     

Second-harmonic generation in “Cubic” PLZT ceramics

The temperature-dependent optical second-harmonic generation in thermally depoled La-doped lead zirconate titanate (PLZT) ceramics of composition 8/65/35 and 9/65/35 has been studied. A microscopic model for the anomalous phase transition behaviour of these compounds is suggested.     

Ac behaviour and phase transition in PLZT 6/80/20 ferroelectric ceramics

The Ac behaviour of PLZT 6/80/20 ferroelectric ceramics was analyzed around and above the phase transition. Two relaxation processes are identified, showing that the so-called ‘universal relaxation law’ holds for the ceramics. A critical point in the values of the Ac conductivity, around the temperature corresponding to the maximum of the dielectric losses, is observed below the transition temperature due to the relaxor behaviour. The frequency dependence of the Ac conductivity at various temperatures and the hysteresis loops show classical relaxor behaviour with a diffuse phase transition.     

Bilateral switching of the modulated electrooptic contrast in PLZTN ceramics

Spatial distribution of ferroelectric polarization in Pb(Zr,Ti)O₃ ceramics doped with lanthanum and niobium has been investigated using scanning electrooptic confocal microscopy. Measurement of the modulated electrooptic signal in presence of a sinusoidal electric field reveals ferroelectric domains at microscopic scale not accessible to conventional polarized optical imaging. Both electrooptic images and local electrooptic loops have been observed after applying a bias electric field to the ceramic samples, being initially non-ferroelectric macroscopically. Bilateral reversal of the electrooptic contrast during the poling cycle has been detected and explained taking into account non-180° switching processes in ferroelectric grains of arbitrary orientation.     

Measurement of the birefringence properties of the reflecting surface of an interferential mirror

We have set up an apparatus to measure the ellipticity acquired by a linearly polarized light beam after a single quasi-normal reflection on an interferential mirror. Results of measurements performed on a standard 10 cm diameter interferential mirror are presented.     

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.     

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.     

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.     

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.     

Structural and dielectric properties of (La, Bi) modified PZT ceramics

Polycrystalline samples of Pb_0.9(La_1−zBi_z)_0.1(Zr_0.55Ti_0.45)_0.975O₃ [referred as PLBZT] (z=0.0, 0.3, 0.5 and 0.7) have been synthesized by a high-temperature solid-state reaction technique. X-ray diffraction analysis suggests the formation of single-phase compounds with monoclinic structure. The dielectric studies of the compounds as a function of temperature (room temperature, RT to 350 °C) at frequency (1, 10 and 100 kHz) show that the compounds undergo a phase transition of diffuse type. Diffusivity (γ) study of phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. Measurement of dc resistivity (ρ) as a function of temperature (RT to 350 °C) at a constant biasing field suggests the compounds have negative temperature coefficient of resistance.     

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.     

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.     

Diffuse phase transition of Fe doped lead ytterbium tantalate ceramics

The effect of different concentration of Fe on the phase transition behavior of Lead ytterbium tantalate is investigated by dielectric and differential scanning calrimetry measurements. The samples are prepared through solid state reaction method and it has been found that the sintering temperature significantly lowered when the proportion of Pb(Fe_1/2Ta_1/2)O₃ increased. It has been observed that the doping in small amounts (0≤x≤0.2) of Fe could meliorate the dielectric and ferroelectric properties. The diffuseness in the mode of phase transition increases and the phase transition temperature decreases as a function of Fe content. It is revealed that the dielectric data and heat capacity data follow a similar trend in the variation of the mode of phase transition and phase transition temperatures. The phase transition temperature values obtained from the heat capacity measurement well agreed with the values obtained from dielectric measurement.     

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     

Faraday rotation in a single-mode fiber with controlled birefringence

Faraday rotation within a polarization non-conserving single-mode optical fiber is used for ac magnetic field measurement. For that purpose the intrinsic fiber birefringence on the path to and from the Faraday rotation section is controlled. As control devices externally applied stress birefringence and phase retarder plates were used. With respect to magnetic field measurements on high electric potentials all control devices are placed at fiber input and output only.     

Structural and dielectric properties of phosphorous-doped PLZT ceramics

In the present work we have reported the unique effects of P₂O₅-doped PLZT ceramics with composition (Pb_0.92La_0.08)(Zr_0.65Ti_0.35)O₃ +x wt% of P₂O₅ (wherex = 1, 3 and 5) prepared chemically by co-precipitation method. X-ray diffraction studies suggest that the prepared compound was very fine (10–25 nm), homogeneous and of rhombohedral symmetry. The apparent density of samples decreased with the P⁵⁺ additions. Studies of dielectric constant and dielectric loss as a function of frequency (10–1000 kHz) and temperature suggest that the compound undergoes diffuse type of phase transition without any sign of relaxor behaviour. With increasingx, dielectric constant was found to decrease appreciably, whereas Curie temperature (T_C) was found to increase     

Influences of compressive stress and aging on dielectric properties of sodium bismuth titanate ceramics

The influences of compressive stress and aging on dielectric properties of undoped and Fe-doped sodium bismuth titanate (NBT) ceramics were investigated. The dielectric properties were decreased significantly with the compressive stress applied parallel to the electric field direction, while the changes were reversed with the stress applied perpendicularly. In addition, lower changes of the dielectric properties with stress were observed in Fe-NBT ceramics, likely caused by an enhanced relaxor characteristic with the acceptor doping, which also reduced the aging rate in the ceramics. Finally, the aging behavior of the NBT and Fe-NBT ceramics followed the slightly stretched exponential law, and the aging rate in both ceramics was found to decrease with frequency, a result of the pinning of the polarization components. These observations clearly confirmed the role of the acceptor dopant in enhancing the relaxor ferroelectric characteristics in the NBT-based ceramics.     

Properties of vanadium-doped SrBi4Ti4O15 ferroelectric ceramics

Using the standard solid-state reaction method, several vanadium-doped ferroelectric ceramics of type SrBi_4−x/3Ti_4−xV_xO₁₅ (SBTV-x) were synthesized. The vanadium doping content, x, ranges from 0.000 to 0.06. The crystal structure of SrBi₄Ti₄O₁₅ is not affected by V-doping. The electric breakdown voltage of the samples increases with V content. Meanwhile, V-doping results in a notable enlargement of remnant polarization (2P_r). The 2P_r of STBV-0.03 reaches a very large value, which is over 50 μC/cm² and is nearly twice greater than that at zero doping. The Curie temperatures of V-doped samples decrease slightly in comparison with that of SrBi₄Ti₄O₁₅. V-doping can improve the electric properties of SrBi₄Ti₄O₁₅ without sacrificing its thermal stableness.