Electric-field modulation measurements of complex electro-optic effects in poled polymer

Received: 10 July 1998/Revised version: 21 September 1998     

Amorphization and grain size effect on milled PbTiO3 studied by Raman scattering and visible photoluminescence emission

Raman scattering and photoluminescence (PL) studies on milled PbTiO₃ are presented in this paper. The results suggest that the visible PL emission could be related to both the localized states in the interface between the amorphous layer and the crystalline core and the amorphous layer itself. The Raman spectrum of PbTiO₃ milled for a long time showed the vibrational density of states, and a detailed analysis of the soft mode allowed us to conclude that the PbTiO₃ crystalline core did not experience any structural phasetransition. Received: 2 February 2001 / Accepted: 23 July 2001 / Published online: 17 October 2001     

Flexible ceramic–polymer composite films with temperature-insensitive and tunable dielectric permittivity

BaTiO₃–polymer composite layers have been produced by the spin-on technique (thickness 3–10 μm). The dielectric permittivity of the layers at room temperature can be tuned from 2.8 to approximately 33 by varying the ceramic filling from 0 to 60% by volume. The dielectric properties of the films are almost insensitive to temperature variations in the range 20–180 °C. Free-standing composite layers with ceramic content ≤50% are flexible without noticeable change of permittivity after repeated mechanical bending. Received: 22 November 2001 / Accepted: 24 November 2001 / Published online: 23 January 2002     

Preparation and characterization of poled nanocrystal and polymer composite PZT/PC films

Pb(Zr_0.52Ti_0.48)O₃ (PZT) nanocrystals and transparent polycarbonate (PC) composite thin films with useful properties for ferroelectric, piezoelectric and electro-optic devices were prepared by a spin-coating technique. Ultra-fine PZT (∼40–50 nm) nanocrystals with pure perovskite tetragonal phase were synthesized by a hydrothermal method. The structure and morphology of the composite thin films were studied by means of X-ray diffraction and scanning electron microscopy. To obtain the optimum electro-optic properties of poled composite films, the poling condition under an external electric field was optimized through the dielectric properties of PZT and PC polymer and effective field intensity theory. The electro-optic coefficient of the poled PZT/PC composite film is estimated to be 30.5 pm/V. The transparency spectra were measured and the optical band gaps of the unpoled, poled at 145 °C and poled at 165 °C composite thin films are estimated to be 4.26 eV, 4.21 eV and 4.18 eV, respectively. The measured dielectric constants of PZT/PC are in good agreement with the calculated values for the composite with a very small PZT volume fraction, based on the Onsager effective-field theory. This offers a reliable and indirect way to predict the dielectric constant of nanocrystals. PACS 81.40.Tv; 78.66.Sq; 78.66.Vs     

Electro-optic characterization of tetragonal (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystals by a modified Sénarmont setup

The refractive indices of tetragonal (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) single crystals were measured with a prism coupler and their linear electro-optic (EO) properties were investigated from 20 to 80 °C by the automated scanning Sénarmont system with an ac field. The composition and temperature effect on the EO coefficients were also discussed. It has been found that their EO coefficients are much larger than that of widely used LiNbO₃ single crystal and the calculated half-wave voltages are also much lower, which enable the operation at lower voltages and the smaller device dimensions. Since the excellent EO properties are very stable and such high quality single crystals with large-size have been obtained, the PMN-xPT single crystals are a very promising candidate for EO modulation applications. By linking to the polarization-related quadratic EO coefficients, we find that the linear EO properties are related with the spontaneous polarization and dielectric constants.     

Visible and infrared dispersion of the refractive indices in periodically poled and single domain Nd:Mg:LiNbO3 crystals

3 crystals are measured in the transparency region (with the accuracy ±0.0002) and for the upper phonon polariton branch (with the accuracy ±0.003–±0.05), from 0.44 μm up to 10.5 μm. The method of spontaneous parametric light scattering is used for measurement of the ordinary refractive index dispersion in the mid-infrared region and for determination of the domain grating period d=5.6±0.2 μm in the periodically-poled crystal. Received: 29 January 1997/Revised version: 10 July 1997     

Raman characterization of H:LiNbO3 waveguides

In this work we present polarized Raman measurements of z-cut multimode H:LiNbO₃ waveguides in the κ_i- and β_i-phase states. A spectrum with unexpected form and behaviour, originating from the layer at the very surface, was observed in samples with various degrees of Li–H substitution. Possible reasons for the presence of this strange spectrum are considered. Received: 16 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Germania/ormosil hybrid materials derived at low temperature for photonic applications

We report on germania/organically modified silane (ormosil) hybrid materials produced by the sol–gel technique for photonic applications. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide have been used as precursors for the hybrid materials. Planar waveguide films with a thickness of about 2 μm have been prepared by a single spin-coating process and low-temperature heat treatment from these high germanium content hybrid materials. Atomic force microscopy, thermal gravimetric analysis, UV–visible spectroscopy, and Fourier-transform infrared spectroscopy have been used to investigate the optical and structural properties of the films. The results have indicated that a dense, low absorption, and high transparency (in the visible range) waveguide film could be achieved at a low temperature. A strong UV-absorption region at short wavelengths ∼200 nm, accompanied by a shoulder peaked at ∼240 nm, has been noticed due to the neutral oxygen monovacancy defects. The propagation mode and loss properties of the planar waveguide films have also been investigated by using a prism-coupling technique. Received: 5 November 2002 / Revised version: 27 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +65-67909081, E-mail: ewxque@ntu.edu.sg     

Photoluminescence characteristics of neodymium oxide nanocrystal/titania/ormosil composite sol-gel thin films

Neodymium (III) oxide nanocrystal/titania/organically-modified silane (ormosil) composite thin films have been prepared using a chemical approach consisting of a combination of inverse microemulsion and sol-gel techniques at low temperature. Transmission electron microscopy shows that the neodymium (III) oxide nanoparticles have a needle-like nanocrystal structure. A strong room temperature emission at 1064 nm, corresponding to the ⁴ F _3/2?⁴ I _11/2 transition, has been observed as a function of the heat treatment temperature used for the production of the composite thin films. In addition to this emission, two other main emissions at 890 nm and at 1336 nm have also been observed. In particular, there was a clear shoulder peak at 1145 nm, probably be due to the host matrix, which was observed in all the measured samples and this shoulder peak gave a maximum intensity after heat treatment at 300 °C. Received: 6 September 2000 / Accepted: 15 November 2000 / Published online: 20 June 2001     

Reflectivity of cadmium sulphide nanocrystal films grown by the Langmuir–Blodgett technique

0 ) in the reflectivity spectra has been found to be blue-shifted with respect to that for bulk CdS. The appearance of the E₁(A) peak at about 5 eV is evidence for the hexagonal nature of the particles with the c axis normal or nearly normal to the plane of the film. This suggests the epitaxial nature of the grown particulate films. Another reflectivity peak has been observed at about 320 nm. However, its origin is not yet clear. Received: 26 May 1998/Accepted: 28 May 1998     

Recording grooves for fiber-chip coupling in PMMA using KrF excimer laser radiation: experiment and simulation

The fabrication of deep grooves with a high aspect ratio in poly-methylmethacrylate using excimer laser radiation (248 nm) and a cylindric lens has been investigated. The form of the grooves and their dimensions are evaluated with respect to the recording parameters. The patterns and their widths and depths may be adapted in a wide range depending on the particular application. As an example grooves for coupling monomode optical silica fibers were fabricated. Simulations were performed in order to explain the pattern formation. In a simple model the thermo-optic etching process may be simulated using a Gaussian-shaped laser beam and the FDBPM approach. Received: 7 December 1998 / Revised version: 12 January 1999 / Published online: 12 April 1999     

Structural and optical characterization of Er3+/Yb3+-doped LiNbO3

We report the dependence of the unit-cell parameters and the extraordinary and ordinary refractive indices of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals. Both properties depend in a non-monotonic manner on the Er³⁺/Yb³⁺ content. A singularity was observed at concentrations of 1.1-1.2 mol. % in the crystal (0.6-0.7 mol. % in the melt). In the same way the Er and Yb concentration influences the periodically poled lithium niobate formation. The observed behavior of refractive indices and unit-cell parameters of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals could be explained in terms of the RE³⁺-ion concentration affecting the Li-vacancy concentration and the RE³⁺-ion positions in the crystal. Received: 21 May 2001 / Revised version: 22 August 2001 / Published online: 23 October 2001     

Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements

40 S₄₀Se₂₀, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc’s procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As₄₀S₄₀Se₂₀ thin films were also studied, using both transmission and reflection spectra. Received: 11 February 1998/Accepted: 16 February 1998     

Photoinduced transformations and holographic recording in nanolayered a-Se/As2S3 and AsSe/As2S3 films

Received: 13 October 1998 / Accepted: 9 December 1998 / Published online: 24 February 1999     

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) by spectroscopic ellipsometry

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq₃) have been studied in the spectral range from 1.55 eV to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq₃ deposited on unheated substrates and on substrates kept at 100 °C have been determined. The optical functions have been modeled using point-to-point fitting, with the conventional oscillator model and modified oscillator model. It has been found that point-to-point fitting gives the best agreement with the experimental data, and that the modified oscillator model yields better agreement with the experimental data than the conventional oscillator model. Received: 3 September 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +852-2559/8738, E-mail: dalek@eee.hku.hk     

Modeling and recording characteristics of a dye-doped polymer bilayer film

The optical, thermal, and structural behavior of a laser-irradiated dye-in-polymer bilayer film consisting of an elastic and a plastic layer was investigated by a computer modeling and recording experiment. The results of numerical calculations describe absorption of optical energy in the recording layers, the heat flow, and the resultant bump formation throughout the multilayer. Recording performance of the material is strongly affected by the dye concentration and the thickness of the plastic layer. The calculated and measured properties of the media are compared, and the relationship between the dye concentration, thickness of plastic layer, and thermal, structural, and recording performance are described. Received: 9 March 1999 / Revised version: 10 May 1999 / Published online: 29 July 1999     

Optical characterization of thin thermal oxide films on copper by ellipsometry

A complete optical characterization in the visible region of thin copper oxide films has been performed by ellipsometry. Copper oxide films of various thicknesses were grown on thick copper films by low temperature thermal oxidation at 125 °C in air for different time intervals. The thickness and optical constants of the copper oxide films were determined in the visible region by ellipsometric measurements. It was found that a linear time law is valid for the oxide growth in air at 125 °C. The spectral behaviour of the optical constants and the value of the band gap in the oxide films determined by ellipsometry in this study are in agreement with the behaviour of those of Cu₂O, which have been obtained elsewhere through reflectance and transmittance methods. The band gap of copper oxide, determined from the spectral behaviour of the absorption coefficient was about 2 eV, which is the generally accepted value for Cu₂O. It was therefore concluded that the oxide composition of the surface film grown on copper is in the form of Cu₂O (cuprous oxide). It was also shown that the reflectance spectra of the copper oxide–copper structures exhibit behaviour expected from a single layer antireflection coating of Cu₂O on Cu. Received: 19 July 2001 / Accepted: 27 July 2001 / Published online: 17 October 2001     

Growth and characterization of oriented cadmium sulphide nanocrystals under Langmuir-Blodgett monolayer of arachidic acid

Cadmium sulphide nanocrystals were grown at room temperature (20 °C) under arachidic acid monolayers floating over an aqueous solution of CdCl₂ inside an enclosed Langmuir-Blodgett set-up, through slow infusion of H₂S gas. X-ray diffraction spectra suggest an oriented growth of the crystallites. The particle sizes were found to increase with duration of exposure to the H₂S gas. Atomic force microscopy indicated that the particles were nearly circular pellets with uniform morphology throughout. In Raman spectra, the FWHM of the LO phonon was found to be large (≈20 cm^-1) for all the films grown with different exposure times in H₂S gas, and was found to reduce to 8 cm^-1 after annealing a typical sample at 500 °C for 45 min. Received: 30 September 1998 / Accepted: 29 March 1999 / Published online: 11 August 1999     

Material tensor parameters of LiNbO3 relevant for electro- and elasto-optics

The complete set of self-consistent parameters of nominally undoped LiNbO₃ crystals of congruent composition that describe the electro-optic, piezoelectric, elasto-optic, elastic, and dielectric response has been determined by numerically evaluating available measurements. The parameters were determined at room temperature and consist of the low-frequency clamped dielectric constants ε^S _ij, elastic stiffness constants at constant electric field C^E _ijkl, piezoelectric stress coefficients e_ijk, elasto-optic constants at constant electric field p^E _ijkl, and clamped electro-optic coefficients r^S _ijk. It is shown that the complete set is required for calculating the effective electro-optic coefficients and dielectric constants in photorefractive applications of LiNbO₃. Received: 4 January 2002 / Revised version: 1 February 2002 / Published online: 14 March 2002     

Reversible and athermal photo-vitrification of As50Se50 thin films deposited onto silicon wafer and glass substrates

Photo-vitrification of As₅₀Se₅₀ thin films deposited onto silicon wafer and glass substrates has been studied using X-ray diffraction, far-infrared, and differential infrared spectroscopies. The optical study of this photo-amorphization effect has been carried out by two different methods enabling the determination of the average thickness and refractive index of a wedge-shaped thin film. The refractive-index behaviour of the as-evaporated, crystallized, and photo-vitrified As₅₀Se₅₀ films is analyzed within the single-oscillator approach. The optical-absorption edge is described using the non-direct transition model, and the optical energy gap is calculated. In the course of the vitrification of an As₅₀Se₅₀ thin film deposited on a silicon substrate the photo-oxidation of the film has been additionally detected and arsenic trioxide micro-crystals were formed on the surface of the film. Such oxidation has not been observed with As₅₀Se₅₀ films deposited on glass substrates, which demonstrates that the photo-vitrification phenomenon depends also on the type of substrate. Finally, it is concluded from the optical study that a reversible photo-darkening effect accompanies the photo-induced vitrification phenomenon. Received: 3 August 1998 / Accepted: 13 January 1999 / Published online: 7 April 1999