Structural evolution and dopant occupancy preference of yttrium-doped potassium sodium niobate thin films

Sodium potassium niobate (KNN) is the most promising candidate for lead-free piezoelectric material, owing to its high Curie temperature and piezoelectric coefficients among the non-lead piezoelectric. Numerous studies have been carried out to enhance piezoelectric properties of KNN through composition design. This research studied the effects of yttrium concentrations and lattice site occupancy preference in KNN films. For this research, the yttrium-doped KNN thin films (mol% = 0, 0.1, 0.3, 0.5, 0.7 and 0.9) were fabricated using the sol-gel spin coating technique and had revealed the orthorhombic perovskite structures. Based on the replacement of Y³⁺ ions for K⁺/ Na⁺ ions, it was found that the films doped with 0.1 to 0.5 mol% of yttrium had less lattice strain, while films with more than 0.5 mol% of Y³⁺ ions had increased strain due to the tendency of Y³⁺ to occupy the B-site in the perovskite lattice. Furthermore, by analysing the vibrational attributes of octahedron bonding, the dopant occupancy at A-site and B-site lattices could be identified. O-Nb-O bonding was asymmetric and became distorted due to the B-site occupancy of yttrium dopants at high dopant concentrations of >0.5 mol%. Extra conduction electrons had resulted in better resistivity of 2.153× 10⁶ Ω at 0.5 mol%, while higher resistivity was recorded for films prepared with higher concentration of more than 0.5 mol%. The introduction of Y³⁺ improved the grain distribution of KNN structure. Further investigations indicated that yttrium enhances the surface smoothness of the films. However, at high concentrations (0.9 mol%), the yttrium increases the roughness of the surface. Within the studied range of Y³⁺ _, the film with 0.5 mol% Y³⁺ represented a relatively desirable improvement in dielectric loss, tan δ and quality factor, Q_m.     

Body fitted grid generation with moving boundary and its application for dielectric waveguides

The stress birefringence and mode coupling effects in polarization-preserving fibres are most important problems which need to be improved. For a realization of some optical devices, the dielectric waveguide with sinusoidally varying circular cross-section has been investigated. It becomes very important to analyse the electromagnetic field distribution in a dielectric waveguide with a time-dependent moving boundary. This paper shows that numerical methods can simulate the effect of the external disturbance on the dielectric waveguide from time to time. The author has discussed body fitted grid generation with moving boundary for the Poisson and Laplace equations.^1,2 We have extended this technique for Maxwell's equation. The technique employs a kind of an expanded numerical grid generation. As the author adds the time component to grid generation, the time dependent co-ordinate system which coincides with a contour of moving boundary can be transformed into a fixed rectangular co-ordinate system. We could show the electric distribution in the waveguide time by time to verify the possibility of an application for an optical fibre. This technique makes it possible not only to analyse the effect of the external disturbance in a coherent optical communication system but also to fabricate optical devices.     

Second-harmonic distortion in vertical-cavity surface-emittinglasers with lateral loss effects

The influence of lateral loss effects on the second-harmonic distortion (SHD) of vertical-cavity surface-emitting lasers is investigated theoretically. Two parameters, differential confinement factor and differential cavity loss, are defined for the consideration of self-focusing and diffraction loss. It is found that SHD increases at low modulation frequencies due to the presence of differential cavity loss, especially for lasers with small core radius     

Electrical switching in lithium niobate thin films

The ferroelectric switching properties of thin films of lithium niobate (LiNbO3) on silicon have been investigated. The polarization is shown to be partially reversible with an applied electric field at room temperature. The samples were films of LiNbO3 which were magnetron sputter deposited on silicon substrates. A double pulse method was used to investigate the ferroelectric switching properties of the films. Evidence for partial stable switching of the LiNbO3 film was observed at a field of roughly 500 kV/cm.     

Ferroelectric lead iron-chromium-nickel niobate films

Abstract

The fabrication of ferroelectric films of modified lead iron niobate by a multiple magnetron sputtering technique with a subsequent rapid thermal annealing at 800°C for 5 seconds is reported. Since the magnetic properties of pure iron preclude its use in magnetron sputtering, a non-magnetic stainless steel was used as one of the target materials resulting in a ferroelectric of composition Pb[(Fe_0.7Cr_0.2Ni_0.1)_0.5Nb_0.5]O₃. The reaction sequence involved in the formation of the ferroelectric perovskite phase has been identified. The films exhibit unsaturated ferroelectric hysteresis loops with a remanent polarization of 15 μC/cm² and a coercive field of 100 kV/cm. The room temperature dielectric constant and dielectric loss at 1 kHz were 640 and 0.1, respectively. The dielectric constant showed a dielectric anomaly as a function of temperature in the form of a broad maximum around 90°C confirming the ferro-para electric phase transition. The films were highly insulating with a room temperature conductivity of ≈1 X^?12 Ω^?1 cm^?1, and an activation energy of 0.8 eV.     

Theory of design parameters for quasi-phase-matched waveguides andapplication to frequency doubling in polymer waveguides

A theory of dispersion in single-mode symmetric waveguides is presented for phase-matching second harmonic generation (SHG) of the fundamental modes, based on the approximate analytical waveguide theory of Botez. The theory is used to derive new equations for the maximum phase-matching distance allowed when there are random fluctuations in waveguide thicknesses, under critical and noncritical phase-matching conditions. The theory is also used to calculate the overlap integral as a function of the waveguide parameters V^ω and V^2ω. A new expression is derived for the efficiency of SHG in waveguides in terms of waveguide parameters that can be used to optimize SHG. Theoretical results are presented for typical LiNbO₃ and polymer waveguides. Quasi-phase-matched (QPM) waveguides are fabricated from nonlinear optical (NLO) polymers using the techniques of periodic poling and bleaching, and channel waveguides are printed by the bleaching of the NLO polymers. The NLO polymers are characterized for their refractive indexes, optical loss, NLO coefficients, and bleaching characteristics. Phase-matched SHG results are presented for the different fabrication methods over a distance of 0.5 cm, and an assessment is given of the relative strengths and weaknesses of the different fabrication approaches     

Coupled resonator optical waveguides

Properties of the recently introduced family of coupled resonator optical waveguides (CROWs) are reviewed, particularly with reference to CROWs designed as planar waveguides in two-dimensional photonic crystal slabs to enhance nonlinear interactions and develop novel all-optical information processing devices. Topics covered include: pulse propagation both in the nondispersive approximation and to all orders of dispersion, and the coupled mode theory of nonlinear optics with pulses in CROWs and its applications to second-harmonic generation and wave coupling via field-induced refractive-index gratings. We also review recent experimental progress in the fabrication and characterization of CROWs, and applications of the CROW concept to fiber gratings and microwave waveguides     

Electrooptic coefficient variation in proton exchanged and annealedlithium niobate samples

A new technique to measure relative changes in the electrooptic (EO) coefficient of a material is presented. It is used to measure the spatial variation of the EO coefficient in x-cut lithium niobate caused by the annealed proton exchange technique. The lithium niobate samples are proton exchanged at different temperatures for varying times. The measurements presented indicate significant degradation of the EO coefficient immediately after the proton exchange process. The samples are annealed at 375°C to enable recovery of the EO coefficient. Samples processed in benzoic acid for 30 min at a temperature of 240°C show immediate recovery of the EO coefficient upon thermal annealing. Samples processed in benzoic acid for 5 h at a temperature of 180°C show a slower recovery of the EO coefficients requiring longer periods of thermal annealing even though the diffusion depth is the same as in the previous case. Samples processed in benzoic acid for 5 h at a temperature of 240°C exhibit significant cracking which worsens progressively upon thermal annealing. The EO coefficient cannot be restored in all areas even after a long 8-h anneal step     

Thin film lithium niobate for use in silicon based devices

The incorporation of a thin film of lithium niobate (LiNbO3) in a conventional MOS (metal-oxide-semiconductor) structure gives the possibility of two fundamentally different types of computer memory architectures. One, based on ferroelectric switching involves the reorganization of charge in the transistor channel to compensate for the change in polarization. Another, based on the bulk photovoltaic effect, involves a shift in the transistor threshold with exposure to differing intensities of incident light. With the use of a molybdenum liftoff process, transistors have been fabricated in which LiNbO3 replaces the usual gate oxide of an MOS transistor. Transistor parameters such as the transconductance, output conductance, and amplification for these devices are reported.     

Characteristics of lithium niobate based capacitors and transistors

Abstract

The electrical properties of thin film (<1000 Å) capacitor devices of lithium niobate grown on silicon and platinum and of thicker film metal-ferroelectric-semiconductor field effect transistors (MFSFET) with lithium niobate as the gate material were measured. Dielectric constants of the thin films on silicon were as high as 27, while those for films on platinum were as high as 49. The MFSFET structures showed good FET properties, and demonstrated a channel current modulation consistent with switching of the ferroelectric gate by pulsing.     

Dielectric and pyroelectric anisotropy in the melt-quenched barium bismuth niobate ceramics

BaBi₂Nb₂O₉ textured ceramics were fabricated via melt-quenching followed by high temperature (800–1000 °C) sintering process. The resulting ceramics possessed a {00l} texture with the a–b plane of the grains oriented parallel to the major face of the quenched plate. The influence of the sintering temperature on the orientation factor (f) and microstructure was evaluated via X-ray powder diffraction (XRD) and Scanning electron microscopy (SEM). The orientation factor was found to increase with increase in the sintering temperature and reached a maximum value of 0.47 for the samples sintered at 1000 °C for 10 h. Relative density and the grain-size of the ceramics were found to increase with increase in the sintering temperatures. The effect of texture on the dielectric and pyroelectric properties was evaluated. The measurements performed along the direction perpendicular to the pressing axis exhibited superior values than that of the direction parallel to the pressing axis. The observed anisotropy in the physical properties was attributed to the increased contribution from the highly polarizable a–b planes which are oriented in the direction perpendicular the pressing axis.     

Light localizations in photonic crystal line defect waveguides

In this paper, we discuss unique light localizations in photonic crystal line defect waveguides based on two different concepts. The first concept is an additional defect doping that breaks the symmetry of the line defect. Even though such a defect is open to the line defect, the optical field is well confined around the defect at cutoff frequencies of the line defect. This expands the design flexibility of microcavities and allows effective mode controls such as the single-mode operation. The lasing action of such cavities in a GaInAsP photonic crystal slab was experimentally observed by photopumping at room temperature. The second concept is a chirping of the waveguide structure. The photonic band of a waveguide mode has a band edge, at which the group velocity becomes zero. The band-edge condition shifts in a chirped line defect waveguide, so guided light reaches a zero group velocity point and is localized. A macroscopic behavior of this phenomenon was experimentally observed in a waveguide fabricated into a silicon-on-insulator substrate. In addition, a microscopic behavior was theoretically investigated, which suggested its applicability to a group delay device.     

The role of temperature in quantum-cascade laser waveguides

The role of temperature on the properties of quantum-cascade laser (QCL) waveguides is investigated. One-dimensional waveguide parameters are obtained using a transfer-matrix technique and the complex dielectric constants of the waveguide layers are calculated using a semi-classical Drude–Lorentz model. To model the effect of temperature on the waveguide parameters, a temperature dependent electron mobility is incorporated within the Drude–Lorentz framework. It is shown that by including the effect of temperature, a significant improvement in the agreement with experiment of the waveguide loss and hence the laser threshold current density can be achieved.     

Optical intensity induced shutter in photochromic-doped sol-gelgel-glass waveguides

The dynamic response, modeling and performance of a new optical intensity induced shutter have been studied. Hybrid optical waveguides made of optical fibers and photochromic doped gel-glass have been prepared by sol-gel process. A novel theoretical model explaining their behavior is shown, Experimental validation of the model is also shown. Design parameters of the devices, optical tunability and their application as extrinsic fiber optic intensity induced shutter (EFIS) are theoretically and experimentally demonstrated     

Self-phase modulation-based integrated optical regeneration in chalcogenide waveguides

We demonstrate integrated all-optical 2R regenerators based on Kerr optical nonlinearities (subpicosecond response) in chalcogenide glass waveguides with integrated Bragg grating filters. By combining a low loss As/sub 2/S/sub 3/ rib waveguide with an in-waveguide photo-written Bragg grating filter, we realize an integrated all-optical 2R signal regenerator with the potential to process bit rates in excess of 1 Tb/s. The device operates using a combination of self phase modulation induced spectral broadening followed by a linear filter offset from the input center wavelength. A nonlinear power transfer curve is demonstrated using 1.4 ps pulses, sufficient for suppressing noise in an amplified transmission link. We investigate the role of dispersion on the device transfer characteristics, and discuss future avenues to realizing a device capable of operation at subwatt peak power levels.     

A review of lithium niobate modulators for fiber-opticcommunications systems

The current status of lithium-niobate external-modulator technology is reviewed with emphasis on design, fabrication, system requirements, performance, and reliability. The technology meets the performance and reliability requirements of current 2.5-, 10-, and 40-Gb/s digital communication systems, as well as CATV analog systems. The current trend in device topology is toward higher data rates and increased levels of integration. In particular, multiple high-speed modulation functions, such as 10-Gb/s return-to-zero pulse generation plus data modulation, have been achieved in a single device     

Planar photonic crystal coupled cavity waveguides

We present absolute transmission measurements of coupled cavity waveguides defined within planar photonic crystals. We investigate a range of cavity types and also vary the spacing between cavities. Modal analysis of the individual cavities reveals the symmetries that determine the coupling between adjacent cavities. Enhanced transmission is demonstrated by modifying the photonic crystal lattice. We highlight the need for correct impedance matching at the waveguide input in order to improve the transmission.     

GaInAsP-InP distributed feedback waveguides for all-opticalswitching

All-optical switching characteristics of waveguide devices having a nonlinear distributed feedback structure are reported theoretically and experimentally. These devices were composed of a strip-loaded GaInAsP-InP waveguide and a Bragg reflector, which were suitable for optical integrated circuit. In the experiments, all-optical and gate operation and set-reset operation for two output ports were successfully demonstrated     

Slow light bimodal interferometry in onedimensional photonic crystal waveguides

Strongly influenced by the advances in the semiconductor industry,the miniaturization and integration of optical circuits into smaller devices has stimulated considerable research efforts in recent decades.Among other structures,integrated interferometers play a prominent role in the development of photonic devices for on-chip applications ranging from optical communication networks to point-of-care analysis instruments.However,it has been a longstanding challenge to design extremely short interferometer schemes,as long interaction lengths are typically required for a complete modulation transition.Several approaches,including novel materials or sophisticated configurations,have been proposed to overcome some of these size limitations but at the expense of increasing fabrication complexity and cost.Here,we demonstrate for the first time slow light bimodal interferometric behaviour in an integrated single-channel one-dimensional photonic crystal.The proposed structure supports two electromagnetic modes of the same polarization that exhibit a large group velocity difference.Specifically,an over 20-fold reduction in the higher-order-mode group velocity is experimentally shown on a straightforward all-dielectric bimodal structure,leading to a remarkable optical path reduction compared to other conventional interferometers.Moreover,we experimentally demonstrate the significant performance improvement provided by the proposed bimodal photonic crystal interferometer in the creation of an ultra-compact optical modulator and a highly sensitive photonic sensor.     

Temperature dependent Raman scattering study of order-disorder lead zinc niobate

Abstract

Raman spectra have been measured in lead zinc niobate (PZN)- lead titanate (PT) solid solution single crystals (facets undefined) as functions of temperature (70K–580K) and composition (x ' 0.02, 0.085, and 0.11) into its parallel and perpendicular configurations. The presence of complex multiband Raman spectra is inconsistent with Pm3m space group symmetry and is an indication of short-range ordering on the B-site. Raman bandwidths are substantially large and nearly independent of temperature indicating a static structural disorder rather than a dynamic dipolar disorder. No soft mode like behavior is observed in the measured frequency range. Abrupt change in wavenumber (frequency) and width of certain bending mode (270 cm_?1) at 410 K (140°C) is interpreted as an evidence for a reversible structural phase transition (C-R) at this temperature. The sequence of structural phase transition of C-T-R was also observed in one of the compositions (x ' 0.085), though diffuse and less obvious. In addition, the spectroscopic properties are found to be dependent on the substitution of titanium ion reflecting in the temperature of phase transition.