Second-harmonic reflectors on 128 degrees LiNbO3

In this work, we study theoretically the operation of long surface acoustic wave reflectors, comprising a large number of electrodes, at the fundamental and second harmonic frequencies on the 128/spl deg/ LiNbO/sub 3/ substrate for various electrode thicknesses and metallization ratios. Numerical simulations utilizing tailored test structures and time gating indicate that the reflectivity of the second-harmonic reflectors can be very high for certain geometries. Furthermore, our simulations suggest that inside the stopband the total losses for the second harmonic are of the same order as those for operation at the fundamental harmonic.     

Phases of the SAW reflection and transmission coefficients for short reflectors on 128 degree LiNbO3

We study numerically the phase of surface acoustic waves reflected by or transmitted through short reflectors comprising only 1-3 aluminium electrodes on 128 degree YX-cut lithium niobate (LiNbO3). The electrodes have a finite thickness, and they are either open-circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted surface acoustic wave (SAW). Using artificial enhancement of time resolution in conjunction with the fast Fourier transform (FFT) and time-gating, calculation of the Y-parameters in a relatively wide frequency range allows us to determine the phase of the reflection and transmission coefficients.     

Short reflectors operating at the fundamental and second harmonics on 128 degree LiNbO3

In this work, we study numerically the operation of surface acoustic wave (SAW) reflectors comprising a small number of electrodes on the 128 degree YX-cut lithium niobate (LiNbO3) substrate. The electrodes have a finite thickness, and they are either open circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted SAWs. Using the fast Fourier transform (FFT) and time-gating techniques, calculation of the Y-parameters in a wide frequency range with rather a small step allows us to determine the reflection coefficients, and to estimate the energy loss due to bulk-wave scattering. The detailed dependences of the attenuation and reflectivity on the metallization ratio and the electrode thickness are given for the classic 128 degree-cut of LiNbO3.     

Narrowband Bragg reflectors in Ti:LiNbO3 optical waveguides

Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides to obtain a narrow (0.05 nm) reflectance spectrum with a > 20 dB dip in the transmittance spectrum. These results were realized at a wavelength of 1542.7 nm for TE polarization on an x-cut, y-propagating substrate with gratings etched to a depth of approximately 93 nm in a 105 nm thick silicon film over a length of 12.5 mm. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated by using a model for contradirectional coupling that includes an attenuation coefficient. The values for coupling constants kappa and amplitude attenuation constants alpha of samples etched for different time durations to control the grating depths are obtained from the model through the use of the depth of the dips in the transmittance spectra and the spectral widths of the reflectance peaks. It is concluded that the corrugated Si overlay film increases the insertion loss by approximately 2.7 dB, and the loss is not significantly affected by the grating depth.     

Y128°LiNbO3上的单指无内反射叉指换能器和滤波器的实验研究

1、引言 叉指换能器的指间反射是影响声表面波器件性能的主要二阶效应之一.分裂指结构使反射问题得到较好的解决.但分裂指结构在相同的工艺下所能达到的最高工作频率比单指结构降低了一倍,因此不少学者试图寻求一种用单指结构抑制反射的方法[1,2,3].1985年陈东陪和Haus[4]运用变分原理与藕合模理论相结合,统一分析了金属栅阵各种反射机制对反射信号的贡献.我们进一步将这一理论推广到各向异性晶体的实际情况,具体分析了各种栅阵的反射特性,在此基础上提出了各种可能的无内反射叉指换能器结构.     

LiTaO3 and LiNbO3 Epitaxial Films

The preparation of film-forming peroxide solutions of LiTaO₃ and LiNbO₃ precursors is described. The solutions are used to produce LiTaO₃ films on single-crystal corundum and LiNbO₃ substrates by the spinning process and dip coating, followed by heat treatment, and LiNbO₃ films on single-crystal corundum substrates by the spinning process.     

LiNbO3薄膜的研究进展

LiNbO3薄膜具有优良的压电、热释电、声光及非线性光学特性，在光电子学及集成光学等领域有许多重要应用，着重介绍LiNbO3的结构，性质与薄膜制备技术，以及在光通信领域中的应用。     

Electron-Beam Poling on Ti:LiNbO(3)

The periodic domain inversion by direct electron-beam (EB) bombardment on Ti:LiNbO(3) is presented. Gratings with a 6.6-mum period are achieved. The inverted patterns are observed after chemical etching by use of a scanning electron microscope, and they exhibit a high resolution, as expected. Next, the influence of the EB parameters on the inversion phenomenon is developed for both LiNbO(3) and Ti:LiNbO(3). In this way we can provide an explanation of the phenomenon of domain inversion with an EB, which is not completely understood. Finally, quasi-phase-matched second-harmonic generation is presented in bulk LiNbO(3) by use of a Nd:YAG laser light. These experiments allowed us to achieve the characteristics of the inverted domains along the crystal in particular.     

Coupling characteristics of LiNbO3 directional couplers

An analytic model of a Ti:LiNbO3 channel waveguide has been developed to predict conveniently the coupling coefficient of directional couplers based on near-field intensity measurements. This model is compared to experimental results at wavelengths of 0.63 and 0.81 microm and indicates an accuracy of predicting coupling coefficients within a factor of 2 without prior knowledge of the fabrication parameters.     

Local polarization reversal in LiNbO3 crystals

Local polarization reversal has been investigated in a thin surface layer of a ferroelectric and preliminary results are presented for lithium niobate. The polarization reversal is accomplished by using a moving pointed electrode. Pis’ma Zh. Tekh. Fiz. 23, 54–58 (June 12, 1997)     

Photorefractive effect in LiNbO3 directional couplers

Experimental results on the photorefractive effect in Ti-indiffused LiNbO3 directional couplers at GaAs and He-Ne wavelengths are presented. The photorefractive effect has been observed as drift in the switching state of this device at GaAs wavelengths (0.81 and 0.85 Am) for intensities (powers) as low as 32 W/cm 2 (3.2 MW). Analysis of this effect indicates refractive-index changes as large as 8 X 10-4 may be occurring. Measurements also suggest the possibility of two time constants in the relaxation of the photorefractive effect in these devices.     

Vibrational properties of the LiNbO3 z-surfaces

The existence of localized vibrational modes both at the positive and at the negative LiNbO3 (0001) surface is demonstrated by means of first-principles calculations and Raman spectroscopy measurements. First, the phonon modes of the crystal bulk and of the (0001) surface are calculated within the density functional theory. In a second step, the Raman spectra of LiNbO(3) bulk and of the two surfaces are measured. The phonon modes localized at the two surfaces are found to be substantially different, and are also found to differ from the bulk modes. The calculated and measured frequencies are in agreement within the error of the method. Raman spectroscopy is shown to be sensitive to differences between bulk and surface and between positive and negative surface. It represents therefore an alternative method to determine the surface polarity, which does not exploit the pyroelectric or piezoelectric properties of the material.     

Photocatalytic hydrogen generation with Ag-loaded LiNbO3

In this contribiution LiNbO₃ and Ag-loaded LiNbO₃ photocatalysts were tested in the reaction of hydrogen evolution. The silver modified samples contained different loading of co-catalyst in the range of 0.5–4 wt%. It was essential to optimize the sample composition to achieve an efficient hydrogen evolution. The optimal sample contained 2 wt% of silver. The detailed analysis indicated that silver was deposited on the surface of LiNbO₃ in the form of Ag₂O. Therefore, it is supposed that Ag₂O was responsible for the enhanced photocatalytic activity in the studied reaction. The crystallographic phases and optical and vibronic properties were examined by X-ray diffraction (XRD) and diffuse reflectance (DR) UV-Vis and resonance Raman spectroscopic methods, respectively. Morphology of the produced samples were studied using a high-resolution transmission electron microscope (HRTEM).     

Surface acoustic waves on X-cut LiNbO/sub 3/

Surface acoustic wave (SAW) parameters on X-cut LiNbO₃ were calculated and measured. For the XY20deg cut, a free-surface SAW velocity, a square of the electromechanical coefficient and a temperature coefficient of frequency are equal to 3727 m/s, 1.6% and -65 ppm/K, respectively. For the XY120deg cut, the above parameters are equal to 3403 m/s, 4.1% and -75.7 ppm/K, respectively. These parameters are compared with that of YZ and 128degYX cuts     

Extraordinary-mode refractive-index change produced by the linear electro-optic effect in LiNbO(3) and reverse-poled LiNbO(3)

To examine aspects of an integrated photonic electric-field sensor, we calculate electro-optically induced refractive-index change in regular and reverse-poled LiNbO(3). Specifically, for y-propagating extraordinary modes, we determine how index change depends on electric-field magnitude and direction. To accomplish this, changes in index-ellipsoid shape and orientation are found by the use of a numerical eigenvalue procedure to diagonalize the impermeability tensor; then, refractive index is calculated by the use of a vector reference-frame transformation and a small perturbation approximation. A general formula is inferred from calculations for specific field directions. Electro-optic coefficients for reversepoled LiNbO(3) are obtained by application of a tensor reference-frame transformation to those of LiNbO(3). The index-calculation procedure has utility beyond the problem that is considered.     

XPS-investigation of proton exchanged waveguides in LiNbO3

An X-ray photoelectron spectroscopy (XPS) study of X-cut LiNbO₃ proton exchanged in pure and lithium benzoate buffered benzoic acid melt has been performed. Binding energies and full width at half maximum for the Li1s, O1s and Nb3d peaks in the depth range 0–10 nm were investigated by using Ar ion sputtering. The largest changes with increasing depth are established for the surface atomic layers between 0 and 1 nm. Li-enrichment of the surface is observed when both buffered and pure benzoic acid melts are used.     

LiNbO3--a new material for artificial photosynthesis

The solid-gas phase photo-assisted reduction of carbon dioxide (artificial photosynthesis) was performed using ferroelectric lithium niobate and titanium dioxide. Illumination with a high-pressure mercury lamp and visible sunlight showed that lithium niobate achieved unexpectedly high conversion of CO(2) to products despite the low levels of band-gap light available. The high reaction efficiency of lithium niobate is explained by its strong remnant polarization (70 μC/cm(2)), allowing a longer lifetime of photo-induced carriers as well as an alternative reaction pathway.     

Polarization dependence in Ti-diffused LiNbO3 directional couplers

The effect of design and fabrication parameters on the dependence on polarization of the splitting ratio in directional couplers produced by Ti diffusion in LiNbO3 has been investigated experimentally at a wavelength of 1.54 microm. The parametric study was carried out at a diffusion temperature of 1025 degrees C, which was found to suppress outdiffusion effects. The directional couplers were produced by use of various combinations of waveguide separation, Ti film thickness, and diffusion time. Of particular interest was the identification of parameter sets for which the sum of TE and TM splitting ratios equals unity, as required for electro-optic and acousto-optic tunable filter designs with relaxed beam-splitter requirements. Directional couplers that closely match this criterion were obtained by diffusion of 3.5-mm-long, 7-microm-wide Ti strips separated by 11 microm for 11 h. It was found that a bending angle of 0.6 degrees for input and output waveguides produces lower transmission loss for both polarizations than a 1.0 degree bend angle (>1-dB loss reduction).