Half-thickness inversion layer high-frequency ultrasonic transducers using LiNbO/sub 3/ single crystal

Half-thickness inversion layer high-frequency ultrasonic transducers were fabricated using lithium niobate (LiNbO₃) single crystal plate. The transducers developed for this study used a 36deg rotated Y-cut LiNbO₃ thin plate with an active element thickness of 115 mum. The designed center frequency was in the range of 30 to 60 MHz. Half-thickness inversion layer was formed after the sample was annealed at a high temperature, and it is shown that the inversion layer thickness can be controlled by the temperature. Silver powder/epoxy composite and parylene were used as acoustic matching layers. A lossy silver epoxy was used as the backing material. Using an analytical method, the electrical impedance for different inversion layer ratios was determined. The measured resonant frequency was consistent with the modeled data. Even-order higher frequency broadband ultrasonic transducers with a center frequency at 60 MHz were obtained using half-thickness inversion layer of LiNbO₃ single crystal     

Proton-exchanged Z-cut LiNbO3 waveguides for surfaceacoustic waves

Proton-exchanged (PE) waveguides in Z-cut LiNbO₃ have been fabricated using benzoic acid. Secondary-ion mass spectrometry (SIMS) measurements show that the distribution of hydrogen in the PE Z-cut LiNbO₃ samples exhibits a step-like profile with the diffusion constant D₀ and the activation energy Q of about 2.82×10⁸ μm²/h and 87.76 kJ/mol, respectively. On the other hand, the important parameters for the design of surface acoustic wave (SAW) devices are measured and discussed. The results show that the phase velocity and electromechanical coupling coefficient decrease with the increase of kd, where k is the wavenumber and d is the waveguide depth. The variation of insertion loss becomes saturated at about kd=0.068 with a maximum increase of about 4~5 dB. The temperature coefficient of delay calculated from the frequency change of the output of SAW delay line shows an evident increase in the PE layer. Moreover, the effects of postannealing can result in a restoration of the decreased velocity and an improvement of the insertion loss     

Spectroscopic investigation of Nd ion in LiNbO3, MgO:LiNbO3 and LiTaO3 single crystals relevant for laser applications

The polarized absorption and luminescence properties of Nd³⁺ doped isostructural LiNbO₃, MgO:LiNbO₃ and LiTaO₃ nonlinear bulk single crystals are reported. Pump-probe experiments associated with the Judd-Ofelt approach are used to estimate two types of room temperature cross sections: polarized emission cross sections of the dominant ⁴F_3/2 → ⁴I_1//2 transition near 1085 and 1093 nm and polarized excited-state absorption cross sections in the same spectral domain and in the green spectral range corresponding to self frequency doubling. Self frequency-doubling results are also given in Nd:LiNbO₃ and Nd:MgO:LiNbO₃ versus sample temperature.     

Study of diamond-like carbon films on LiNbO3

Diamond-like carbon (DLC) films have been successfully deposited on Y-cut LiNbO₃ substrates using the plasma enhanced CVD technique. A thin interlayer of SiC between the DLC films and the LiNbO₃ is necessary to ensure a good adhesion of the DLC films to the LiNbO₃ substrate. The physical properties and structural network of the DLC films have been investigated in detail. It is observed that the film hardness is increased with increasing the film thickness, as is the adhesion of the DLC films to the LiNbO₃ substrates. The effect of accelerating surface acoustic wave by the DLC films has been confirmed.     

Liquid phase epitaxy of LiNbO3 thin films for integrated optics

LiNbO₃ thin films have been obtained by liquid phase epitaxy from a Li₂OV₂O₅ flux. Phase diagram, supersaturation domain, and growth conditions have been investigated. The propagation of light has been demonstrated in an Ag substituted LiNbO₃ film grown on a c plate LiNbO₃ substrate.     

Modification of UV-VIS optical absorption properties caused by MgO incorporation in MgO-doped LiNbO3 crystals

MgO-doped LiNbO₃ single crystals were investigated in the UV-VIS range to study the effect of MgO in the LiNbO₃ host. Congruent LiNbO₃ has an optical absorption edge of 3.75 eV. Analysis of the absorption edge gave an estimate of the indirect band gap for MgO-LiNbO₃ solid solutions. The band gap shows an MgO threshold effect in the absorption coefficient. An additional indirect transition at 2.2–2.5 eV was revealed when MgO was incorporated into the lithium niobate structure. This indirect transition also has a MgO threshold effect around 5 mol% MgO.     

Multistrip couplers for surface acoustic wave sensor application

We present a new surface acoustic wave gas sensor utilizing a multistrip coupler on LiNbO₃ and LiTaO₃ substrates and using copper phthalocyanine as a sensor layer for NO₂. The sensor signal originates from surface conductivity changes induced by the adsorbed NO₂. This variation in conductivity leads to a changing coupling efficiency of the multistrip coupler and thus to variations in the insertion loss of the device. Rayleigh and shear wave devices with operating frequencies of 170 and 243 MHz have been tested. A sensitivity of better than 1 ppb NO₂ in air was achieved     

Pulsed laser deposition of La0.67Ca0.33MnO3 thin films on LiNbO3 and surface acoustic wave studies

Surface Acoustic Waves on piezoelectric substrates can be used to investigate the dynamic conductivity of thin films in a non-contact and very sensitive way, especially at low conductivities. Here, we report on such surface acoustic wave studies to characterize thin manganite film like La_0.67Ca_0.33MnO₃, exhibiting a Jan Teller effect with a strong electron phonon interaction and a metal insulator transition at high temperatures.

We report on the deposition of La_0.67Ca_0.33MnO₃ on piezoelectric substrates (LiNbO₃ in different crystal cuts, employing a pulsed laser deposition technique. The structural quality of the thin films are examined by X-Ray Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy. For the electrical characterization, we employ the surface acoustic wave technique, accompanied by conventional direct current resistance measurements for comparison.     

Formation and properties of proton-exchanged and annealed LiNbO3 waveguides for surface acoustic wave

The proton-exchanged (PE) and annealed PE (APE) z-cut LiNbO₃ waveguides were fabricated using H₄P₂O₇. The positive strain, c-axis lattice constant change (Δc/c), was calculated to be about +0.43%, which was almost independent of the exchanged conditions. The penetration depth of H measured by secondary ion mass spectrometry (SIMS) exhibited a step-like profile, which was assumed to be equal to the waveguide depth (d). The surface acoustic wave (SAW) properties of PE and APE z-cut LiNbO₃ samples were investigated. The phase velocity (V_p) and electromechanical coupling coefficient (K ²) of PE samples were significantly decreased by the increase of kd, where k was the wavenumber (2π/λ). The insertion loss (IL) of PE samples was increased by the increase of kd and became nearly constant at kd>0.064. The temperature coefficient of frequency (TCF) of PE samples allowed an apparent increase with kd, reaching a maximum at kd=0.292, then slightly decreased at higher kd. The effects of annealing resulted in a restoration of V_p and an improvement of IL     

Scanning electron microscopy observation of surface acoustic wavepropagation in the LiNbO3 crystals with regular domainstructures

This paper reports a scanning electron microscopy investigation of surface acoustic wave propagation in the LiNbO₃ crystals with regular domain structures. The regular domain structures in the LiNbO ₃ crystals were formed by the method of the thermo-electric treatment after growth. We investigated two modes of the interaction between the surface acoustic waves and regular domain structures in the LiNbO₃ crystals: excitation of the surface acoustic waves by the curved regular domain structure, and propagation of the surface acoustic waves along and across the regular domain structures. It is shown that the regular domain structure in the first case can be used as an interdigital transducer for excitation of the surface acoustic waves through the longitudinal piezoelectric effect. In the second mode of the interaction we observed that the regular domain structure can be used as an acoustic wave-guide in the process of the propagation of the traveling surface acoustic waves along the regular domain structure. Also, we demonstrate the reflection of the surface acoustic waves from the domain walls in the process of surface acoustic wave propagation across the regular domain structure     

The study of self-erasing phenomenon in reduced Zn:Fe:LiNbO3 crystal

It is experimentally demonstrated that the self-erasing phenomenon, diffraction efficiency grows gradually to its maximum and then decays a great extent during the recording process, existed in reduced Zn:Fe:LiNbO₃ crystal. To analyze this phenomenon, the absorption spectra were measured by UV–Visible Spectrophotometer in the region of 200–700 nm, and the nonlinear dependence of photoconductivity on light intensity were studied. The cooperative action of the two kinds of carriers and their own shallow or deep traps was considered to explain the self-erasing phenomenon in reduced Zn:Fe:LiNbO₃ crystal.     

Phononic crystals based on linbo/sub 3/realized using domain inversion by electron-beam irradiation

We report in this paper about the realization of domain inversion in z-cut lithium niobate by electron beam irradiation in order to perform phononic crystals. The fabrication of these phononic crystals on z-cut LiNbO₃, which is, in our case, a periodic repetition of voids and LiNbO₃, was achieved by domain inversion followed by wet etching, taking advantage of the large difference in etching rate between z+ and z- faces. A pertinent choice of irradiation conditions such as accelerating voltage, beam current, and charge density was determined and optimized. Two-dimensional structures at the micrometer scale were then realized on z-cut LiNbO₃. We demonstrate the achievement of hexagons with diameters between 2 mum and 18 mum, with a very important depth close to 30 mum, which depends on the etching time and the hole size. The obtained structures, which exhibit a filling fraction varying from 1% to 64%, were characterized before etching by polarizing microscope to visualize the inverted domains. After HF etching, scanning electron microscopy was used to observe the obtained phononic structures. Taking into account the obtained filling fraction values and the size of created hexagons, the frequency band gap of these structures is expected at a range of 200 to 350 MHz. As expected in this frequency range, we have proven experimentally the existence of the phononic band gap on z-cut LiNbO₃ by combination of a realized phononic crystal with a surface acoustic wave (SAW) device.     

Light-induced absorption changes in iron-doped LiNbO3

LiNbO₃: Fe and LiNbO₃: Fe,Me (Me = K, Mg, Zn) crystals are illuminated with frequency-doubled pulses of a Q-switched Nd: YAG laser. We detect light-induced absorption changes at high pulse intensities (I > 10⁹Wm^-2) utilizing cw probe lasers of different wavelengths (λ^p = 488, 633, and 785 nm). Intensity dependences as well as relazation processes after illumination are investigated. Absorption changes increase with increasing light intensity. Strongest effects are observed in reduced crystals. Doping with magnesium or zinc and an increasing lithium content, respectively, diminishes light-induced absorption changes. Our results can be described by a two-center charge transport model.     

Electric Field Sensing Scheme Based on Matched $ hbox{LiNbO}_{3}$ Electro-Optic Retarders

In this paper, a wideband-electric-field-sensing scheme that uses optically matched integrated optics electrooptic devices and coherence modulation of light is described. In a coherence modulation scheme, the integrated optics sensor detects the electric field and imprints it around an optical delay. The optical delay is generated by a birefringent optical waveguide in a lithium niobate (LiNbO₃) integrated optics two-wave interferometer. The modulated optical delay, acting as an information carrier, is transmitted through an optical fiber channel. At the receiver, light is demodulated by a second integrated optics two-wave interferometer, which also introduces a second optical delay. The optical delays on the sensor and demodulator are matched at the same value. The integrated optics demodulator measures the autocorrelation of light around the optical delay value, and the imprinted electric field is recuperated as a linear variation of the received optical power. The matching of the sensor and demodulator allows a direct detection of the electric field, giving a unique feature to this fiber-integrated optics scheme. The experimental setup described here uses two pigtailed LiNbO₃ electrooptic crystals: one acting as the electric field sensor and the other acting as the optical demodulator. The wideband sensing range on the experimental setup corresponds to frequencies between 0 and 20 kHz.     

Theoretical study on SAW characteristics of layered structuresincluding a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%     

High temperature stable GHz-range low-loss wide band transducersand filter using SiO2/LiNbO3, LiTaO3

GHz-range low-loss transducers and filters are required for communication systems, especially mobile telephone communication systems. Many types of low insertion-loss transducers and filters utilizing the high electromechanical coupling coefficient (K²) materials such as LiNbO₃ and LiTaO₃ have been developed. Unfortunately, these materials have large temperature coefficients of the frequency (TCF). In this paper, SAW substrates with high coupling coefficients and low propagation attenuations and small temperature coefficient of frequency in the GHz-range are theoretically and experimentally investigated. The experimental results show very low propagation loss of 0.02 dB/λ ₀ and larger K² than those of the substrates of LiNbO₃ and LiTaO₃ at the TCF of below -5 ppm/°C at 1~2 GHz-range. The low-loss filter results using internal reflection types of IDT show the insertion loss of about 2.9 dB at 1 GHz and 4.9 dB at 2 GHz under the TCF's of 0 and +20 ppm/°C. These materials are applicable for devices at GHz-range because SiO₂ thickness is very thin such as below 1 μm and the center frequency shift of the filter versus SiO₂ thickness is very small     

High-performance guided-wave acoustooptic Bragg cells inLiNbO3- and GaAs-based structures

We present a theoretical study of guided-wave acoustooptic (AO) Bragg cells produced by a single surface acoustic wave (SAW) in PE:LiNbO ₃/LiNbO₃- and ZnO/GaAs/AlGaAs-based structures. A rigorous modeling of the SAW propagation in multilayered structures has been used and the analysis of the AO interaction has been performed by using a generalized multimode formulation of the coupled-mode theory. High-performance configurations, particularly advantageous in wide-band optical communication and signal processing applications are presented     

Optical spectroscopy of Dy ions in LiNbO3

A systematic spectroscopic study of Dy³⁺ doped LiNbO₃ is presented. The energy position of the Stark levels and their symmetry character is given for most of the multiplets. Luminescence of this system has been investigated in the visible and infrared. The only emitting state in this region is the metastable 4F_9/2 multiplet whose life time is temperature independent and with a value of 186 μs. Evidence about Dy³⁺ multicentres is also discussed.     

Surface and bulk properties of Cu nanocluster composites in LiNbO3

Implantation of 60 keV Cu⁻ ions into LiNbO₃ resulted in formation of complex nanocomposites consisting of metal Cu nanoparticles distributed among nanodomains of the host medium. The nanodomains of the host medium correspond to regions of a few hundred naometers in diameter with a constant refractive index. Distributions of the Cu nanoparticles and nanodomains in the implanted region do not correlate with each other. Variations of linear and non-linear optical absorption of the nanocomposites are mainly determined by the change of chemical composition and structure of implanted regions of LiNbO₃.     

A method of determining acoustical physical constants for piezoelectric materials by line-focus-beam acoustic microscopy

We developed a new method of determining acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of piezoelectric materials with high accuracy. This method acquires velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface, measured by line-focus-beam (LFB) acoustic microscopy, and bulk velocities of longitudinal and shear waves, measured with planewave transducers replacing the LFB device in the same system, together with the dielectric constants and density measured independently, for a small number of specimens. For LiNbO₃ and LiTaO₃ crystals, we demonstrated that we could accurately determine the constants by choosing proper propagation directions of LSAWs and bulk waves for three principal X-, Y-, and Z-cut specimens and one rotated Y-cut specimen [(104) plate for LiNbO₃ and (012) plate for LiTaO₃]. The accuracy is nearly the same as that for the constants determined only from the bulk wave velocities