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.     

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO3 substrates

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO₃) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N₂ concentration, and sputtering pressure. Highly c-axis-oriented AlN films to fabricate the AlN/LiNbO₃-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N₂ concentration of 60%, RF power of 165 W, and substrate temperature of 400°C. A dense pebble-like surface texture of c-axis-oriented AlN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K²) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm/°C     

Shapes of isolated domains and field induced evolution of regular and random 2D domain structures in LiNbO3 and LiTaO3

The variety of the shapes of isolated domains, revealed in congruent and stoichiometric LiTaO₃ and LiNbO₃ by chemical etching and visualized by optical and scanning probe microscopy, was obtained by computer simulation. The kinetic nature of the domain shape was clearly demonstrated. The kinetics of domain structure with the dominance of the growth of the steps formed at the domain walls as a result of domain merging was investigated experimentally in slightly distorted artificial regular two-dimensional (2D) hexagonal domain structure and random natural one. The artificial structure has been realized in congruent LiNbO₃ by 2D electrode pattern produced by photolithography. The polarization reversal in congruent LiTaO₃ was investigated as an example of natural domain growth limited by merging. The switching process defined by domain merging was studied by computer simulation. The crucial dependence of the switching kinetics on the nuclei concentration has been revealed.     

Equivalent networks for SAW interdigital transducers

An equivalent network approach is described for the analysis of surface-acoustic-wave interdigital transducers. Circuit parameters can he theoretically determined by applying the finite-element method to an infinite array. In this approach, all the effects of piezoelectric perturbation, mechanical perturbation, and energy storage are taken into account, To show the validity and usefulness of this approach, examples are computed for both single- and double-electrode interdigital transducers and one-port resonators. For single- and double-electrode interdigital transducers on 128°Y-X LiNbO₃, X-112°Y LiTaO₃,₄₅°X-Z Li₂B₄O₇, and ST-X quartz substrates, the dependence of excitation characteristics on electrode thickness and metallization ratio is investigated in detail. For a 128°Y-XLiNbO ₃ substrate, frequency responses of various one-port resonators are also investigated     

Sol–gel synthesis and luminescence of unexpected microrod crystalline Ca5La5(SiO4)3(PO4)3O2:Dy phosphors employing different silicate sources

Ca₅La₅(SiO₄)₃(PO₄)₃O₂ doped with Dy³⁺ were synthesized by sol–gel technology with hybrid precursor employed four different silicate sources, 3-aminopropyl-trimethoxysilane (APMS), 3-aminopropyl-triethoxysilane (APES), 3-aminopropyl-methyl-diethoxysilane (APMES) and tetraethoxysilane (TEOS), respectively. The SEM diagraphs show that there exist some novel unexpected morphological structures of microrod owing to the crosslinking reagents than TEOS as silicate source for their amphipathy template effect. X-ray pictures confirm that Ca₅La₅(SiO₄)₃(PO₄)₃O₂:Dy³⁺ compound is formed by a pure apatitic phase. The Dy³⁺ ions could emit white light in Ca₅La₅(SiO₄)₃(PO₄)₃O₂ compound, and the ratio of Y/B is 1.1, when the Dy³⁺ doped concentration is 1.0 mol%.     

High-Q AlN/SiO2 symmetric composite thin film bulk acoustic wave resonators

High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO₂-AlN-SiO₂ sandwiched between electrodes have been developed. Acoustic isolation was achieved by means of deep silicon etching to obtain membrane type thin film bulk acoustic wave resonators (TFBARs). Three different device versions were investigated. The SiO₂ film thicknesses were varied (0 nm, 70 nm, 310 nm, and 770 nm) while the piezoelectric AlN film had a constant thickness of 1.2 μm. The sputter-deposited AlN film grown on the amorphous, sputter-deposited SiO₂ layer exhibited a d_33,f of 4.0 pm/V. Experimental results of quality factors (Q) and coupling coefficients (k_t²) are in agreement with finite element calculations. A Q of 2000 is observed for the first harmonic of the 310 nm oxide devices. The most intense resonance of the 770 nm oxide device is the third harmonic reaching Q factors of 1450. The temperature drift reveals the impact of the SiO₂ layers, which is more pronounced on the first harmonic, reducing the TCF to 4 ppm/K for the 3rd harmonic of the 310 nm oxide devices.     

SAW ring filters with insertion loss of 1 dB

This paper presents low loss ring SAW filters on 49°YX, 64°YX, 128°YX LiNbO₃ with reflective multistrip couplers (RMSCs). Using the RMSCs with 3 electrodes per λ (λ is the SAW wavelength at the center frequency) and the self-matching approach, when the static capacitance of the IDT is compensated by the acoustic radiation susceptance, the ring filters have shown very low insertion loss of 0.8-1 dB, 3-dB fractional bandwidth of 2-5% with very low ripple of 0.1 dB, stopband attenuation over 50 dB at 10-33% offset from the center frequency of 45 MHz. In a 50 ohm system, 148, 164, 172 MHz ring filters on 128°YX for low power transceivers have provided an insertion loss of 1 dB, 1 dB bandwidth of 1.8-2 MHz, stopband attenuation over 55 dB at ±25 MHz offset from the center frequency. Two cascaded filters at 164.5 MHz have shown insertion losses below 3 dB and stopband attenuation over 90 dB at ±25 MHz, offset from the center frequency. The chip size is 5×4×0.7 mm     

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.     

Structural and dielectric properties in the system LiTaO3-WO3

Phase transition in the (1-x)LiTaO₃-xWO₃ solid solution system has been studied by means of X-ray diffraction and dielectric measurements. Nonstoichiometric solid solutions with a LiTaO₃ structure are formed in the range of 0 < x ≤ 0.4, and the introduction of WO₃ into LiTaO₃ causes cation vacancies in the Lisites. The axial ratio (c/a) of the hexagonal cell and the ferroelectric Curie temperature decrease with the increase of x. A trirutile compound LiTaWO₆ (x=0.5) exhibits photochromism.     

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%     

Optimized Stoneley wave device by proper choice of glass overcoat

The characteristics of a Stoneley wave propagated along an interface between a piezoelectric material and an isotropic material were investigated both theoretically and experimentally. First, the condition for the existence of Stoneley waves was shown for various piezoelectric materials. A rule of thumb for selecting the combination of the two materials was obtained. Then, LiTaO₃ was selected as the piezoelectric material and SiO₂ was selected the isotropic material. After the calculation of the Stoneley wave characteristics, actual devices were fabricated and measured. The experimental results were found to be in good agreement with the theory; zero slope temperature and high electromechanical coupling coefficient ( K²=1.5%) were obtained for Stoneley wave propagation between SiO₂/X-148° LiTaO₃. As a result, future surface-acoustic-wave (SAW) devices can be made without any package     

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     

Optical properties of Er-doped SiO2-GeO2-Al2O3 planar waveguide fabricated by sol–gel processes

In this paper, we report silica based planar waveguides doped with Er³⁺, and co-doped with GeO₂ and Al₂O₃. These sol–gel derived planar waveguides were fabricated on SOS (silica on silicon) using multiple spin-coating and rapid thermal processing (RTP). Investigation has been made on their characteristics in terms of their application in optical amplification and lasing, including photoluminescence (PL), fluorescence lifetime, refractive index, propagation loss, surface roughness, Fourier transform infrared (FTIR) spectrum and X-ray diffraction (XRD) analysis. The propagation loss of a 20-layer planar waveguide was measured to be about 1.6 dB/cm for TE₀ and 2.2 dB/cm for TM₀ mode. A strong emission transition (⁴I_13/2→⁴I_15/2) at 1.536 μm with a lifetime of 3.6 ms has been obtained for an optimized molar composition of 90SiO₂: 10GeO₂: 20AlO_1.5: 1ErO_1.5.     

Characterization of surface acoustic wave propagation in multi-layered structures using extended FEM/SDA software

This paper describes the characterization of SAW propagation in layered substrate and overlayered structures. The software based on the finite element method and spectral domain analysis was newly developed and applied to the characterization of SAW propagation under an infinitely-long Al interdigital transducer on a rotated Y-cut LiTaO₃/sapphire substrate. Because of the finite LiTaO₃ thickness, a series of spurious resonances appears. It is shown that the excitation strength of the spurious resonances changes with frequency as well as the rotation angle, which reflects the frequency and rotation angle dependence of the energy leakage. Next, the analysis was carried out for SAWs propagating in a SiO₂ layer/Al IDT/42°YX-LiTaO₃ structure. It is shown that the influence of the SiO₂ layer is significantly dependent on the location where the SiO₂ layer is deposited. In particular, it is shown that when the SiO₂ layer is deposited only on top of the electrodes, the SAW reflectivity increases compared with when the SiO₂ layer is deposited between and on top of electrodes.     

Microwave dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low-temperatures

The microwave dielectric properties and microstructures of Ba(Mg_1/3Ta_2/3)O₃ (BMT) ceramics sintered at low temperatures with 2–3 wt.% NaF additives were investigated. BMT ceramics sintered at 1340 °C for 3–12 h showed dielectric constants (_r) of 25.5–25.7, Qf values of 41 500–50 400 GHz and temperature coefficients of the resonator frequency (τ_f) of 10.9–21.4 ppm °C⁻¹. The variation of sintering time almost had no effect on the dielectric constant. The Qf value increased and the τ_f decreased with increasing sintering time. The ordering degree of Mg²⁺ and Ta⁵⁺ at B-sites increased with increasing sintering time.     

A comparative study of the UV optical and structural properties of SiO2, Al2O3, and HfO2 single layers deposited by reactive evaporation, ion-assisted deposition and plasma ion-assisted deposition

Growing requirements for the optical and environmental stability, as well as the radiation resistance against high-power laser radiation, especially for optical interference coatings used in the ultraviolet spectral range, have to be met by new, optimised, thin-film deposition technologies. For applications in the UV spectral range, the number of useful oxide thin film materials is very limited due to the higher absorption at wavelengths near to the electronic bandgap of the materials. Applying ion-assisted processes offers the ability to grow dense and stable films, but in each case careful optimisation of the deposition process (evaporation rate, substrate temperature, bombarding gas, ion energy and ion current density) has to achieve a balance between densification of the layers and the absorption. High-quality coatings and multilayer interference systems with SiO₂ as the low-index material can be deposited by various physical vapour deposition technologies, including reactive e-beam evaporation, ion-assisted deposition and plasma ion-assisted deposition. In order to improve the degradation stability of dielectric mirrors for use in UV free-electron laser optical cavities, a comparative study of the properties of SiO₂, Al₂O₃ and HfO₂ single layers was performed, and was addressed to grow very dense films with minimum absorption in the spectral range from 200 to 300 nm. The films were deposited by low-loss reactive electron-beam evaporation, by ion-assisted deposition using a ‘Mark II’ ion source, and by plasma ion-assisted deposition using the advanced plasma source. Optical and structural properties of the samples were studied by spectral photometry, infrared spectroscopy, X-ray diffraction and reflectometry, as well as by investigation of the surface morphology. The interaction of UV radiation with photon energy values close to the bandgap was studied. For HfO₂ single layers, laser-induced damage thresholds at 248 nm were determined in the 1-on-1 and 1000-on-1 test modes as a function of the deposition technology and film thickness.     

Al2O3纤维对SiO2基陶瓷型芯的烧缩阻滞

向SiO₂基体粉料中添加Al₂O₃纤维,采用热压注法制备Al₂O₃/SiO₂陶瓷型芯。分析Al₂O₃纤维含量对陶瓷型芯性能的影响。研究结果表明：Al₂O₃纤维含量对Al₂O₃/SiO₂陶瓷型芯的线收缩率、体积密度和抗弯强度均有较大的影响。当Al₂O₃纤维含量大于1wt%时,Al₂O₃/SiO₂陶瓷型芯的线收缩率大幅度降低,稳定在0.335%左右,体积密度随之降低,稳定在1.790 g · cm^-3左右;当Al₂O₃纤维含量为1wt%时,陶瓷型芯抗弯强度达最大值20.48 MPa。分析了Al₂O₃纤维对Al₂O₃/SiO₂陶瓷型芯烧结收缩的阻滞作用机制。     

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     

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     

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.