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The propagation properties of leaky surface acoustic waves (LSAWs) and longitudinal-type LSAWs (LLSAWs) on a LiNbO3 (LN) or LiTaO3 (LT) thin plate bonded to an AT-cut quartz or c-plane sapphire (c-Al2O3) substrate with a high phase velocity were investigated. It was theoretically revealed that when the LN or LT thin-plate thickness is less than one wavelength, the particle displacement of LLSAWs was concentrated in the thin plate and the electromechanical coupling factor (K2) was increased to two to three times that in the single substrate. Furthermore, for 36° Y-cut X-propagating LT/c-Al2O3 with an LT thin-plate thickness of 0.35 λ and X-cut 36° Y-propagating LN/c-Al2O3 with an LN thin-plate thickness of 0.19 λ, the values of K2 for an LSAW and an LLSAW were experimentally found to increase from 5.6 and 10.4% in the single substrate to 11.5 and 19.7% in the thin-plate bonded structure, respectively.

The propagation properties and resonance characteristics of leaky surface acoustic waves (LSAWs) and longitudinal-type LSAWs (LLSAWs) on a LiTaO3 (LT) thin plate bonded to an AT-cut quartz substrate were investigated experimentally. For the LSAWs and LLSAWs, the bonded structures of 36°Y-cut X-propagating LT (36°YX-LT)/AT-cut 90°X-propagating quartz (AT90°X-quartz) and X-cut 31°Y-propagating LT (X31°Y-LT)/AT-cut 45°X-propagating quartz (AT45°X-quartz) were fabricated, respectively. For the LSAW on 36°YX-LT/AT90°X-quartz, the electromechanical coupling factor (K2) of 11.1% was obtained at an LT thin-plate thickness of 0.25 wavelength, whereas K2 for a single LT substrate was measured to be 5.7%. For the LLSAW on X31°Y-LT/AT45°X-quartz, K2 increased from 2.8% for the single LT substrate to 7.2% at an LT thin-plate thickness of 0.14 wavelength. Furthermore, K2 of approximately 12% and the temperature coefficient of frequency (TCF) of 0 ppm/°C were theoretically obtained simultaneously for the LSAW on 36°YX-LT/AT-cut 90°X-quartz at a certain thin-plate thickness.

A waveguide-type acoustooptic modulator (AOM) driven by a surface acoustic wave (SAW) in a tapered crossed-channel waveguide on a 128°-rotated Y-cut LiNbO3 substrate has been proposed for an optical wavelength of 1.55 µm. In this study, to clarify the conditions for a higher diffraction efficiency and a lower driving power, the diffraction properties of the waveguide-type AOM were measured and simulated. First, an AOM with an AO interaction region length of 3 mm was fabricated and the diffraction efficiency of 65% was obtained. Next, the measured values of the SAW power required for 100% diffraction (P 100) for the driving frequencies of 125 MHz and 200 MHz were found to be in agreement with the calculated P 100, which shows that there is an optimum driving frequency. Furthermore, optical frequency domain ranging using a frequency-shifted-feedback fiber laser with the waveguide-type AOM was demonstrated. Finally, the diffraction properties of the waveguide-type AOM are simulated using a beam-propagation method (BPM) and compared with the experimental results.

To obtain a bonded structure with low attenuation for longitudinal leaky surface acoustic waves (LLSAWs), the propagation and resonance properties on a LiTaO3 (LT) or LiNbO3 thin plate bonded to an X-cut quartz substrate were theoretically analyzed. The attenuation of an X-cut 31°Y-propagating LT (X31°Y-LT)/X32°Y-quartz (X32°Y-Q) was calculated to be 0.0005 dB/λ at the normalized LT thin plate thickness h/λ = 0.062 (λ: wavelength) and was lower than that on an X31°Y-LT/AT45°X-Q. Using a finite element method, for the X31°Y-LT/X32°Y-Q, the admittance ratio and Q factor were improved to 120 dB and 53 400 from 62 dB and 1000 for the X31°Y-LT/AT45°X-Q, respectively. Then, the propagation and resonance properties were measured. For the X31°Y-LT/X32°Y-Q, the measured electromechanical coupling factor (K2) and Q factor increased to 5.6% and 280 from 1.8% and 32 for the single LT, respectively. The temperature coefficient of frequency of the LLSAW was measured to be −26.2 ppm °C−1.

Highly X-axis-oriented tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on a SiO2 substrate using an RF-magnetron sputtering system with a metal tantalum target and an O2-radical source. The degree of orientation, Rayleigh-type surface acoustic wave properties, and surface morphology were evaluated. The deposition condition with the substrate temperature T S of 700 °C and O2 flow rate of 10 ccm was found to be optimum for obtaining a strongly piezoelectric property. Under the optimum condition, the coupling factor of the oriented Ta2O5 thin film with a normalized thickness h/λ of 0.21 was determined to be 0.88% and was 75% of the reported value. The diffraction angle of the preferential peak under the optimum condition was equal to that of the (200)-plane spacing d (200) in the unit cell of monoclinic Ta2O5. A larger plane spacing with Δd/d (200)=2.9% exists preferentially at T S=600 °C, and the piezoelectricity is considered to be zero or very weak. When T S was higher than 700 °C and the O2 flow rate was more than 8 ccm, a smooth surface with the rms roughness of approximately 8–10 nm was obtained. A correlation was found in which a strongly piezoelectric property was obtained when the thin film had a smooth surface.

A longitudinal-type leaky surface acoustic wave (LLSAW) has large inherent attenuation owing to the continuous radiation of two bulk waves. The attenuation of an LLSAW can be reduced by loading with an aluminum nitride (AlN) thin film with a higher bulk wave velocity than the substrate. In this study, to further reduce the attenuation of the LLSAW, we investigated the propagation properties of an LLSAW on an X-cut 36°Y-propagating LiNbO 3 (X36°Y-LN) substrate with an AlN thin film deposited by an RF magnetron sputtering system with a long-throw sputter cathode, in which the substrate is not exposed to plasma directly, by measurement using an interdigital transducer (IDT) pair with a wavelength λ of 8.0 or 4.8 µm. The insertion loss and bulk wave radiation loss into the substrate from the IDTs were markedly decreased by loading with the AlN thin film. When the film thickness was 0.250 λ for λ = 8.0 µm, the measured propagation loss decreased approximately tenfold from 0.28 dB/λ for the sample without the film to 0.03 dB/λ with the film. Furthermore, the temperature coefficient of delay was also reduced by loading with the AlN thin film.

In this paper, first, the surface acoustic wave (SAW) propagation mode and a method of analyzing the propagation property are introduced briefly. Then, typical composite substrate structures that have been developed to obtain high-performance SAW devices are reviewed. Furthermore, the recent results obtained by the author and research colleagues on the propagation and resonance properties of leaky SAW (LSAW) and longitudinal-type LSAW on dissimilar-material bonded structures comprising a LiTaO3 (LT) or LiNbO3 thin plate with a thickness of less than 1 λ (λː wavelength) and a quartz substrate are described. The control of attenuation and the cause of large coupling factor of LSAWs by utilizing layered structures were also discussed. For the bonded 4 inch wafer of 36°YX-LT/AT90°X-quartz with a thin-plate thickness of 0.3 λ, an admittance ratio of 81 dB, a fractional bandwidth of 4.2%, and resonance and antiresonance factors of approximately 1500 with markedly improved properties compared with a single 36°YX-LT substrate were obtained experimentally at 2.2 GHz.