P. J. Vella

High‐resolution spectroscopy for optical probing of continuously generated surface acoustic waves

Under conditions of continuous acoustic generation, light scattered from oppositely directed surface acoustic waves on y‐cut z‐propagating lithium niobate was frequency analyzed with a high‐resolution Fabry‐Perot interferometer. The following parameters were measured at 105 MHz: acoustic power standing‐wave ratio, reflection coefficient for both sending and receiving transducers, low‐power acoustic damping coefficients, surface wave propagation velocity, and nonlinear harmonic growth. We note that the method used has an advantage over other methods since it is to a large extent independent of the optical quality of the propagation surface.

Nonlinear surface‐wave interactions: Parametric mixing and harmonic generation

The nonlinear interaction of surface acoustic waves propagating in the same direction has been treated from the rigorous theory of thermoelasticity. Exact expressions were derived for the nonlinear volume forces and surface stresses, and the nonlinear cross sections were calculated under two separate assumptions. First, the work done by the nonlinear forces and surface stresses on a normal‐mode surface wave was evaluated and assumed to be totally converted into surface‐wave energy. In the second approach, a linear analytic solution was formulated which simultaneously satisfies the nonlinear wave equation and maintains a stress‐free boundary condition. Numerical values for the strength of second‐harmonic generation were calculated for 24 materials. The characteristics of nonlinear surface‐wave generation were compared with those of nonlinear bulk‐wave generation and significant differences were found. Coupled amplitude equations were derived to treat the case of multiharmonic generation. Finally, a charact...

Enhanced second‐harmonic generation by counter‐propagating guided optical waves

Second‐harmonic light generated by mixing oppositely propagating guided waves in LiNbO3 waveguides was enhanced ∼500‐fold by varying the refractive index of the medium above the LiNbO3 surface.

Probing of surface acoustic wave devices with large‐diameter laser beam

The theory of dark field imaging technique as applied to acousto‐optic interactions has been developed and then used in studies of linear and nonlinear surface acoustic waves and volume acoustic waves. The theory is consistently expressed in terms of phase transmittance function. Experimental results show how the technique can be applied in the case of multimode propagation to unfold the complex frequency spectra of spurious modes in surface wave devices, and how to discriminate between different harmonics in the case of nonlinear acoustic propagation.

Surface‐wave harmonic generation on y‐z, x‐z, and 41 1/2‐x lithium niobate

Optical probing was used to evaluate the coefficient for surface‐wave harmonic generation on three high‐coupling cuts of lithium niobate, namely, y‐z, x‐z, and 41 1/2‐x. Surface‐wave attenuation and acoustic beam diffraction profiles of the fundamental and second harmonic were also measured.

Probing of acoustic shear wave radiation in surface wave devices

Simultaneous Rayleigh and shear wave generation has been achieved using interdigital transducers on a yz LiNbO3 sample at a frequency of 104 MHz. A large‐diameter laser beam of 5145‐A wavelength and 1‐W cw power was transmitted through the sample and was modulated by two acoustic waves. The double‐modulated laser beam causes fringes at the recording plane, which were used for an accurate determination of the shear wave velocity vs and of the shear wave acoustic radiation angle α into the crystal. It was found that vs = 3.547 ×103 m/sec for α = 14.3°. The shear wave conversion loss of the transducer was approximately 6 dB.

Bulk wave generation by surface interdigital transducers operating near resonance

The fraction of electrical power converted into spurious bulk waves at a surface interdigital transducer operating on and near surface wave resonance was measured. Bulk wave generation on y‐z lithium niobate was found to be strong below, at, and above resonance and to increase with decreasing number of finger pairs.

Parametric coupling of bulk acoustic waves at surface interdigital transducers

Acoustic radiation by interdigital transducers on yz LiNbO3 has been studied experimentally in the region of strong longitudinal bulk wave generation. The measured relative intensity of the transducer first and second spatial harmonics and the number and type of acoustic modes generated establish the existence of acoustic generation by parametric periodic coupling.

Optical probing of surface acoustic wave generation under interdigital transducers

High‐resolution optical spectroscopy was used to study the growth of continuously excited surface acoustic waves under 15‐ and 25‐finger‐pair aluminum interdigital transducers on y‐cut z‐propagating lithium niobate. The observed exponential growth of the surface wave with distance is in agreement with simple coupled‐mode theory. The coupling parameter leads to a direct evaluation of the effective electromechanical coupling constant K. For yz LiNbO3 a value of K2=0.030±0.003 was found.

Optical diffraction studies of sound waves in lithium niobate

The acousto-optic interaction in the Raman-Nath regime was used to study sound waves propagating in the principal planes of piezoelectric lithium niobate. Complete cuts of the inverse phase velocity surfaces were obtained for surface waves on reflection and for three bulk waves and one surface wave on transmission through the surface acoustic wave (SAW) propagation surface. It is also shown how simple diffraction can be used to measure the group velocity direction and the electro-mechanical coupling coefficient for surface waves. A modified schlieren technique was used to visualize bulk waves generated by a finite transducer and to observe shear waves reflecting and mode converting to longitudinal waves at an interface.