As a result of the first project stage (2023–2024), a theoretical analysis of acoustic wave propagation in dielectric plates characterized by strong anisotropy (tellurium, barium titanate, lithium iodate, paratellurite) was carried out using the transfer matrix method. Dispersion relations were constructed for acoustic waves of various types (antisymmetric and symmetric Lamb waves, waves with shear-horizontal polarization) and orders (zeroth and higher orders) for different crystallographic orientations of the studied materials. It was found that paratellurite (TeO₂), tellurium (Te), and lithium iodate (LiIO₃) support branches corresponding to backward waves. However, no such waves were found in barium titanate within the studied frequency range. It was observed that the width of the frequency range of existence of these waves is narrower than in materials such as lithium niobate and potassium niobate, which appears to be related to the low piezoelectric activity of the aforementioned materials.
Using the transfer matrix method, a theoretical analysis was performed of the characteristics of backward acoustic wave existence in layered structures of the type “strongly anisotropic substrate (h₂) / piezoelectric film (h₁).” Tellurium, paratellurite, and lithium iodate were used as substrates, while lithium niobate (LiNbO₃), potassium niobate (KNbO₃), and piezoceramic materials PZT and CTS19 were used as films. It was shown that as the film thickness increases, the phase velocity of the backward wave at a fixed frequency increases and the zero group velocity point shifts toward lower frequencies. For most of the layered structures considered, increasing the film thickness leads to a reduction in the width of the frequency range of backward acoustic wave existence. However, using the example of layered structures with a tellurium substrate and lithium niobate or potassium niobate films, it was shown that it is possible to increase the width of the frequency range of backward acoustic wave existence when the film thickness is increased in the h₁/h₂ range from 0 to 0.04. When CTS19 and PZT films are used, a stronger shift of the zero group velocity point toward lower frequencies is observed compared to lithium niobate and potassium niobate films, which can be explained by the higher density of the piezoceramic materials.
Using the finite element method, a theoretical analysis was carried out and dispersion relations of the backward acoustic wave A₁ were obtained under the condition of an ideal conducting screen approaching one of the surfaces of the piezoelectric plate. It was shown that, at a fixed hf parameter, the approach of the metallic screen leads to an increase in the phase velocity of the backward branch of the A₁ wave.
Based on the data obtained, an electrode structure topology for interdigital transducers was designed, and an experimental sample was fabricated — a set of interdigital transducers with different spatial periods (wavelengths 1.0–2.0 mm, step 0.1 mm) formed on the surface of a lithium niobate plate 350 µm thick, using projection photolithography and DC magnetron sputtering.
An experimental setup was developed and built for determining the electromechanical coupling coefficient of backward acoustic waves in piezoelectric plates with a large electromechanical coupling coefficient. The main components of the setup are a vector network analyzer, a precision micrometer screw with a non-rotating spindle, and a lithium niobate plate with a set of interdigital transducers fabricated on its surface.
The effect of the distance from the back surface of the piezoelectric plate (opposite to the side carrying the electrode structures) to the conducting screen (in the range 0–500 µm with an accuracy of ±0.5 µm) on the frequency dependences of the S₁₁ parameters of the samples was experimentally studied. It was shown that the approach of the metallic screen to the piezoelectric surface causes a shift of the backward wave resonance frequency toward lower values of hf (where h is the plate thickness and f is the wave frequency), and consequently an increase in the phase velocity of the backward acoustic wave.
The experimentally confirmed effect — for the first time — of an increase in the phase velocity of backward acoustic waves upon approach of a metallic screen to the piezoelectric surface can be explained as follows. Apparently, the electric field accompanying the backward wave penetrates deeply into the piezoelectric plate. The approach of the metallic screen leads to localization of the electric field near one side of the plate, which in turn causes a stiffening of the elastic moduli and, accordingly, an increase in phase velocity. A similar effect was previously observed for Bleustein–Gulyaev waves when the conductivity of a surface layer on a piezoelectric was varied. However, in the case of Bleustein–Gulyaev waves, a point of maximum localization of the electric field near the surface is reached, after which the phase velocity begins to decrease due to the growing influence of the short-circuiting effect on the tangential electric fields. In the present case, the electric field does not reach maximum localization, and therefore no phase velocity reduction effect occurs. It would be worthwhile to conduct additional studies involving direct metallization of the surface by forming an ideally conducting film on the piezoelectric surface. This will be done at the next stage of the project.
Using the transfer matrix method, the phase and group velocities, energy flow angle, and polarization for the identified backward waves were calculated for the first time as a function of a 1-degree temperature change. Dependences of the temperature coefficient of velocity and the temperature coefficient of delay were obtained for the backward wave in the hf range of 1.7–1.9 in a lithium iodate plate. It was found that this parameter is an order of magnitude higher for backward waves than for forward waves in piezoelectric plates. This appears to be related to the proximity of these waves to the cutoff frequency, which leads to their strong dependence on changes in the propagation medium parameters. This conclusion is supported by the fact that the temperature coefficient of velocity (TCV) has its largest value at the beginning of the backward wave existence frequency range, reaches a minimum in the middle at an hf value of approximately 1750 m/s, and increases with further frequency increase. This finding is of interest for the development of highly sensitive temperature sensors, but requires further investigation.
Using the transfer matrix method, the dependences of the temperature coefficient of velocity and the temperature coefficient of delay were calculated for the backward waves previously identified in structures comprising a lithium iodate or paratellurite plate with a piezoelectric film of lithium niobate or zinc oxide. It was found that in this case as well, the temperature coefficient of velocity of the backward wave remains very large. These results require experimental verification.