# Propagation of Upward and Downward Interface Acoustic Waves in Fused Silica/ZnO/SU-8/Fused Silica-Based Structures

## TL;DR

This paper shows how two types of acoustic waves can coexist in a layered material structure, which could lead to new sensor and acoustic device designs.

## Contribution

The coexistence of upward and downward interface acoustic waves in a SiO2/ZnO/SU-8/SiO2 structure is demonstrated both theoretically and experimentally.

## Key findings

- Upward and downward interface acoustic waves coexist in SiO2/ZnO/SU-8/SiO2 structures.
- The SU-8 layer thickness strongly influences IAW characteristics like mode number and propagation loss.
- Three distinct IAW modes were experimentally observed, matching theoretical predictions.

## Abstract

What are the main findings?
The main findings of this work is the coexistence of an upward- and downward-propagating interface acoustic wave (IAW) in SiO2/ZnO/IDT/SU-8/SiO2 which has been demonstrated both theoretically and experimentally. The former wave is predominantly localized in the SiO2/ZnO domain: it is excited within the SiO2/ZnO/IDT region and interacts with the SU-8/SiO2 overcoat. The latter is mainly confined to the SiO2/SU-8/ZnO domain: it is excited within the SiO2/SU-8/IDT/ZnO region and interacts with the SiO2 overcoat. The waves have quite different velocity, propagation loss and electroacoustic coupling coefficient.

The main findings of this work is the coexistence of an upward- and downward-propagating interface acoustic wave (IAW) in SiO2/ZnO/IDT/SU-8/SiO2 which has been demonstrated both theoretically and experimentally. The former wave is predominantly localized in the SiO2/ZnO domain: it is excited within the SiO2/ZnO/IDT region and interacts with the SU-8/SiO2 overcoat. The latter is mainly confined to the SiO2/SU-8/ZnO domain: it is excited within the SiO2/SU-8/IDT/ZnO region and interacts with the SiO2 overcoat. The waves have quite different velocity, propagation loss and electroacoustic coupling coefficient.

What is the implication of the main finding?
The findings presented herein establish a solid foundation for the development of next-generation, multi-frequency IAW-based devices, ranging from package-less acoustic components to microfluidic platforms and gas or optical sensors engineered for reliable operation in harsh environments.

The findings presented herein establish a solid foundation for the development of next-generation, multi-frequency IAW-based devices, ranging from package-less acoustic components to microfluidic platforms and gas or optical sensors engineered for reliable operation in harsh environments.

The propagation of interfacial acoustic waves (IAWs) along a SiO2/ZnO/SU-8/SiO2 multilayer structure is theoretically predicted and experimentally validated. A two-dimensional finite-element analysis was performed using COMSOL Multiphysics, revealing that key IAW characteristics—such as the number of supported modes, propagation losses, and acoustic field distribution—are strongly influenced by the thickness of the intermediate SU-8 adhesive layer. In particular, the presence of the SU-8 layer enables the existence of IAW modes with opposite localization, namely upward- and downward-propagating IAWs. To validate the theoretical predictions, experimental measurements were carried out on delay lines fabricated on SiO2/ZnO/SU-8/SiO2 layered structures, revealing the propagation of three distinct IAW modes. The first two modes correspond to the downward and upward fundamental IAWs, while the third mode is a second-order mode identifiable as a downward leaky IAW (LIAW). The experimental results show excellent agreement with the theoretical predictions and establish a solid foundation for the future development of multifrequency IAW-based devices, including package-less acoustic components, microfluidic platforms, and gas and optical sensors designed for operation under harsh environmental conditions.