# Propagation and imaging of mechanical waves in a highly-stressed single-mode phononic waveguide

## TL;DR

This paper presents a highly-stressed silicon nitride membrane waveguide that supports single-mode mechanical wave propagation with low loss, enabling robust phononic circuits for advanced signal processing and quantum applications.

## Contribution

The authors demonstrate a novel single-mode phononic waveguide supporting low-loss, out-of-plane mechanical waves, with direct imaging confirming theoretical mode profiles.

## Key findings

- Supports single-mode propagation over a specific frequency band
- Achieves low propagation loss (~1 dB/cm) at ~5 MHz
- Mode profiles match theoretical predictions

## Abstract

We demonstrate a single-mode phononic waveguide that enables robust propagation of mechanical waves. The waveguide is a highly-stressed silicon nitride membrane that supports the propagation of out-of-plane modes. In direct analogy to rectangular microwave waveguides, there exists a band of frequencies over which only the fundamental mode is allowed to propagate, while multiple modes are supported at higher frequencies. We directly image the mode profiles using optical heterodyne vibration measurement, showing good agreement with theory. In the single-mode frequency band, we show low-loss propagation ($\sim1$~dB/cm) for a $\sim5$~MHz mechanical wave. This design is well suited for phononic circuits interconnecting elements such as non-linear resonators or optomechanical devices for signal processing, sensing or quantum technologies.