Imaging of spatial mode profiles is crucial for understanding the behavior of mechanical resonators. The recent development of phononic circuits has increased the demand for a fast imaging method based on principles of coherent detection. However, it becomes complicated to perform measurements on a large surface area. Here, we present a frequency-detuned collimated-beam interferometry measurement scheme with an in-plane spatial resolution of approximately $6\text{μ}\mathrm{m}$, which can provide information about the phase dynamics of the entire mechanical oscillation cycle on a timescale of a few seconds. We employ a stroboscopic pulse-probing method to resolve high-frequency vibrational motion with a standard CMOS camera. We use this setup to image megahertz frequency resonant mode profiles present in a valley-Hall topological triangular cavity, over an area of more than $20{\mathrm{mm}}^2$. We relate the obtained data to numerical simulations of the topological edge modes to reveal the relation between backscattering and the mode profile distribution. The presented protocol can become a staple for characterizing mesoscopic mechanical resonators.

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• Acoustic phonons
• Imaging & optical processing
• Optomechanics
• Phononic crystals
• Topological effects in acoustic systems
• Topological materials