All-optical probing of GHz acoustic waves in multiferroic MEMS

Acoustic wavelengths are about 5 orders of magnitude shorter than EM waves of the same frequency. This can be a critical advantage in the quest to further miniaturize devices that have dimensions linked to their characteristic resonance frequencies. Therefore, many novel multiferroic (MF) devices rely on the transduction of signals from EM radiation to surface and bulk acoustic waves, and yet acoustic wave interactions in complex MF heterostructures are poorly understood. Here, an all-optical interferometric imaging system is developed for two-dimensional scanning imaging of acoustic wave amplitudes with sub-angstrom surface displacement sensitivity and submicron spatial resolution. Measurement capability has been verified with a 50-MHz piezoelectric SAW bandpass filter, with the frequency spectrum showing similar behavior to the S21 transmission measured with a network analyzer, and with a linear dependence on the input power. Spatial dependence of the acoustic waves also behaves as expected, with the largest amplitude near the input electrode. Next, the system has been demonstrated to detect acoustic waves in MF heterostructure-based MEMS devices with frequencies up to 3 GHz. These detections pave the way for fast and reliable troubleshooting of new device designs involving acoustic waves, in which it may not be immediately clear to where or by what mechanism power is being dissipated.