Sagnac interferometry with proper phase compensation for visualizing surface acoustic wave propagation on magnetic thin film

Recently, magnons coupled to surface acoustic waves (SAWs) have attracted much attention due to their ability to propagate over long distances [1]. In particular, optically pumped SAWs offer a promising approach to initiating magnon-phonon coupling, which has been studied extensively [2]. To further explore the magnon-phonon coupling, it is crucial to separately capture spatiotemporal images of both SAWs [3] and magnons. In this study, we utilize the Sagnac interferometry with proper phase compensation to achieve clear separation of spatiotemporal SAW images on a permalloy thin film sample.

We utilized an asynchronous optical sampling system, incorporating two frequency-stabilized pulsed lasers, for fast time-resolved measurement of the SAW dynamics. The surface displacement due to SAW propagation was probed using the Sagnac interferometry, including two optical paths. To correct phase differences due to slight misalignment between the paths, we employed liquid crystal variable retarders. As a result, we obtained time-resolved 2D images of SAWs and clearly visualized their spread over several nanoseconds following optical pumping.

We also discuss the future potential of this system for examining the relationship between magnons and phonons.

References:

1) B. Casals, N. Statuto, M. Foerster, et al., Phys. Rev. Lett. 124, 137202 (2020).

2) C. Berk, M. Jaris, W. Yang, et al., Nat Commun 10, 2652 (2019).

3) K. Maezawa, S. Fujii, K. Yamanoi, et al., Phys. Rev. Appl. 21, 044047 (2024).