The concept of topological insulators for the electronic system has been extended to various aspects of physics including optical and acoustic waves. In acoustics, substantial improvements in waveguiding efficiency have been demonstrated in topological phononic crystals. The interface between two phononic crystal structures both based on a two-dimensional hexagonal structure but having different topological natures may have a topologically protected interface mode which allows a high efficiency wave propagation even at sharp corners.[1]

In this paper, we present the time-resolved two-dimensional imaging of Lamb-like acoustic waves propagating along the interface between two different types of valley Hall topological phononic crystals. Each phononic crystal consists of a two-dimensional hexagonal array of micron-scale voids formed on a self-supported GaAs thin slab of 1 micron thickness.[2] Each void has a three-fold rotational symmetry, but slightly (5 degrees) rotated clockwise or counter-clockwise so that the mirror symmetry which the original unrotated structure possesses is broken. These two rotated structures may have different topological natures and the interface between them may accommodate localized modes which serve for waveguiding. The acoustic waves in the frequency range 550-600 MHz are generated by an electric signal using an inter-digital transducer located adjacent to the phononic crystal structure. The acoustic field is optically monitored using periodic ultrashort light pulses and an optical interferometer with micron spatial resolution and picosecond temporal resolution.[3,4] We observe a high efficiency acoustic wave propagation along a Z-shaped folded waveguide.

This study proves that the topological effect is feasible for achieving high efficiency waveguiding in the sub-GHz frequency region, and that the time-resolved acoustic wave imaging is capable of clarifying the properties of topological acoustic devices.

References:

[1] Jiuyang Lu, Chunyin Qiu, Liping Ye, Xiying Fan, Manzhu Ke, Fan Zhang, Zhengyou Liu, Nat. Phys. 13, 369 (2017).

[2] Daiki Hatanaka, Hiroaki Takeshita, Motoki Kataoka, Hajime Okamoto, Kenji Tsuruta, Hiroshi Yamaguchi, Nano Lett. 24, 5570 (2024).

[3] Takehiro Tachizaki, Toshihiro Muroya, Osamu Matsuda, Yoshihiro Sugawara, David H. Hurley, Oliver B. Wright, Rev. Sci. Instrum. 77, 43713 (2006).

[4] Osamu Matsuda, Shohei Ueno, Motonobu Tomoda, Paul H. Otsuka, Oliver B. Wright, Appl. Phys. Lett. 125, 052201 (2024).