3P1-8
Structural Design of Topological Surface Acoustic Waveguides
with Pillar-shaped Phononic Crystals
Yuito Ohashi1͊, Yusuke Hata1, Masaaki Misawa2 and Kenji Tsuruta1*(1Okayama Univ; 2Fukuoaka Inst. Technol;)
1. Introduction
Recent years have seen technological innovation
toward Beyond 5G/6G. One of the challenges in thetarget is to increase the peripheral components which
can stably operate in the high frequency ranges. In
particular, various attempts have been made toimprove the efficiency and multifunctionality ofcommunication devices using surface acousticwaves (SAW) operating in the GHz band. Amongthem, topological phononic crystals,[1],[2],[3],[4],[5]which have the characteristics of a conductor on theboundary surface and an insulator inside, haveattracted much attention recently for their ability toefficiently manipulate SAW. We have been recently
attempting to design highly efficient elastic
waveguides using topological phononic crystals,[6],[7],[8],[9]and are currently focusing on pillar-type
phononic crystals. It has been recently reported thatthe fabrication of nanoscale pillar structures onsemiconductor substrates enables the design oftopological SAW waveguides in the MHz band.[10] In
the present study we aim to design topological SAW
devices that operate at higher frequencies and in
multiple frequency bands, while using the same
design scheme as we have developed. As a
preparatory step toward the GHz band design, we
first explored the structure of millimeter-scale
topological phononic crystals for the purpose ofcontrolling surface acoustic waves in the kHz band.

Fig. 1 Structural parameters of phononic crystals withsymmetric (right) and asymmetric (middle) pillar
structures (middle) pillars arrays in the unit cell.

Fig.2 Band diagram symmetry (left) and asymmetry(right) of topological phononic crystals and bandgap expansion

2. Band Gap Control and Topological Acoustic
Waveguides Design
A phononic crystal consisting of two pillars with acylinder on the upper edge on a polypropylene
substrate as the unit cell of a period of 8.4 mm wasnumerically designed using the finite element
method. Figure 1 shows the phonon dispersion for
two pillars with the same shape (left) and two pillars
with different shapes(right). By breaking thesymmetry, a band gap emerges between in the twobulk bands, blocking sound waves in the respective
frequency region, as shown in Fig.2. When thesymmetry of the pillars is broken and crystals ofdifferent orientations are combined to form aboundary plane, a localized band with edge statecharacters appears in the gap, as depicted in Fig 3.

Fig. 3 Supercell structure (top left), edge mode
displacement shapes (bottom left), and phonon band
diagram of a supercell (right).

By numerical simulation based on the finite
element method, we demonstrate a surface wave
propagation in a linear waveguide with the boundary
surface discussed above. Figure 4 shows apreliminary result of the edge-mode propagation
along the boundary. Here a surface acoustic wave wasinjected at the left edge of the waveguide by excitinga vibrational mode with displacements along the xand z-directions. The present simulation confirmedthat the incident surface acoustic waves propagate in
the linear waveguide through the topological edgestate as shown in Fig. 4. In the presentation, we will
discuss more details of these results.

Fig. 4 Numerical simulation of edge mode propagation ina waveguide with an interface between two opposing
pillar phononic structures. (Displacement in zdirection only)

3. Conclusion
In conclusion, we have shown that surface acoustic
waves can be controlled in a single material by using
a columnar resonance structure. By tuning thestructural parameters of the unit cell, topologicaledge modes appear near the third-order band gap. In
the waveguide formed at the topological interface,
we were able to confirm the surface acoustic wave
along the waveguide at the edge mode frequency.
Actual fabrication of a phononic structure using a 3D
printer as well as an experimental verifications of theedge propagations will further be conducted.
Acknowledgement
This work was supported in part by the JSPS
KAKENHI Grant Numbers 21H05020.
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