Valley-Selective Topological Corner States in Sonic Crystals.

Area of Science:

• Condensed Matter Physics
• Acoustics
• Materials Science

Background:

• Higher-order topological insulators (HOTIs) exhibit unique topological boundary states (corner/hinge states) often dictated by crystalline symmetries.
• Current HOTI boundary states lack flexibility, being fixed to specific locations and difficult to switch or select.
• Exploiting topological phases is crucial for developing robust topological waveguides and novel electronic/photonic devices.

Purpose of the Study:

• To experimentally realize topologically switchable and valley-selective corner states.
• To investigate the interplay between higher-order topology and the valley degree of freedom.
• To demonstrate flexible control and manipulation of wave localization in a two-dimensional sonic crystal.

Main Methods:

• Fabrication of a two-dimensional sonic crystal exhibiting second-order topological properties.
• Utilizing the valley degree of freedom in conjunction with higher-order topology.
• Experimental observation of topological switching and valley-selective corner states via spatial scanning of the sound field.

Main Results:

• Demonstrated a valley higher-order topological insulator (HOTI) with nontrivial bulk polarization.
• Observed valley-dependent corner states enabling flexible wave localization control.
• Successfully switched corner states on/off and selected valleys, confirmed by sound field measurements.
• Designed complex patterns with tunable corner states based on valley selection.

Conclusions:

• The study establishes a novel valley HOTI, showcasing switchable and valley-selective corner states.
• This work provides fundamental insights into the synergy between higher-order topology and valley physics.
• The findings pave the way for lower-dimensional valleytronics with potential applications in integrated acoustics and photonics.