Deep-Subwavelength Holey Acoustic Second-Order Topological Insulators

Area of Science:

• Acoustics
• Condensed Matter Physics
• Materials Science

Background:

• Higher-order topological insulators (HOTIs) exhibit unique topological phases with energy confinement at hinges and corners.
• These properties make HOTIs promising for acoustic wave studies and control.
• Current applications often require compact, subwavelength designs for audible and low frequencies.

Purpose of the Study:

• To propose and demonstrate a holey HOTI for compact and subwavelength acoustic wave manipulation.
• To achieve deeply confined acoustic states significantly smaller than the wavelength.
• To investigate the robustness of these states against defects and their performance across multiple frequencies.

Main Methods:

• Fabrication of a holey higher-order topological insulator.
• Experimental observation of acoustic states confined to the corners of the HOTI.
• Testing the resilience of these states against structural defects.
• Demonstration of topologically protected sound propagation in different frequency regimes.

Main Results:

• A holey HOTI was successfully designed and implemented.
• Deeply confined corner states, 50 times smaller than the wavelength, were sustained.
• The acoustic states exhibited remarkable resilience against defects.
• Topologically protected sound was demonstrated across three distinct frequency bands.

Conclusions:

• The proposed holey HOTI enables robust, subwavelength acoustic confinement.
• These findings pave the way for advanced acoustic imaging beyond the diffraction limit.
• The material offers a promising platform for future acoustic devices and applications.