Demonstration of a quantized acoustic octupole topological insulator

Xiang Ni; Mengyao Li; Matthew Weiner; Andrea Alù; Alexander B Khanikaev

doi:10.1038/s41467-020-15705-y

Demonstration of a quantized acoustic octupole topological insulator

Nat Commun. 2020 Apr 30;11(1):2108. doi: 10.1038/s41467-020-15705-y.

Authors

Xiang Ni^{1

2

3}, Mengyao Li^{1

2}, Matthew Weiner^{1

2}, Andrea Alù^{4

5

6}, Alexander B Khanikaev^{7

8}

Affiliations

¹ Department of Electrical Engineering, Grove School of Engineering, City College of the City University of New York, 140th Street and Convent Avenue, New York, NY, 10031, USA.
² Physics Program, Graduate Center of the City University of New York, New York, NY, 10016, USA.
³ Photonics Initiative, Advanced Science Research Center, City University of New York, New York, NY, 10031, USA.
⁴ Department of Electrical Engineering, Grove School of Engineering, City College of the City University of New York, 140th Street and Convent Avenue, New York, NY, 10031, USA. aalu@gc.cuny.edu.
⁵ Physics Program, Graduate Center of the City University of New York, New York, NY, 10016, USA. aalu@gc.cuny.edu.
⁶ Photonics Initiative, Advanced Science Research Center, City University of New York, New York, NY, 10031, USA. aalu@gc.cuny.edu.
⁷ Department of Electrical Engineering, Grove School of Engineering, City College of the City University of New York, 140th Street and Convent Avenue, New York, NY, 10031, USA. akhanikaev@ccny.cuny.edu.
⁸ Physics Program, Graduate Center of the City University of New York, New York, NY, 10016, USA. akhanikaev@ccny.cuny.edu.

Abstract

Recently introduced quantized multipole topological insulators (QMTIs) reveal new types of gapped boundary states, which themselves represent lower-dimensional topological phases and host symmetry protected zero-dimensional corner states. Inspired by these predictions, tremendous efforts have been devoted to the experimental observation of quantized quadrupole topological phase. However, due to stringent requirements of anti-commuting reflection symmetries, it is challenging to achieve higher-order quantized multipole moments, such as octupole moments, in a three-dimensional structure. Here, we overcome this challenge, and experimentally realize the acoustic analogue of a quantized octupole topological insulator using negatively coupled resonators. We confirm by first-principle studies that our design possesses a quantized octupole topological phase, and experimentally demonstrate spectroscopic evidence of a hierarchy of boundary modes, observing 3^rd order topological corner states. Furthermore, we reveal topological phase transitions from higher- to lower-order multipole moments. Our work offers a pathway to explore higher-order topological states in 3D classical platforms.