Topologically protected zero refraction of elastic waves in pseudospin-Hall phononic crystals

Abstract

Zero-angle refraction of elastic waves in metamaterials has attracted attention for its extraordinary wave collimation properties. However, earlier implementations relied on the specific flat equifrequency curve of the phononic crystals suffer from a narrow range of incident angles or operating bandwidths, which severely hinders the exploration and design of functional devices. Here, we propose an elastic near-zero refractive index metamaterial of a triangular lattice to realize topological zero refraction with arbitrary angles of incidence and wide working frequency range. Topological robustness of the zero-angle refraction of pseudospin-Hall edge state against defects is experimentally demonstrated. Furthermore, tunable wave mode conversion associated with the zero-angle refraction is revealed and discussed. These results provide a paradigm for the simultaneous control of the refraction properties of longitudinal and transverse waves that can be employed for designing the topological elastic antennas and elastic wave collimator.

Tunable graphene phononic crystal

“…Summarizing, we have shown a tunable speed of sound and realized an upshift of the defect mode resonance under pressure, whilst maintaining its localization. Such a more-than-four-fold frequency increase is unprecedented and remains elusive in any other phononic systems [21][22][23][24][25][26][27][28][29][30][31][32] .…”

“…There have been recent efforts to realize a PnC with dynamically tunable frequency [21][22][23][24][25][26][27][28][29][30][31][32] . Frequency tunability may unlock new regimes of guiding, filtering, and focusing phonons.…”

“…It would furthermore allow to resonantly couple to an external optical or mechanical excitation and thus realize sensing applications with mechanical qubits and studies on quantum entanglement 22 . Yet, the mechanical resonances in PnCs are determined by material constants and the crystal geometry [23][24][25][26][27] , which cannot be varied easily. In principle, the mode frequencies can be controlled by changing the temperature 28,29 or by an external magnetic field 30,31 .…”

Topological surface wave metamaterials for robust vibration attenuation and energy harvesting

“…Mechanical metamaterials [3] whose configurations can be artificially fabricated and modulated have been proven to manipulate the propagation of mechanical waves with new effects such as wave isolation [4][5][6][7][8][9], topological insulator [10][11][12][13][14][15][16][17][18][19], Fano resonance [20][21][22][23], chirality [24], focusing and imaging [25], among others. Especially, surface wave metamaterials [1,[26][27][28][29][30][31][32][33][34][35][36] are designed mainly for transmission suppression of low-frequency surface wave in certain frequency ranges based on the conception of hybridized bandgaps.…”

Abnormal topological refraction into free medium at sub-wavelength scale in valley phononic crystal plates

“…Perhaps the most concerned issue is how the topological edge waves would be transmitted into the surrounding homogeneous medium. Various outcoupling effects, such as topological positive/negative, near-zero refraction and wave splitting have recently been reported in valley phononic crystal (PnC)s [18,[21][22][23], Weyl PnC [28], and QSHE inspired PnCs [29,30].…”