Theoretical and experimental study of gradient-helicoid metamaterial

Resonance based acoustic metamaterials and metasurfaces exhibit outstanding phase modulation capabilities with high transmission coefficient, but it is rather confined to the neighborhood of fundamental or harmonic resonant frequencies. Based on the impedance-matched gradient material, we theoretically and experimentally study a gradient index helicoid metamaterial that can modulate sound wavefront with nearly perfect transmission over a broad bandwidth. Taking advantage of the linear spectral phase shifting, a thin flat metalens is constructed to demonstrate sound focusing with the extraordinary transmission for more than 1/3 octave band. It is anticipated that this engineered material may be used in ultrasonography, ultrasound surgery, and DNA fragmentation. Moreover, with the flexible adjustment of effective parameters, it can be an ideal alternative to the anisotropic inhomogeneous medium and has much better stiffness.