Single-layer elastic metasurface with double negativity for anomalous refraction

Elastic metasurfaces are artificial thin layers composed of sub-wavelength structures designed to manipulate wave propagation such as anomalous refraction/reflection. Despite recent active researches, achieving a really thin metasurface has been a challenge, since it has been almost impossible to design a single unit to satisfy both the 2 pi phase span and the full transmission. In this paper, we revealed the way to achieve both conditions by a single unit so that a really thin elastic metasurface is possible. Here, we found that the single unit should have not only positive, but negative effective parameters to achieve both the 2 pi phase span and the full transmission. To realize such a single unit, we design resonance-based metasurface units consisting of two types of resonators: vertical and horizontal resonators, which individually modulate effective stiffness and mass, respectively. By adjusting each resonator, any desired effective parameters from negative to positive values can be tuned, achieving both the 2 pi phase span and the full transmission. Numerical validations are carried out for the proposed metasurface unit. Finally, various single-unit elastic metasurfaces are designed with the proposed metasurface unit for anomalous refraction. Since extremely thin elastic metasurface consisting of a single unit is possible, we expect our research to be able to open a new field in wave tailoring applications.