Elastic metasurfaces with tailored initial phase for broadband subwavelength focusing

Subwavelength focusing using elastic metasurfaces has attracted widespread interest in the past decade, yet most of the designs available for energy focusing tend to have downsides such as narrow frequency bands and low modulation efficiency. In order to broaden the operating frequency, the existing strategies are dedicated into active control or nonlocal design, which unavoidably increases the complexity of the system. As for passive waveguide structures, a structural re-design of the unit cell or a complex optimization is normally required. Achieving broadband focusing performance without changing the original unit cell configuration is still highly desired. Here, a new broadband design paradigm based on the initial phase dependency of elastic metasurfaces is proposed. The initial phase dependency of a zigzag-type metasurface is systematically investigated and tailored for broadband subwavelength focusing. Numerical simulations and experimental measurements show that the operating frequency range has a significant dependency on the initial phase. A selection criterion of an appropriate initial phase is provided to facilitate the band broadening. The trend of the initial phase selection with respect to the operating frequency band is mathematically and theoretically evaluated by introducing the concepts of phase deviation and equivalent phase velocity difference. This work proposed a new avenue for broadband elastic wave focusing without increasing system complexity, which is of great significance for understanding the coupling mechanism between the initial phase dependency and broadband performance. This design is expected to serve as a general strategy to enable various steering scenarios in metasurfaces and different wave systems.