Strain‐Enabled Phase Transition of Periodic Metasurfaces

Abstract

metasurfaces operate like their analogues of solid-state matters. [2] This understanding is incessantly nourishing the field of metasurfaces. Among them, periodic metasurfaces, where meta-atoms are arranged in periodic lattices, are one of the most prevailing structures (Figure S1, Supporting Information). [1,[7][8][9][10][11][12] Like natural crystals, periodic metasurfaces support 2D Bragg modes due to long-range coherent interactions between meta-atoms, leading to a strong dispersion of the effective permittivity or permeability, dependent on the geometry of lattices. As a result, metasurfaces with various 2D lattices offer a broad spectrum of interesting optical properties. The engineered dispersion yields high-Q (quality factor) resonance reflection and/ or absorption, and has enabled them to be ideal platforms for chemical sensing [11,13] and the coupling with absorptive and excitonic materials. [11,[14][15][16][17][18][19][20][21] Furthermore, the frequency of emerging resonance sensitively depends on lattices' geometry and periodicity, [8,13] offering accessible degrees of freedom to dynamically control the resonances of metasurfaces, and thus the regime of strong coupling. These degrees of freedom form the design principle for active metasurfaces, crucial for the photonic and communication systems. [22][23][24][25] Active metasurfaces are driven in various ways, including electrical gating, optical pumping, mechanical actuation, etc. [24,26,27] Deformable metasurfaces, whose optical responses are directly tuned through spatial re-arrangement of metaatoms by mechanical actuation, constitute a large category of active metasurfaces, [12,28] which have benefitted from the maturation of micro-electro-mechanical (MEMS) systems. These optical responses result in functional mechanical reconfigurable metasurfaces, such as adaptive metalens, [28,29] tunable holograms, [30] and variable gratings. [31] In particular, periodic lattices, which are inherently sensitive to the spacing between meta-atoms, have shown remarkable tuneability in structural colors and lasing wavelength under strains. [12,[32][33][34][35] Although the deformation of periodic metasurfaces under strain has been reported frequently, [12,[32][33][34] there remain no universal ways to describe strain-induced lattice deformation of periodic metasurfaces. As a result, one may neglect the accessible structures, and fail to discover interesting optical properties of metasurfaces under mechanical actuation.Here, we report a universal way to access arbitrary phases of periodic metasurfaces by applying strains. We introduce the crystallographic terminology of 2D Bravais lattices, [36] Phase transitions are universal in solid-state matters, as well as in periodic electromagnetic metasurfaces-the photonic analogues of crystals. Although such transitions dictate the properties of active metasurfaces, universal ways to describe the structure transition of periodic metasurfaces have not yet been established. Here, the authors report the strain-enabled phase tr...

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