Mechano-optical metasurfaces

Abstract

Tunable metasurfaces enable active and on-demand control over optical wavefronts through reconfigurable scattering of resonant nanostructures. Here, we present novel insights inspired by mechanical metamaterials to achieve giant tunability in mechano-optical metasurfaces where the mechanical metamaterial and optical metasurfaces are integrated in a single nanopatterned material. In a first design, judiciously engineered cuts in a flexible substrate enable large, strain-induced extension of the inter-particle spacing, tuning a high quality-factor resonance in a silicon nanoparticle array across a very broad spectral range. In a second design, we eliminate the substrate and demonstrate a nanopatterned silicon membrane that simultaneously functions as a mechanical metamaterial and an optical metasurface with large tunability. Our results highlight a promising route toward active metasurfaces, with potential applications in tunable filters, reconfigurable lenses, and dynamic wavefront shaping.

Figures (5)

• Figure 1: Mechanical deformation and tunable optical response of an idealized kirigami metasurface. (a-c) Schematics of 3$\times$3 unit cell sections of the kirigami metasurface under increasing levels of strain: minimal strain ($\theta=1$°) (a), medium strain ($\theta=15$°) (b), and large strain ($\theta=44$°) (c). Each elliptical nanoparticle is centered on a rectangular tile measuring 250$\times$400 nm. (d) With increasing strain in the x-direction, the tiles — and thus the particles — are rotated by an angle $\theta$ (solid, blue) and displaced, leading to changes in the inter-particle spacing (dashed, orange). (e) Reflectance spectra for the arrays shown in (a-c) illustrating the giant tunability of the high quality factor resonant peak across the visible spectrum from green ($\sim$500 nm) through yellow ($\sim$580 nm) to red ($\sim$650 nm), with zoomed-in sections near these wavelengths to highlight the Fano lineshape. (f) Strain dependence of the resonance wavelength (blue triangles) and quality factor $Q$ (orange circles).
• Figure 2: Electric field intensity profiles, normalized to the source intensity, for the idealized kirigami metasurface. Gray (arrow) lines indicate the electric field lines in the plane. Overlaid white arrows indicate the effective dipole moments. (a) The undeformed state ($\theta=0$°) displays negligible field enhancement. (b) Small strain condition ($\theta = 15^\circ$) and (c) large strain condition ($\theta = 44^\circ )$ show enhanced localized field intensities.
• Figure 3: Mechanical deformation and tunable optical response of the beam-linked metasurface(a-c) Schematics of 2$\times$2 unit cell sections of the kirigami-inspired metasurface: the initial undeformed state (a) shows no rotation of the elliptical nanoparticles, while moderate strain ($\theta=10$°) (b), and large strain ($\theta=38$°) (c) show increasing rotation and inter-particle spacing.(d) Particle rotation angle $\theta$ (solid, blue) and displacement (dashed, orange) as functions of strain in the x-direction. (e) Simulated stress distribution for $\theta=60$°, showing strong localization of the stress at the beams. (f) Reflectance spectra for the arrays shown in (a-c), illustrating the large tunability of the resonant peak across the visible spectrum from yellow ($\sim$580 nm) through orange ($\sim$610 nm) to red ($\sim$640 nm). (g) Strain-dependence of the resonance wavelength (blue triangles) and quality factor $Q$ (orange circles), with a notable overall decrease in comparison to Fig. \ref{['Fig1']}f.
• Figure 4: Electric field intensity ($|E|^2$) profiles, normalized to the source intensity, for the beam-linked metasurface. Gray (arrow) lines indicate the electric field lines in the plane. Overlaid white arrows indicate the effective dipole moments. a The undeformed state ($\theta=0$°) already displays significant field enhancement. (b) Moderate strain condition ($\theta = 30^\circ$) and (c) high strain condition ($\theta = 60^\circ )$ also show field enhancement.
• Figure 5: Finite size effects of the realistic kirigami metasurface. (a) A finite size metasurface (20x20 unit cells) with rigid mechanical handholds on either side, showing variations in local deformation. The color of the particles indicates deviation in the local strain with respect to the bulk strain. (b) Zoomed-in view of the local strain distribution near the handholds highlighting localized deformation. (c) Local strain distributions across the structure for three strain values, each showing a strain plateau with roughly equal lateral size (between vertical dashed lines). (d) Comparison of the reflectance spectra between the finite array (solid blue curves) and an infinite array (dashed orange curves), illustrating how non-uniform stretching affects the overall optical properties, resulting in a reflectance peak with reduced amplitude and larger linewidth for the finite array.