Stress‐Induced 3D Chiral Fractal Metasurface for Enhanced and Stabilized Broadband Near‐Field Optical Chirality

Abstract

Metasurfaces comprising 3D chiral structures have shown great potential in chiroptical applications such as chiral optical components and sensing. So far, the main challenges lie in the nanofabrication and the limited operational bandwidth. Homogeneous and localized broadband near‐field optical chirality enhancement has not been achieved. Here, an effective nanofabrication method to create a 3D chiral metasurface with far‐ and near‐field broadband chiroptical properties is demonstrated. A focused ion beam is used to cut and stretch nanowires into 3D Archimedean spirals from stacked films. The 3D Archimedean spiral is a self‐similar chiral fractal structure sensitive to the chirality of light. The spiral exhibits far‐ and near‐field broadband chiroptical responses from 2 to 8 µm. With circularly polarized light (CPL), the spiral shows superior far‐field transmission dissymmetry and handedness‐dependent near‐field localization. With linearly polarized excitation, homogeneous and highly enhanced broadband near‐field optical chirality is generated at a stably localized position inside the spiral. The effective yet straightforward fabrication strategy allows easy fabrication of 3D chiral structures with superior broadband far‐field chiroptical response as well as strongly enhanced and stably localized broadband near‐field optical chirality. The reported method and chiral metasurface may find applications in broadband chiral optics and chiral sensing.

Chiral Plasmonics and Their Potential for Point-of-Care Biosensing Applications

“…Despite the advantages of this type of chiral plasmonic structure, setting nanoparticles at customized regions for high chiral interactions constitutes a major hurdle for practical biosensing applications. A recent approach to circumvent this latter issue is the use of plasmonic chiral metasurfaces [46][47][48][49][50][51][52][53][54], where several 2D plasmonic arrays with symmetry broken along the third dimension have been developed. These metasurfaces have their chiral plasmonic near-fields directly accessible to nearby molecules on the surface.…”

Kirigami/origami: unfolding the new regime of advanced 3D microfabrication/nanofabrication with “folding”

“…9i), which exhibited extraordinary chiral dissymmetry in transmission and broadband near-field chiroptical responses from 2 to 8 µm (Fig. 9j) 109 . With the wide adoption of FIB-based nano-kirigami and related techniques, generation of useful 3D nanogeometries to enrich the field of metasurfaces for the exploration of new physics and novel applications can be envisioned.…”

“…h Measured LCP and RCP transmission spectra of three-arm pinwheels arranged in a hexagonal lattice and corresponding CD spectrum 108 . i SEM image of a 3D chiral fractal metasurface and j its LCP and RCP transmission spectra 109 . Scale bars: 1 μm.…”

“…i, j Reprinted from ref. 109 with permission from Wiley polymer and mechanically compressed and released the structure with a fibre tip, as shown in Fig. 10g 108 .…”

Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

“…In this technique [ 112 ], the focused ion beam irradiates a freestanding metal/dielectric films. The 3D Archimedean spirals are stacked from a bilayer planar film of Au/Si 3 N 4 (50 nm Au and 50 nm, respectively), as schematically shown in Figure 5 i.…”