Materials and 3D Designs of Helix Nanostructures for Chirality at Optical Frequencies

Abstract

The capability to fully control the chiro‐optical properties of metamaterials has drawn considerable attention due to developments in metamaterial fabrication techniques and a deeper understanding of the light–matter interaction, enabling a number of applications from integrated photonics to life sciences. From simple geometrical studies on single helix shaped metamaterials, both the design complexity and nanofabrication capability are steadily increasing allowing to extend their properties to the visible regime and beyond. In this work, helix‐shaped micro‐ and nanostructures are reviewed, which have the possibility to modulate their optical response in the linear regime at optical frequencies by spatial arrangement or by constitutive material. An overview of different theories describing the general optical operation of chiral media and a discussion on experimental results obtained by the state‐of‐the‐art helix structures realized by different fabrication techniques is provided. Finally, advanced designs for improved functionality envisaged to match practical applications or to enable new optical multifunctionalities are compared. Even if many applications have been envisaged only theoretically, the continuous effort and progress shown by the research community engaged in this field suggests that helical chiral metamaterials could represent a disruptive breakthrough for advanced optical devices.

Large‐Area 3D‐Printed Chiral Metasurface Composed of Metal Helices

“…For instance, the direct laser writing technique, featuring submicrometer resolution, can be used for printing polymer substrates for subsequent formation of double‐layer grating of metallic ribbon microhelices. The metal master‐molds with 3D “helical” sub‐micrometer relief can be formed by the focused ion and electronbeam‐induced deposition methods . Such molds can be used multiple times for cast replication of flexible PDMS substrates and, then, assembling such substrates into larger‐area metasurfaces.…”

“…Such a helix has coils located too close together so that it becomes a very difficult problem to print it. As a rule, in printing wire‐type 3D helices the helix pitch is chosen two times exceeding the helix diameter ( P / D ≈ 2, α = 33°) . In the case of our six‐coil helices (Figure e–g), we have P / D = 2, and the length of the screw line going along the centerline of the thin‐film ribbon in a coil is 4.1 mm.…”

“…In the past decade, great interest has been expressed in 3D microstructures and nanostructures used as electrically active resonator elements in metamaterials, photonic crystals, and transformation optics . On the basis of 3D helices and other structures, chiral metamaterials with an optical activity several hundred times exceeding that of natural optical crystals were fabricated . Chiral metamaterials are highly promising materials for ultrathin polarizers, modulators, filters, nonreflective and highly absorptive coatings, negative refraction index materials, measuring and medical apparatuses, and for many other applications …”

Intelligent nanophotonics: merging photonics and artificial intelligence at the nanoscale

“…An object is chiral if it cannot be superimposed to its mirror image. Due to its universal existence in nature, chirality has aroused enormous research interests [140][141][142][143][144]. From a nanophotonic point of view, designing chiral nanostructures that respond to chiral light (usually left/right circularly polarized (LCP/RCP) light) differently is of both fundamental and application-wise importance.…”

Light polarization management via reflection from arrays of sub-wavelength metallic twisted bands

“…Further theoretical (numerical) studies followed, focusing on the parameter optimization in terms of transmission, contrast and bandwidth [8][9][10]. In [11], upon comparing metal and dielectric helices, it was discovered that a trade-off exists between the transmission efficiency and bandwidth in this class of circular polarizers and other all-dielectric structures with interesting polarizing properties have also been demonstrated [12][13][14][15][16].…”