Tapered N-helical metamaterials with three-fold rotational symmetry as improved circular polarizers

Abstract

Abstract:Chiral helix-based metamaterials can potentially serve as compact and broadband circular polarizers. We have recently shown that the physics of structures composed of multiple intertwined helices, so called N-helices with N being an integer multiple of 4, is distinct from that of structures made of single circular helices (N = 1). In particular, undesired circular polarization conversion is strictly eliminated for N = 4 helices arranged on a square lattice. However, the fabrication of such structures for infrared/visible operation wavelengths still poses very significant challenges. Thus, we here revisit the possibility of reducing N from 4 to 3, which would ease micro-fabrication considerably. We show analytically that N = 3 helices arranged on a hexagonal lattice exhibit strictly vanishing circular polarization conversion. N = 3 is the smallest option as N = 2 obviously leads to linear birefringence. To additionally improve the circular-polarizer operation bandwidth and the extinction ratio while maintaining high transmission for the wanted polarization and zero conversion, we also investigate by numerical calculations N = 3 helices with tapered diameter along the helix axis. We find operation bandwidths as large as 2.4 octaves.

Optical and Infrared Helical Metamaterials

“…By decreasing, for example, the helix radius, the extinction ratio can be increased, however, at the cost of a decreased bandwidth. Tapered N = 3 helices simultaneously lead to an increased extinction ratio and increased bandwidth of up to values of more than two octaves [33]. The numerically calculated data are depicted in Figure 3(c).…”

“…For this design, however, losses are crucial. The necessity of losses in the system can be explained by a simple discussion exploiting symmetry arguments [32,33]. In fact, by recovering threefold or fourfold rotational symmetry, one eliminates circular polarization conversions, as illustrated in Figure 3(b).…”

“…Unfortunately, conversions are not eliminated solely in transmission but also in reflection. In reflection, on the other hand, the diagonal elements of the Jones matrix are generally equal because of reciprocity reasoning [33]. Therefore, for these so-called N-helical metamaterials where circular polarization conversions are eliminated, the reflectance spectra for both circular polarizations are equal and the undesired polarization cannot simply be reflected.…”

“…(C) By arranging N = 3-helices in a hexagonal lattice, circular polarization conversions can also be eliminated. By furthermore tapering the helix radius, bandwidth and extinction ratio can be optimized simultaneously, as depicted in the bottom panel (adapted with permission from [33]. Copyright 2014 OSA).…”

Strong Rotational Anisotropies Affect Nonlinear Chiral Metamaterials

“…This makes them attractive for applications involving CPL. [6,8,[37][38][39] Thus, the nonlinear optical response of helical metamaterials is of particular interest as they already demonstrate strong linear chiroptical effects. However, until recently it has been very difficult to fabricate high quality helical metamaterials for use at optical frequencies.…”