Twisted optical metamaterials for planarized ultrathin broadband circular polarizers

Abstract

optical metamaterials are usually based on planarized, complex-shaped, resonant nanoinclusions. Three-dimensional geometries may provide a wider set of functionalities, including broadband chirality to manipulate circular polarization at the nanoscale, but their fabrication becomes challenging as their dimensions get smaller. Here we introduce a new paradigm for the realization of optical metamaterials, showing that three-dimensional effects may be obtained without complicated inclusions, but instead by tailoring the relative orientation within the lattice. We apply this concept to realize planarized, broadband bianisotropic metamaterials as stacked nanorod arrays with a tailored rotational twist. Because of the coupling among closely spaced twisted plasmonic metasurfaces, metamaterials realized with conventional lithography may effectively operate as three-dimensional helical structures with broadband bianisotropic optical response. The proposed concept is also shown to relax alignment requirements common in three-dimensional metamaterial designs. The realized sample constitutes an ultrathin, broadband circular polarizer that may be directly integrated within nanophotonic systems.

Symmetry breaking and optical negative index of closed nanorings

“…We fabricated two layers of shifted gold nanorings on quartz substrates with planarized SU-8 photoresist as spacer. Multiple electron beam lithography processes were employed for the alignment between the layers 22,23 .…”

Ultra-Compact Multi-Band Chiral Metamaterial Circular Polarizer Based on Triple Twisted Split-Ring Resonator

“…In effect, the coupling mechanisms have been studied in detail at microwave and optical frequencies. Optical activity and CD effect have also been observed [24,25]. On these peculiarities, the giant CD effect for circular polarization is very important, which has many potential applications, such as antennas [26], liquid crystal displays and remote sensors [27,28].…”

“…However, multi-band or bandwidth expansion is still a main concern which needs improvement. An effective method is stacking metallic-dielectric multilayer planar structure [40], which gives some new ideas for multi-band or broadband device design for polarization transformation [11,25,40,41].…”

Dual-Band Circular Polarizer and Asymmetric Spectrum Filter Using Ultrathin Compact Chiral Metamaterial

“…The unusual polarization functionalities and chirality not found in natural medium arise from the magneto-electric cross coupling of chiral structures due to their bianisotropy and lack of any mirror symmetry. After the extensive theoretical or numerical studies, and experimental realizations, there have been many efforts tending toward the practical applications of the CMMs in manipulating the polarization states of EM wave [16][17][18][19][20][21][22][23][24][25][26][27][28][29], polarization spectral filtering [30], improving the gain and axial ratio of circularly polarized antennas [31], subwavelength focusing and imaging [32], as well as microwave absorbers [33]. A set of circular polarizers [16][17][18][19][20][21][22][23][24] and pure linear polarization rotators [25][26][27][28][29] have been intensively reported.…”

“…Among them, several chiral structures have been available to tailor the cross coupling between electric and magnetic fields such as the commonly utilized U-shaped structures [1,15,21,29], conjugated metallic pairs and conjugated gammadion resonators combining cut-wire pairs [27,28], twisted rosettes [2,3,5], cross-wire structure (gammadion) and its complementary variations [9,11,20,31], twisted foil structure [7,26], L-shaped chiral structure [8], fishscale pattern [10,11], twisted split ring resonators (SRRs), twist Hilbert patterns, and twisted complementary SRRs with symmetric or asymmetric geometry [12-14, 23, 24, 30, 33], Metallic helix array [16,18], stacked nanorod arrays [19], twist arc structure [22], and I-shaped structure [25]. However, most "meta-atoms" or "meta-molecule" of CMMs are extremely large, this is especially true when arrange the four U-shaped rings in four-fold rotational symmetry (C4) to obtain the optical activity insensitive to the linear polarization of the incident wave.…”