Spin-controlled wavefront shaping with plasmonic chiral geometric metasurfaces

Abstract

Metasurfaces, as a two-dimensional (2D) version of metamaterials, have drawn considerable attention for their revolutionary capability in manipulating the amplitude, phase, and polarization of light. As one of the most important types of metasurfaces, geometric metasurfaces provide a versatile platform for controlling optical phase distributions due to the geometric nature of the generated phase profile. However, it remains a great challenge to design geometric metasurfaces for realizing spin-switchable functionalities because the generated phase profile with the converted spin is reversed once the handedness of the incident beam is switched. Here, we propose and experimentally demonstrate chiral geometric metasurfaces based on intrinsically chiral plasmonic stepped nanoapertures with a simultaneously high circular dichroism in transmission (CDT) and large cross-polarization ratio (CPR) in transmitted light to exhibit spin-controlled wavefront shaping capabilities. The chiral geometric metasurfaces are constructed by merging two independently designed subarrays of the two enantiomers for the stepped nanoaperture. Under a certain incident handedness, the transmission from one subarray is allowed, while the transmission from the other subarray is strongly prohibited. The merged metasurface then only exhibits the transmitted signal with the phase profile of one subarray, which can be switched by changing the incident handedness. Based on the chiral geometric metasurface, both chiral metasurface holograms and the spin-dependent generation of hybrid-order Poincaré sphere beams are experimentally realized. Our approach promises further applications in spin-controlled metasurface devices for complex beam conversion, image processing, optical trapping, and optical communications.

Spin-preserving chiral photonic crystal mirror

“…In contrast, the majority of the experimental demonstrations of chiroptical effects in visible and near infrared range have reported circular dichroism in transmission. 8,[14][15][16][17][18][20][21][22]29,46 Among these, the most notable ones include a demonstration of a chiral metasurface by Zhu et al 8 , which achieved spin filtering with an extinction ratio of ∼ 9 : 1 while almost ∼ 90% of light with the selected helicity was transmitted, and a planarized chiral structure made of multiple layers of twisted metamaterials by Zhao et al 16 . The latter structure exhibits almost ∼ 30% transmission difference (and the extinction ratio of Our PC mirror outperforms most of the chiral mirrors realized to date in most categories, although its operational mechanism introduces some sensitivity to the angle of incidence and its resonant nature gives rise to a relatively narrow operation band and a certain susceptibility to fabrication imperfections.…”

“…For this to occur, according to a long-held notion, the structure has to be composed of complicated three-dimensional chiral elements, such as helices or, alternatively, made of multi-layer patterns carrying structural chirality. 1,7 To date, several demonstrations of plasmon-assisted intrinsic chiroptical responses in metastructures composed of subwavelength array of 3D chiral shapes 7,[13][14][15] or multilayer patterns of mirrorsymmetry-broken structures [16][17][18][19][20][21] have been reported. Although such structures may exhibit strong and wide-band chiaroptical response, their fabrication is not compatible with 2D patterning techniques.…”

A Multi‐Foci Metalens with Polarization‐Rotated Focal Points

“…Optical metasurfaces, 2D counterparts of metamaterials, have provided unprecedented capabilities in the modification of light wavefront, enabling a plethora of novel applications such as generalized Snell's law of refraction, light beam shaping, Spin‐Hall effects, holograms, polarization control and analysis, and nonlinear dynamics . In comparison with traditional bulk lenses that rely on the required gradual phase change accomplished by controlling surface profile of the optical material, metalenses are ultrathin and ultraflat, which is desirable for device miniaturization and system integration.…”

Design and Experimental Demonstration of Impedance-Matched Circular-Polarization-Selective Surfaces with Spin-Selective Phase Modulations

“…For example, at the cost of increased complexity in fabrication, a curved CPSS has been demonstrated in [26] where the CPSS is physically shaped to obtain the necessary reflection phase shifts from the wave propagation. On the other hand, planar phase-gradient chiral metasurfaces have also been demonstrated that take advantage of the geometrical phase shift [27][28][29][30]. In particular, these phase-gradient chiral metasurfaces realize spin-selective phase modulations by rotating individual chiral unit cells to acquire the Pancharatnam-Berry phase shifts along their surfaces [31].…”