Giant chiroptical responses routinely occur in three dimensional chiral metamaterials (MMs), but their resonance elements with complex subwavelength chiral shapes are challenging to fabricate in the optical region. Here, we propose a new paradigm for obtaining strong circular conversion dichroism (CCD) based on extrinsic 2D chirality in multilayer achiral MMs, showing that giant chiroptical response can be alternatively attained without complex structures. Our structure consists of an array of thin Au squares separated from a continuous Au film by a GaAs dielectric layer, where the Au squares occupy the sites of a rectangular lattice. This structure gives rise to a pronounced extrinsically 2D-chiral effect (CCD) in the mid-infrared (M-IR) region under an oblique incidence, where the 2D-chiral effect is due to the mutual orientation of the Au squares array and the incident light propagation direction; the large magnitude of CCD due to the large difference between left-to-left and right-to-right circularly polarized reflectance conversion efficiencies.

Many previously observed strong chiroptical effects are limited to chiral metamaterials with complex three-dimensional building blocks. Recently, chiral metamirrors consisting of planar elements are proposed to selectively reflect one particular circularly polarized light (CPL) while absorbing the other, with the reflected one either preserving or altering its spin state. However, they are limited by complicated subwavelength chiral profiles of their unit cells, which are challenging to fabricate in the visible-near-infrared (NIR) region. We present an extrinsically chiral metamirror that reflects one CPL preserving its handedness while strongly absorbing the other, reaching a circular conversion dichroism (CCD) in reflectance as large as 0.43 in the NIR region. Our polarization-conserving mirror possesses an Au pillar array and a continuous Al film separated by a SiO 2 dielectric layer, where the Au pillars are periodically arranged by a rectangular lattice. The rather simple geometry of mirror significantly simplifies its fabrication in the high frequency region. The pronounced CCD originates from a chiral triad of the achiral resonator array and light's propagation direction. Such a chiral mirror provides an efficient methodology toward handedness-selective modulation of the CPL and finds applications in planar photonic devices such as molecular spectroscopy, quantum information, and polarimetric imaging.

Control of the polarization of light is highly desirable for detection of material’s chirality since biomolecules have vibrational modes in the optical region. Here, we report an ultrafast tuning of pronounced circular conversion dichroism (CCD) in the mid-infrared (M-IR) region, using an achiral phase change metamaterial (PCMM). Our structure consists of an array of Au squares separated from a continuous Au film by a phase change material (Ge2Sb2Te5) dielectric layer, where the Au square patches occupy the sites of a rectangular lattice. The extrinsically giant 2D chirality appears provided that the rectangular array of the Au squares is illuminated at an oblique incidence, and accomplishes a wide tunable wavelength range between 2664 and 3912 nm in the M-IR regime by switching between the amorphous and crystalline states of the Ge2Sb2Te5. A photothermal model is investigated to study the temporal variation of the temperature of the Ge2Sb2Te5 layer, and shows the advantage of fast transiting the phase of Ge2Sb2Te5 of 3.2 ns under an ultralow incident light intensity of 1.9 μW/μm2. Our design is straightforward to fabricate and will be a promising candidate for controlling electromagnetic (EM) wave in the optical region.

Separating enantiomers is vital in chemical syntheses, life sciences, and physics. However, the usual chemical processes are inefficient. Recently, plasmonic nanostructures have drawn considerable attention for manipulating nanoparticles; however, only a few approaches are proposed to discriminate between entities that differ in terms of their handedness. This is because the chiral polarizability is much smaller than the electric polarizability, and therefore the non‐chiral gradient force dominates over the chiral gradient force. This limit means that the enantioselective sorting of chiral nanoparticles is a formidable challenge. A plasmonic nanostructure consisting of a disc‐double split ring resonator exhibiting a dipole–octupole (DO) Fano resonance (FR) is designed and fabricated. It is theoretically demonstrated that such a DO‐FR can markedly enhance the chiral gradient force on the paired enantiomers. The coaxial channel of the resonator possessing high chirality density gradients around the DO‐FR is derived. This provides an enhanced chiral gradient force that dominates over the non‐chiral gradient forces on sub‐10 nm chiral nanoparticles. Enantiomeric pairs can thus experience distinct potential wells in terms of signs. This proposed structure may advance the techniques of enantiopurification and enantioseparation, bringing a new perspective to state‐of‐the‐art all‐optical enantiopure synthesis.