Metasurfaces, the phase‐engineered quasi‐2D interfaces, have attracted intensive interest due to their great capabilities in manipulating the reflection, refraction and transmission of electromagnetic waves. Here, we demonstrate the design and realization of a gradient chiral metamirror tailored for spin‐selective anomalous reflection based on the theory of Pancharatnam‐Berry phase. Asymmetric split ring resonators are employed as the basic meta‐atoms for strong circular dichroism. Dispersionless phase discontinuities are achieved by adjusting the orientation of the meta‐atoms, and spin‐dependent absorption is realized by introducing a chiral resonance. Theoretical results predict both broadband beam deflection and spin‐selective absorption for circularly polarized waves in a designer metamirror. Experimental verification of this bifunctional performance is implemented at microwave frequencies and the measured results agree well with the simulation ones. Such chiral metamirrors could pave an avenue towards spin‐selective modulation of the wavefront and might find promising applications in planar electromagnetic devices.

We predict the existence of confined transverse electric (TE) phonon polaritons in an ultrathin hexagonal boron nitride (hBN) slab below hBN's second transverse optical frequency. The skin depth of TE phonon polaritons can be decreased to subwavelength scale by increasing the thickness of hBN to several nanometers. Due to the strong spatial confinement, these TE phonon polaritons, different from TE graphene plasmons, can stably exist even when the permittivities of the superstrate and substrate are largely different.These revealed advantages of TE phonon polaritons might lead to potential applications of hBN in the manipulation of TE waves, such as the design of novel waveguides, polarizers, and the exploration of negative refraction between TE polaritons.

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The ability to control the polarization of light at the extreme nanoscale has long been a major scientific and technological goal for photonics. Here we predict the phenomenon of polarization splitting through van der Waals heterostructures of nanoscale thickness, such as graphene-hexagonal boron nitride (hBN) heterostructures, at infrared frequencies. The underlying mechanism is that the designed heterostructures possess an effective relative permittivity with its in-plane (out-of-plane) component being unity (zero); such heterostructures are transparent to the transverse-electric (TE) waves while opaque to the transverse-magnetic (TM) waves, without resorting to the interference effect.Moreover, the predicted phenomenon is insensitive to incident angles. Our work thus indicates that van der Waals heterostructures are a promising nanoscale platform for the manipulation of light, such as the design of polarization beam nano-splitters and epsilon-near-zero materials, and the exploration of superscattering for TM waves while zero scattering for TE waves from deep-subwavelength nanostructures.

Maxwell's equations along with the space-time symmetry in normal conducting materials reveal that the electric permittivity and magnetic permeability tensors of rank 2 are fully determined from the conductivity tensor of rank 2, and therefore one needs a dynamical model only for the latter. We elucidate this unification of response tensors and study its ramifications by explicit constructions for the cubic, tetragonal, and orthorhombic classes of crystals. We next use the Boltzmann transport equation in conjunction with a semiclassical model for the statistics of fermionic charge carriers in the material to obtain an analytical expression for the wave vector and frequency-dependent conductivity tensor and thence the permittivity and permeability tensors for metals possessing spherical and nonspherical Fermi surface. We find, inter alia, the spatial dispersion in various response tensors as an important component for a realistic description of optical properties of metals. We ascertain the efficacy of the present theory by computing frequency and wavevectordependent response tensors and other optical properties of a model metallic system, as a representative example. Final expressions for response tensors are suitable for immediate use in Maxwell's equations to study problems, for example, in subwavelength and near-field optics, plasmonic devices, photonic crystals, and surface-enhanced spectroscopies.