We demonstrate a bi-layer ultrathin anisotropic metasurface which could near-completely convert the circular-polarized electromagnetic wave to its cross polarization. The bi-layer metasurface is composed of periodic 180°-twisted double-cut split ring resonators on both sides of an F4B substrate. At resonance, cross-polarized transmission larger than 94% is observed both in simulations and experiments. The resonant frequency of the metasurface could be effectively tuned by adjusting the geometric parameters of the metasurface, while relatively high conversion efficiency is preserved. The high efficiency and ease of fabrication suggest that the ultrathin metasurface could have potential applications in telecommunications.

Some photonic systems support bound states in the continuum (BICs) that have infinite lifetimes, although their frequencies and momenta are matched to vacuum modes. Using a prototypical system that can be treated analytically, we show that each of these BICs always splits into a pair of new type BIC and lasing threshold mode when a parity-time (PT)–symmetric perturbation is introduced. The radiation loss at the lasing threshold is exactly balanced by the net gain of the particles. These PT symmetry-induced BICs are different from ordinary BICs, as they can be excited by an external source but do not radiate, and they carry a different quality factor divergence rate from that of the ordinary BICs. While most of the attention of PT-symmetric systems is captured by the coalescence of modes at exceptional points, the splitting of ordinary BICs is a new phenomenon that illustrates the rich physics embedded in PT-symmetric systems.

No abstract

The interference between conventional multipoles (e.g., electric and magnetic dipole, electric quadrupole, etc.) is known as the cause of unidirectional backward and forward scattering of nanoparticles. However, an unconventional multipole moment, toroidal dipole moment is generally overlooked in the unidirectional scattering. In this work, we systematically investigate the unidirectional scattering in the system of plasmonic nanoparticles. It is found that the toroidal dipole moment can play a significant role in the unidirectional backward scattering. The structural tunability of the unidirectional scattering is also demonstrated. Our results can find applications in the design of nanoantennas.

No abstract