Terahertz time domain spectroscopy and rigorous simulations are used to probe the coupling between a dark and a bright plasmonic eigenmode in a metamaterial with broken symmetry. The metamaterial consists of two closely spaced split ring resonators that have their gaps in nonidentical positions within the ring. For normal incidence and a fixed polarization both lowest-order eigenmodes of the split ring resonators can be excited, although one of them has to be regarded as dark since coupling is prohibited because of symmetry constraints. Emphasis in this work is put on a systematic evaluation of the coupling effects depending on a spectral tuning of both resonances.

Polarization conveys valuable information for electromagnetic signal processing exhibiting tremendous potential in developing application driven photonic devices. Manipulation of polarization state of an electromagnetic wave has drawn a lot of research interests in many different fields, especially in the terahertz regime. Here, we propose a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces. We experimentally observe a nearly perfect orthogonal polarization conversion with an ultrahigh efficiency, demonstrating a ultrathin terahetz rotator. The Fabry-Perot cavity effect in the tri-layer metasurfaces is attributed to the underlying mechanism of high transmittance and polarization rotation.

Spatial modulation of sound velocity below the wavelength scale can introduce strong frequency-dependent acoustic responses in tailored composite materials, regardless the fact that most natural bulk materials have negligible acoustic dispersions. Here, for the first time, we experimentally demonstrate a metamaterial that traps broadband acoustic waves and spatially separates different frequency components, as the result of dispersion and wave velocity control by designed gradient subwavelength structures. The trapping positions can be predicted by the microscopic picture of balanced interplay between the acoustic resonance inside individual apertures and the mutual coupling among them. With the enhanced wave-structure interactions and the tailored frequency responses, such metamaterial allows precise spatial-spectral control of acoustic waves and opens new venue for high performance acoustic wave sensing, filtering, and nondestructive metrology.

By utilizing the vector nature of light as well as the inherent anisotropy of artificial meta-atoms, we investigate parity time symmetry breaking in polarization space using a metasurface with anisotropic absorption, whose building blocks consist of two orthogonally orientated meta-atoms with the same resonant frequency but different loss coefficients. By varying their coupling strength, we directly observe a phase transition in the eigenpolarization states of the system, across which the long axis of the eigenpolarization ellipses experience a sudden rotation of 45°. Despite the lack of rotational symmetry of the metasurface, precisely at the phase transition, known as the exceptional point, the eigenmodes coalesce into a single circularly polarized state. The $PT$ symmetric metasurfaces are experimentally implemented at terahertz frequencies.