Numerical investigation of terahertz polarization-independent multiband ultrahigh refractive index metamaterial by bilayer metallic rectangular ring structure

Ultrathin metasurface on a 100 nm-thick dielectric membrane absorbs infrared rays

Flat optics based on metasurfaces produce unprecedented two-dimensional planar optical elements that cannot be developed with naturally occurring materials. However, it remains to be shown whether metasurfaces on ultrathin dielectric membranes can be adopted in a broad range of optical elements as flat optics. Here we demonstrate that a fabricated ultrathin metasurface composed of double-sided metal structures on a 100 nm-thick SiNx membrane absorbs infrared rays with a high absorptance of 97.1% at 50.1 THz. This ultrathin metasurface and its fabrication method would be a welcome contribution to a wide range of trailblazing applications, including ultrathin absorbers for imaging and light detection and ranging (LIDAR), directivity control of thermal radiation, and polarization control of vacuum ultraviolet light.

Metalens mounted on a resonant tunneling diode for collimated and directed terahertz waves

Refraction in materials is a fundamental phenomenon in optics and is a factor in the manipulation of light, such as wavefront shaping and beam control. However, conventional optical lenses incorporated in numerous optical sources are made of naturally occurring materials, and material properties predetermine the lens performance. For the development of terahertz flat optics, we experimentally demonstrate a gradient-refractive-index (GRIN) collimating metalens made of our original reflectionless metasurface with an extremely high refractive index, above 10 at 0.312 THz. The planar collimating metalens converts wide-angle radiation from a resonant tunneling diode (RTD) to a collimated plane wave and enhances the directivity of a single RTD 4.2 times. We also demonstrate directional angle control of terahertz waves by moving the metalens in parallel with the incoming wave. The metalens can be simply integrated with a variety of terahertz continuous-wave (CW) sources for 6G (beyond 5G) wireless communications and imaging in future advanced applications. Flat optics based on high refractive index metasurfaces rather than naturally occurring materials can offer an accessible platform for optical devices with unprecedented functionalities.

Reflectionless metasurface with high refractive index in the terahertz waveband

Terahertz flat optics is a design concept for replacing conventional three-dimensional bulky optical components with two-dimensional ultra-thin optical components. However, high refractive index materials suitable for flat optics are frequently subject to high Fresnel reflections due to the cumbersome control of the relative permeability it requires. Here we experimentally demonstrate a reflectionless metasurface with a high refractive index of 5.88 + j1.57, extremely low reflectance of 1.3%, high relative permittivity of 6.73 + j0.85, and the high relative permeability of 5.03 + j2.11 at 2.97 THz. The super-fine ink-jet printer using silver paste ink fabricates the metasurface consisting of 80,036 pairs of cut metal wires on both the front and back of a 5 μm-thick polyimide film. The findings also demonstrate that weak conductors as well as good conductors can be used in the design of reflectionless metasurfaces with a high refractive index in the terahertz waveband. The presented metasurface can offer an accessible platform for terahertz flat optics in 6G (beyond 5G) wireless communications and imaging.

Terahertz focusing metalens of reflectionless meta-atoms with negative refractive indices

Terahertz continuous-wave (CW) sources oscillating around the 1.0 THz band at room temperature have rapidly been developed to bridge the terahertz gap. However, reflectionless metasurfaces suitable for integration with terahertz CW sources as optical components have yet to be developed in the terahertz gap. Here, we propose a terahertz-focusing metalens consisting of reflectionless meta-atoms with a discrete distribution of negative refractive indices from ${-}{1.1}$ to ${-}{2.8}$. The proposed 2D gradient-refractive-index metalens converts an incident terahertz Gaussian beam to a line focus. We also experimentally demonstrate a metasurface of reflectionless meta-atoms with a negative refractive index of ${-}{2.8}$ adopted in the periphery of the metalens. The reflectionless metasurface in the terahertz gap would be a welcome contribution to the rapid growth of terahertz industrial applications with terahertz CW sources. Further, the design approach based on reflectionless meta-atoms with negative refractive indices could be applied to various 2D planar optical components with attractive functionalities such as collimating, arbitrary wavefront shaping, and light vortices.

Collective photonic response of high refractive index dielectric metasurfaces

Sub-wavelength periodic nanostructures give rise to interesting optical phenomena like effective refractive index, perfect absorption, cloaking, etc. However, such structures are usually metallic which results in high dissipative losses and limitations for use; therefore, dielectric nanostructures are increasingly considered as a strong alternative to plasmonic (metallic) materials. In this work, we show light-matter interaction in a high refractive index dielectric metasurface consisting of an array of cubic dielectric nano-structures made of very high refractive index material, Te in air, using computer modelling. We observe a distinct band-like structure in both transmission and reflection spectra resulting from the near-field coupling of the field modes from neighboring dielectric structures followed by a sharp peak in the transmission at higher frequencies. From the spatial distribution of the electric and magnetic fields and a detailed multipole analysis in both spherical harmonics and Cartesian components, the dominant resonant modes are identified to be electric and magnetic dipoles. Specifically at lower frequency (60 THz) a novel anapole-like state characterized by strong-suppression in reflection and absorption is observed, reported very recently as 'lattice-invisibility' state. Differently, at higher frequency (62 THz), strong absorption and near-zero far field scattering are observed, which combined with the field profiles and the multipole analysis of the near-fields indicate the excitation of an anapole. Notably the observed novel modes occur in the simple geometry of dielectric cubes and are a result of collective response of the metasurfaces. Periodicity of the cubic metasurface is shown as the significant material tuning parameter, allowing for the near-field and far-field coupling effects of anapole metasurface. More generally, our work is a contribution towards developing far-fetching applications based on metamaterials such as integrated devices and waveguides consisting of non-radiating modes.