Ultrathin Terahertz Planar Elements

Abstract

two authors contributed equally to this work.Abstract: Conventional optical components shape the wavefront of propagating light by adjusting the optical path length, which requires the use of rather thick lenses, especially for the adjustment of terahertz (THz) radiation due to its long wavelength. Two ultrathin THz planar lenses were designed and fabricated based on interface phase modulation of antenna resonance. The lens thicknesses were extremely reduced to 100 nm, which is only 1/4000 th of the illuminating light wavelength. The focusing and imaging functions of the lenses were experimentally demonstrated. The ultrathin optical components described herein are a significant step toward the development of a micro-integrated THz system. One-Sentence Summary: Ultrathin planar lenses with a thickness of 100 nm were designed and fabricated to implement THz beam focusing and imaging.Terahertz (THz) radiation lies in the frequency range between infrared and microwaves, typically having wavelengths ranging from 10 μm to 3 mm. THz technology is developing rapidly in many independent fields and has many potential applications (1). However, due to the relatively long wavelengths of THz radiation, most THz components, such as lenses and prisms, are on a large scale and are not suitable for system integration.Conventional optical components shape the wavefront of propagating light via gradual phase changes that accumulate along the optical path, usually via alterations to the spatial distribution using the thickness or refraction index of the components. Early optical components possessed continuous curved surfaces to achieve phase modulation, as indicated in Fig. 1A. This continuity determines the bulkiness of the components. Further technological developments utilized the 2π phase jump to reduce component thickness to the wavelength scale, as shown in Fig. 1B. Subsequently, metamaterials with extremely large effective refractive indices have been used to further reduce the thickness of the optical components (2, 3). However, the basic theory for wavefront shaping is still based on phase accumulation along the optical path, and the thickness of the corresponding components is still quite large. The question remains as to whether it is possible to further reduce the thickness of the optical components.Alternatively, phase changes can also be introduced by an optical resonator. Electromagnetic cavities (4-6), nanoparticles clusters (7,8), and plasmonic antennas (9,10) have previously been employed for tailoring phase changes. Recently, a novel method was proposed to introduce a phase discontinuity at the interface between two media (11)(12)(13). In this method, the geometry of planar V-shaped antennas was spatially selected, and the phase shift between the emitted and illuminating lights could be controlled arbitrarily; the generalized laws of reflection and refraction using this method were described. If the antennas are spatially arranged according to a customized phase distribution, an ultrathin planar optical componen...

Plasmonic meta-atoms and metasurfaces

“…Such systems only operate in reflection, but do allow much higher scattering efficiencies (>80%), than the particle--based systems described here (~20%). There have also been several variations of reflectarray and meta--atom based phase--gradient metasurfaces such as arrays of H--shaped elements above a ground plane that have been used to modify the coupling of free space radiation into surface modes 75 ; and 3--layer fishnet structures 76 , arrays of crossed nanorods 77 , and V--shaped cuts in thin metal films 78,79 . Box 2: Phase--gradient metasurface elements The Lorentzian character of the plasmonic resonances supported by V--shaped nano--antennae allows control of the phase of the scattered light over a π phase range by modifying the geometry.…”

Tutorial: Terahertz beamforming, from concepts to realizations

“…Currents that are excited by the co-polarized incident field will therefore radiate in the cross-polarization. For this reason, complementary "V" [180][181][182][183] and "C"-shaped 44,184 metallic resonators, such as that which is shown in Fig. 14(a), are a popular choice, as these shapes are naturally amenable to this form of commutation of current vectors.…”

Spatial Terahertz Modulator

“…However, the wave front modulation devices are still lacking. Recently, a novel technology based on the metasurface has been demonstrated to generate the desired wave front distribution 19,20 . Unfortunately, the specific function of this kind of devices has been determined at the moment of their design and could not be flexibly changed any more.…”