Broadband and high efficiency terahertz metasurfaces for anomalous refraction and vortex beam generation

The applications of metasurfaces are currently a highly active research field due to their extraordinary ability to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturization and integration of metasurface devices. However, these devices work typically under a low efficiency and narrow bandwidth condition. In this work, we design eight multilayered unit cells with similar amplitudes and a phase interval of π/4, which convert the polarization states of the terahertz(THz) waves between two orthogonal directions. The average cross-polarized transmission amplitudes of these cells are all around 0.9 in an ultra-broad frequency range from 0.5 THz to 1.4 THz. Furthermore,unit cells are used to construct both an ultra-thin anomalous refraction metalens and a vortex phase plate. Our simulation results show that the anomalous refraction for the transmitted linear polarization component is comparable to the theoretical prediction, and the maximum error is determined to be below 4.8%. The vortex phase plate can also generate an ideal terahertz vortex beam with a mode purity of 90% and more. The distributions of longitudinal electric field, intensity, and phase illustrate that the generated vortex beam has excellent propagation characteristics and a weak divergence. Simulations of the two types of metasurface devices, based on the eight unit cells, exhibit very high efficiencies in a wide bandwidth. Our research will assist in the improvement in the practical applications of metasurfaces. It also provides a reference for the design of high efficiency and broadband devices that are applied to other frequency ranges.

Terahertz spectroscopy and lattice vibrational analysis of pararealgar and orpiment

Terahertz time-domain spectroscopy(THz-TDS)is an effective nondestructive and noninvasive tool for investigating sulfur-containing pigments.Combined with Raman spectroscopy and vibrational mode analysis,it is significant for artifact identification and conservation.In this work,the terahertz absorption spectra of pararealgar(As_(4)S_(4))and orpiment(As_(2)S_(3))samples mixed with polytetrafluoroethylene(PTFE)are characterized in a range from 0.2 THz to 2.2 THz,and their distinctive peaks are observed,respectively.Meanwhile,qualitative analysis is also implemented by using Raman spectroscopy as a complementary technique.The lattice vibrations are simulated by using solid-state density functional theory(ss-DFT),illustrating different characteristic absorption peaks for specific crystalline structures and dynamic properties.This work provides a reliable database of sulfur-containing pigments for using the THz technology to actually analyze and diagnose cultural relics.

Actively tunable dual-broadband graphene-based terahertz metamaterial absorber

A tunable metamaterial absorber(MA)with dual-broadband and high absorption properties at terahertz(THz)frequencies is designed in this work.The MA consists of a periodic array of flower-like monolayer graphene patterns at top,a SiO_(2)dielectric spacer in middle,and a gold ground plane at the bottom.The simulation results demonstrate that the designed MA has two wide absorption bands with an absorption of over 90%in frequency ranges of 0.68 THz-1.63 THz and 3.34 THz-4.08 THz,and the corresponding relative bandwidths reach 82.3%and 20%,respectively.The peak absorptivity of the absorber can be dynamically controlled from less than 10%to nearly 100%by adjusting the graphene chemical potential from 0 eV to 0.9 eV.Furthermore,the designed absorber is polarization-insensitive and has good robustness to incident angles.Such a high-performance MA has broad application prospects in THz imaging,modulating,filtering,etc.

Hierarchical lichee-like Fe_(3)O_(4) assemblies and their high heating efficiency in magnetic hyperthermia

A nontoxic and biocompatible thermoseed is developed for the magnetic hyperthermia.Two kinds of thermoseed materials:hierarchical hollow and solid lichee-like Fe_(3)O_(4) assemblies,are synthesized by a facile hydrothermal method.The crystal structure of Fe_(3)O_(4) assemblies are characterized by x-ray diffraction,scanning electron microscopy,and transmission electron microscopy.Moreover,the prepared Fe_(3)O_(4) assemblies are used as a magnetic heat treatment agent,and their heating efficiency is investigated.Compared to solid assembly,hollow lichee-like Fe_(3)O_(4) assembly exhibits a higher specific absorption rate of 116.53 W/g and a shorter heating time,which is ascribed to its higher saturation magnetization,larger initial particle size,and the unique hierarchical hollow structure.Furthermore,the magnetothermal effect is primarily attributed to Neel relaxation.Overall,we propose a facile and convenient approach to enhance the heating efficiency of magnetic nanoparticles by forming hollow hierarchical assemblies.