Abstract-Graphene-enabled active metamaterial for dynamical manipulation of terahertz reflection/transmission/absorption

Yin Zhang. Yijun Feng, Junming Zhao,

                                       

https://www.sciencedirect.com/science/article/abs/pii/S0375960120307076

Arbitrarily controlling the reflection/transmission/absorption state of electromagnetic waves, especially their dynamical manipulation, is on high demand due to the growing development of practical optical devices and application systems. In this work, we present a scheme to dynamically manipulating terahertz wave reflection/transmission/absorption using the active metamaterial composed of a layer of metallic structural and two graphene sandwich structures as well as polymer spacers. By adjusting the Fermi level of graphene, this tunable metamaterial can achieve electronically reconfigurable terahertz reflection, transmission and absorption in a highly efficient manner. Furthermore, it also enables continuous dynamical modulation between any two among the three states. The presented metamaterial is a promising candidate for exploring active terahertz devices for dynamically controlling the state of light-matter interactions and have potential applications in optoelectronic devices, wireless communication and frequency selective surface.

Abstract-Effect of aggregation on hydration of HSA protein: Steady-state Terahertz absorption spectroscopic study

BISWAJIT MANNA,  AMITABHA NANDI,  MASAHITO TANAKA, HIROYUKI TOYOKAWA, RYUNOSUKE KUROD, DIPAK K PALIT,

https://www.ias.ac.in/article/fulltext/jcsc/132/0008

Terahertz (THz) absorption behaviour of HSA protein in aqueous buffer solutions has been investigated in the 0.1–2 THz frequency range using a highly intense THz source based on coherent transition radiation (CTR) generated using a femtosecond electron accelerator of 42 MeV energy (for 0.3–2 THz) and a klystron (at 0.1 THz). Like in the cases of other proteins reported earlier, THz absorbance of the protein solutions follow nonlinear behaviour with increasing concentration of HSA protein monitored through the entire frequency range. THz absorbance of the solution initially increases to follow an apparently linear behaviour up to the concentration of *6 9 10-4 mol dm-3 but decreases gradually with further increase in HSA concentration. The linear behaviour in low concentration regime could be explained considering the increase in concentration of the monomer HSA molecules in solution with a well-defined hydration layer of thickness of about 22 A˚ around it. However, the study of dynamic light scattering measurements suggest the presence of increasing number of protein aggregates in solution with increasing concentration of protein. THz absorbance of each of these samples could be calculated to show that absorbance decreases with increasing number of aggregates in solution and also the relative concentrations of the monomer and aggregated particles existing in solutions could be estimated. This work, for the first time, explains the nonlinear change in THz absorbance of protein solutions with increasing protein concentration considering the protein aggregation effect at very high concentration.

Abstract-Borrmann modulation of terahertz absorption in 3D printing microlattices

AIP Advances

Wenya Zhang, Jian Chen, Weixin Lu, Bo Hou

 The schematic picture of the photonic crystal which is a simple cubic lattice of square cross-sectioned rods. (b) The zoomed-in view where a denotes the lattice constant and d denotes the rod size. (c) The photo of the fabricated sample with an overall size of ∼40 × 40 × 7 mm3. (d) The optical microscopy photo (top view) where the lattice constant and the rod size are 580 μm and 130 μm, respectively.

https://aip.scitation.org/doi/10.1063/1.5123655

We have fabricated a three-dimensional (3D) microlattice, composed of orthogonal polymer rods along three dimensions and featured with a sub-millimeter scale size, with a photopolymerizing 3D printer. The reflection and transmission spectra of the microlattice have been measured in terahertz time-domain spectroscopy and compared with numerical simulations. Due to Debye relaxation loss of the constituent photopolymer, the microlattice gives rise to absorption in the measured frequencies. In particular, the absorption modulation is observed across the bandgap, corresponding to the lower/upper band edges, and is a straightforward manifestation of the Borrmann effect.

Abstract-Active Terahertz Shielding and Absorption Based on Graphene Foam Modulated by Electric and Optical Field Excitation

Shi‐Tong Xu, Fei Fan, Jierong Cheng,  Honghui Chen, Wenle Ma, Yi Huang, Shengjiang Chang,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900555

Ultralight materials for broadband terahertz (THz) shielding and absorption are promising in practical THz applications. Here, active THz shielding and absorption properties of 3D graphene foam (GF) controlled by both laser pumping and biased electric field are investigated. The GF can be tuned from OFF‐shielding state to ON‐shielding state when the external field excitations are applied, and 10 dB shielding bandwidth expands from 0 to a broad band of 0.2–1.6 THz. Further researches show that the GF always keeps very low THz reflection either with or without external fields, but its absorption characteristics can be remarkably controlled from 13% to 95.4% at 0.3 THz by the power of the external excitations, and its specific average terahertz absorption performance increases from 3.9 × 103 to 1.95 × 104 dB cm3 g−1. This modulation mechanism reveals that the carrier density in GF increases one order of magnitude from 2.6 × 1014 cm−3 to 3.15 × 1015 cm−3. Finally, the tunable THz shielding and absorption characteristics of this GF device are demonstrated by THz transmission imaging, which shows its great potential applications in active THz imaging, radar, and electromagnetic compatibility.

Abstract-Comparative study on calculated terahertz absorption spectra of different heterostructure materials with external magnetic field

Lei Cao

http://iopscience.iop.org/article/10.1088/2399-6528/aadbba

Room temperature operation and frequency tunability are attractive advantages of terahertz (THz) detectors based on the two-dimensional electron gas (2DEG) in the heterostructure material with external magnetic field. This work compared absorption spectra of four typical heterostructure materials (AlGaN/GaN, InAlN/GaN, AlGaAs/GaAs and SiGe/Si) with the nonlocal magnetoconductivity model at ambient and cryogenic temperatures in the frequency range 0-5 THz. The GaN based materials have the highest absorption amplitude, while the AlGaAs/GaAs material owns the largest frequency shift as the magnetic field increases up to 10 tesla, although superconducting magnets at cryogenic temperature are usually employed to provide that high magnetic field. The numerical results showed that the absorption properties (amplitude and frequency) could be further optimized by other parameters, such as the period and filling factor of the grating coupler, and the barrier thickness.

Abstract-Experimental and Theoretical Study on Terahertz Absorption Characteristics and Spectral De-noising of Three Plant Growth Regulators

Fangfang Qu, 

Yi Pan, 

Lei Lin, 

Chengyong Cai, 

Tao Dong, 

Yong He, 

Pengcheng Nie, 

https://link.springer.com/article/10.1007%2Fs10762-018-0507-1
Accurate identification of terahertz (THz) absorption peaks of biological macromolecules is of great significance in practical applications. In this work, the experimental and theoretical fundamentals of detecting three plant growth regulators (PGRs), including 6-Benzylaminopurine (6-BA), paclobutrazol (PBZ), and maleic hydrazide (MH) were investigated by using THz time-domain spectroscopy (THz-TDS). THz absorption coefficient and refractive index in frequencies of 0.06–4 THz were obtained. The wavelet threshold de-noising (WTD) method was used to remove spectral noise and improve the signal-to-noise ratio (SNR). The density functional theory (DFT) was applied to the molecular characterization and theoretical calculation of PGRs. Experimental results showed that the three PGRs had unique characteristic absorption peaks. Based on the sym4 wavelet function and four-layer wavelet decomposition, the de-noising performance of hard threshold WTD was better than that of soft threshold WTD. The spectra processed by hard threshold de-noising achieved higher peak SNR (6-BA: 40.22, PBZ: 37.73, MH: 34.83) and lower root mean square error (6-BA: 0.41, PBZ:0.40, MH:0.54). In addition, the characteristic absorption and anomalous dispersion of 6-BA were found at 2.08 and 3.00 THz, those of PBZ were shown at 0.71, 1.30, 1.88, and 2.67 THz, and those of MH were shown at 2.34 THz. The absorption peaks in THz spectra processed by hard threshold WTD were demonstrated to be in good agreement with the simulation results of DFT. These results show the effectiveness of WTD in THz spectral de-noising and the feasibility of using THz-TDS to detect PGRs.

Abstract-Interferometric control of dual-band terahertz perfect absorption using a designed metasurface

Ming Kang, Huifang Zhang, Xueqian Zhang, Quanlong Yang, Weili Zhang,  Jiaguang Han

https://journals.aps.org/prapplied/accepted/b3077A50X33E621a909b163840c172b9fbca3365e

Coherent perfect absorber (CPA), a time-reversed counterpart to the laser emission, could cause all energy fed to the system to be absorbed. It can also be as an absorptive interferometer, which could provide applications in controllable optical energy transfer. Here, in order to achieve terahertz CPA, we propose a designed metasurface and experimentally demonstrate that it can serve as a polarization-insensitive CPA at one-frequency channel under normal symmetric excitation, while a transverse electric (TE) CPA at two-frequency channels around oblique ${40}^o$symmetric incidence. Such phenomena in this system can be attributed to Fano resonance consisting of interacting one bright and one dark modes under normal incidence and an additional operative dark mode under oblique symmetric excitation. The experimental results finds good agreement with the fitted coupled mode theory(CMT). Moreover, we show the output amplitude can be effectively tuned from 0 to 1 only by varying the relative phase between the two input waves. The designed CPA could find potential application in effectively controlling absorption for terahertz imaging and terahertz switches.

Abstract-Broadband Tunable THz Absorption with Singular Graphene Metasurfaces

Emanuele Galiffi,  John B. Pendry, Paloma A. Huidobro, 

http://pubs.acs.org/doi/abs/10.1021/acsnano.7b07951

By exploiting singular spatial modulations of the graphene conductivity, we design a broadband, tunable THz absorber whose efficiency approaches the theoretical upper bound for a wide absorption band with a fractional bandwidth of 185%. Strong field enhancement is exhibited by the modes of this extended structure, which is able to excite a wealth of high-order surface plasmons, enabling deeply subwavelength focusing of incident THz radiation. Previous studies have shown that the conductivity can be modulated at GHz frequencies, which might lead to the development of efficient high-speed broadband switching by an atomically thin layer.

Abstract-Electromagnetic energy and negative asymmetry parameter in coated magneto-optical cylinders: Applications to tunable light transport in disordered systems

Tiago J. Arruda, Alexandre S. Martinez, Felipe A. Pinheiro

https://arxiv.org/abs/1712.06129

We investigate electromagnetic scattering of normally irradiated gyrotropic, magneto-optical core-shell cylinders using Lorenz-Mie theory. A general expression for time-averaged electromagnetic energy inside a coated gyroelectric and gyromagnetic scatterer is derived. Using realistic material parameters for a silica core and InSb shell, we calculate the stored electromagnetic energy and the scattering anisotropy. We show that the application of an external magnetic field along the cylinder axis induces a drastic decrease in electromagnetic absorption in a frequency range in the terahertz, where absorption is maximal in the absence of the magnetic field. We demonstrate not only that the scattering anisotropy can be externally tuned by applying a magnetic field, but also that it reaches negative values in the terahertz range even in the dipolar regime. We also show that this preferential backscattering response results in an anomalous regime of multiple light scattering from a collection of magneto-optical core-shell cylinders, in which the extinction mean free path is longer than the transport mean free path. By additionally calculating the energy-transport velocity and diffusion coefficient, we demonstrate an unprecedented degree of external control of multiple light scattering, which can be achieved by either applying an external magnetic field or varying the temperature

Abstract-Mechanism for sub-gap optical conductivity in honeycomb Kitaev materials

Adrien Bolens, Hosho Katsura, Masao Ogata, Seiji Miyashita

https://arxiv.org/abs/1711.00308

Motivated by recent terahertz absorption measurements in α-RuCl3, we develop a theory for the electromagnetic absorption of materials described by the Kitaev model on the honeycomb lattice. We derive a mechanism for the polarization operator at second order in the nearest neighbor hopping Hamiltonian. Using exact results of the Kitaev honeycomb model, we then calculate the polarization dynamical correlation function corresponding to electric dipole transitions, in addition to the spin dynamical correlation function corresponding to magnetic dipole transitions.

Abstract-Terahertz absorption of lysozyme in solution

Daniel R. Martin, Dmitry V. Matyushov

(Submitted on 10 Jun 2017)

https://arxiv.org/abs/1706.03281

Absorption of radiation by solution is described by the solution dielectric constant and can be viewed as a specific application of the dielectric theory of solutions. For ideal solutions, the dielectric boundary value problem separates the polar response into the polarization of the void in the liquid created by the solute and the response of the solute dipole. In the case of a protein as a solute, its nuclear dynamics do not project on significant fluctuations of the dipole moment in the terahertz domain of frequencies and the protein dipole can be viewed as dynamically frozen. Absorption of radiation then reflects the interfacial polarization. Here we apply an analytical theory and computer simulation to absorption of radiation by ideal solutions of lysozyme. Comparison with experiment shows that Maxwell electrostatics fails to describe the polarization of the protein-water interface and the "Lorentz void", which does not anticipate polarization of the solute void by the external field (no surface charges), better represents the data. An analytical theory for the slope of the solution absorption against the volume fraction of the solute is formulated in terms of the cavity field response function. It is calculated from molecular dynamics simulations in good agreement with experiment. The protein hydration shell emerges as a separate sub-ensemble, which collectively is not described by the standard electrostatics of dielectrics.

Abstract-Terahertz absorption in GaN epitaxial layers under lateral electric field

R M Balagula1, M Ya Vinnichenko1, G A Melentev1, M D Moldavskaya1, V A Shalygin1, L E Vorobjev1,D A Firsov1, S N Danilov2 and S Suihkonen3


http://iopscience.iop.org/article/10.1088/1742-6596/741/1/012147
Variation of absorption of terahertz radiation in lateral electric field was investigated in GaN epitaxial layers. Different behaviour of the absorption modulation in electric field was observed for radiation polarized along electric field and perpendicular to it. Joint analysis of optical and transport measurements let us obtain field dependencies of mobility, electron concentration and absorption cross-section. For terahertz radiation polarized perpendicular to the electric field, results are in accordance with Drude model of free electron absorption. Another polarization demonstrates significant deviation that is yet to be studied more thoroughly.