Manipulation Of Terahertz Waves Research Articles

All optically driven memory device for terahertz waves

We demonstrate an all optically driven memory device based on vanadium dioxide ( V O 2 ) for terahertz (THz) waves. By easily tuning the power of illuminating light, a V O 2 memory device is coded in reconfigurable and multi-level states, taking advantage of its hysteretic metal-to-insulator phase transition (MIT). Further, writing with intense femtosecond pulses, the memory effects are performed by non-thermal photo-induced MIT, and resultant 2-bit coding with a write time of 22 µs is demonstrated, yielding a dramatically improved write rate compared to existing thermally controlled V O 2 memory device. The proposed all optically driven V O 2 -based memory device paves the way for actively manipulating THz waves within robust reconfigurable high-speed memory functionality.

Robust and broadband integrated terahertz coupler conducted with adiabatic following

As the key concept in fabricating integrated device, surface plasmon-polaritons (SPPs) have been widely employed to artificially manipulate the electromagnetic waves in metallic surfaces. However, due to the highly structure-dependent resonance of SPPs, it is challengeable to develop a fixed device which can function at wide band. Here, we propose a novel broadband and robust SPPs directional coupler based on the tri-layered curved waveguides working at terahertz (THz) frequencies, where the coupling of SPPs between the adjacent waveguides can be modeled with coupled mode theory. By introducing the stimulated raman adiabatic passage quantum control technique, we achieve the complete transfer of SPPs from the input to the output waveguides in the range of 0.9–1.3 THz, and even considering the propagation loss, the transfer rate is still above 70%. Furthermore, the performance of our device is eminently robust because of its insensitivity to the geometry of structure and the wavelength of SPPs. As a result, our device can tolerate defect induced by fabrication processing and manipulate THz waves at broadband. This finding provides a new theoretical guideline in promoting THz on-demand applications, which is of significance in developing integrated THz devices.

Three‐dimensional printing technologies for terahertz applications: A review

Three-dimensional (3D) printing technologies enables fast prototyping of complex 3D objects with ever improving printing qualities. To date, 3D printing has been found useful in areas such as manufacturing, industrial design, aerospace, dental and medical industries, and many others. In this article, we review recent advances of 3D printing technologies for terahertz (THz) applications. Different 3D printing technologies and printable materials are first discussed and compared. 3D-printed THz components and devices, which are categorized as waveguides/fibers, antennas, and quasi-optical components, are further demonstrated. It is found that the performances and functionalities of 3D-printed THz devices have been greatly enhanced, while the operating frequencies have been increased from the lower end of THz range to over 1 THz region. With further development of novel materials and printing techniques, it is believed that 3D printing technologies will play an important role in the realization of THz components for efficient control and manipulation of THz waves.

Manipulating terahertz wave and reducing radar cross section (RCS) by combining a Pancharatnam–Berry phase with a coding metasurface

We propose a kind of coding metasurface of an ‘M’-shaped metallic pattern coding particle, which can realize the flexible manipulation of terahertz waves and reduce radar cross sections effectively. In this paper, eight kinds of coding particles are obtained by rotating the ‘M’-shaped pattern using the Pancharatnam–Berry phase theory. We design three different coding sequences of 3-bit coding metasurfaces and manipulate the incident terahertz wave to produce two, four and six beams of the reflected terahertz wave, respectively. Both theoretically calculated and numerically simulated scattering patterns of the designed coding metasurfaces demonstrate the expected manipulations. In addition, compared with the bare metal plate, the designed random coding metasurfaces can reduce the radar cross section by at least −10 dB at the frequency range 0.7–1.45 THz.

Active Control of Terahertz Waves Using Vanadium-Dioxide-Embedded Metamaterials

This article describes the active control of terahertz waves using hybrid metamaterials containing embedded vanadium dioxide. VO${}_{2}$ undergoes an insulator-metal phase transition at around 68 \ifmmode^\circ\else\textdegree\fi{}C, causing its conductivity to change by about 5 orders of magnitude, which yields mode switching in the metamaterial. This mode switching, which is analogous to the resonant mode switching seen in plasmonics, can be realized experimentally using thermal, electrical, or optical stimuli. Such a diverse range of stimuli should be very useful for manipulating terahertz waves in practical applications, with a particular eye toward next-generation wireless communication.

Generation of spatiotemporally tailored terahertz wavepackets by nonlinear metasurfaces

The past two decades have witnessed an ever-growing number of emerging applications that utilize terahertz (THz) waves, ranging from advanced biomedical imaging, through novel security applications, fast wireless communications, and new abilities to study and control matter in all of its phases. The development and deployment of these emerging technologies is however held back, due to a substantial lack of simple methods for efficient generation, detection and manipulation of THz waves. Recently it was shown that uniform nonlinear metasurfaces can efficiently generate broadband single-cycle THz pulses. Here we show that judicious engineering of the single-emitters that comprise the metasurface, enables to obtain unprecedented control of the spatiotemporal properties of the emitted THz wavepackets. We specifically demonstrate generation of propagating spatiotemporal quadrupole and few-cycles THz pulses with engineered angular dispersion. Our results place nonlinear metasurfaces as a new promising tool for generating application-tailored THz fields with controlled spatial and temporal characteristics.

Active broadband terahertz wave impedance matching based on optically doped graphene–silicon heterojunction

Broadband terahertz (THz) impedance matching is important for both spectral resolution improvement and THz anti-radar technology. Herein, graphene–silicon hybrid structure has been proposed for active broadband THz wave impedance matching with optical tunability. The main transmission pulse measured in the time domain indicates a modulation depth as high as 92.7% totally from the graphene–silicon interface. The interface reflection from the graphene–silicon junction implies that an impedance matching condition can be actively achieved by optical doping. To reveal the mechanism, we propose a graphene–silicon heterojunction model, which gives a full consideration of both the THz conductivity of graphene and the loss in doped junction layer. The theory fits well with the experimental results. This work proves active THz wave manipulation by junction effect and paves the way for active anti-reflection coating for THz components.

Terahertz wave front manipulation based on Pancharatnam-Berry coding metasurface

We design a coding metasurface based on Pancharatnam-Berry (PB) phase to manipulate terahertz waves, which is simple and flexible. Compared with the previous design of the coding metasurface, the present coding particles can be obtained by using a same size meta-particle with various orientations instead of designing multiple structures or changing specific size parameters. The PB coding metasurfaces composed of U-shaped particles with pre-designed coding sequences can generate multi-bit coding in the terahertz frequencies and control the reflected terahertz waves to the various directions. Both simulation and theoretical calculation scattering patterns of the designed PB coding metasurfaces demonstrate the expected manipulations. Additionally, the bandwidth of radar cross section (RCS) reduction of approaching −15 dB is 1.05THz (range from 0.9THz to 1.95THz). We believe that the proposed design provides a more flexible way for the manipulation of reflected terahertz waves.

Mechanical modulation of terahertz wave via buckled carbon nanotube sheets.

Manipulation of terahertz (THz) wave plays an important role in THz imaging, communication, and detection. The difficulty in manipulating the THz wave includes single function, untunable, and inconvenient integration. Here, we present a mechanically tunable THz polarizer by using stretchable buckled carbon nanotube sheets on natural rubber substrate (BCNTS/rubber). The transmittance and degree of polarization of THz wave can be modulated by stretching the BCNTS/rubber. The experiments showed that the degree of polarization increased from 17% to 97%, and the modulation depth reached 365% in the range of 0.2-1.2 THz, as the BCNTS/rubber was stretched from 0% to 150% strain. These changes can be also used for high strain sensing up to 150% strain, with a maximum sensitivity of 2.5 M/S. A spatial modulation of THz imaging was also realized by stretching and rotating BCNTS/rubber. The theoretical analysis and numerical modeling further confirm the BCNTS/rubber changes from weak anisotropic to highly anisotropic structure, which play key roles in THz wave modulation. This approach for active THz wave manipulation can be widely used in polarization imaging, wearable material for security, and highly sensitive strain sensing.

Terahertz wave manipulation through coupling of spoof plasmonics and Fabry–Perot resonance

The ‘trapped rainbow’ mechanism of graded gratings has been demonstrated only as a type of groove reflection. In this study, we investigate the physical mechanism of the ‘trapped rainbow’ and demonstrate that a small step increase (~1/50λ) in the depth of a single obstructed groove can terminate the propagation of surface terahertz (THz) waves and reflect them back. A single obstructed groove can easily manipulate the properties of THz waves by changing the depth or the refractive index of the groove because the cutoff frequency is highly sensitive to the property of the groove. In addition, waves transmitted and reflected by a single groove can be controlled periodically over a period of a 1/2 − λ increase in depth owing to the interference of surface spoof plasmonics and Fabry–Perot resonance. This is an easy means of controlling surface THz waves and fabricating more compact integrated optical devices such as THz branch waveguides, band pass filters, reflectors, and splitters.

All‐Optical and Ultrafast Tuning of Terahertz Plasmonic Metasurfaces

AbstractMetasurfaces have been widely used to manipulate terahertz waves due to their great potential for achieving unique electromagnetic responses. However, to realize ultrafast and efficient control of the light in active metasurfaces is still a critical challenge. Here, an ultrafast tunable metasurface which consists of ion‐implanted and annealed silicon disk array is experimentally demonstrated in the terahertz range. By utilizing the optical‐pump terahertz‐probe spectroscopy, the absolute transmission modulation up to 38% is realized at the electric dipole resonance under the femtosecond pulse excitation, with a switch‐on time within 20 ps and a recovery time of 300 ps. Furthermore, a theoretical analysis that quantitatively models the time dependence of the electron–hole plasma density and the permittivity of the silicon disks is proposed. Then, the transient transmission spectra of the metasurface are numerically simulated, which agree well with the experimental performances. The silicon‐based metasurface shows significant potential in ultrafast terahertz optics.

Terahertz wave manipulation based on multi-bit coding artificial electromagnetic surfaces

A polarization insensitive multi-bit coding artificial electromagnetic surface is proposed for terahertz wave manipulation. The coding artificial electromagnetic surfaces composed of four-arrow-shaped particles with certain coding sequences can generate multi-bit coding in the terahertz frequencies and manipulate the reflected terahertz waves to the numerous directions by using of different coding distributions. Furthermore, we demonstrate that our coding artificial electromagnetic surfaces have strong abilities to reduce the radar cross section with polarization insensitive for TE and TM incident terahertz waves as well as linear-polarized and circular-polarized terahertz waves. This work offers an effectively strategy to realize more powerful manipulation of terahertz wave.

Bidirectional reconfiguration and thermal tuning of microcantilever metamaterial device operating from 77 K to 400 K

We experimentally report the bidirectional reconfiguration of an out-of-plane deformable microcantilever based metamaterial for advanced and dynamic manipulation of terahertz waves. The microcantilever is made of a bimaterial stack with a large difference in the coefficient of thermal expansion of the constituent materials. This allows for the continuous deformation of microcantilevers in upward or downward direction in response to positive or negative temperature gradient, respectively. The fundamental resonance frequency of the fabricated microcantilever metamaterial is measured at 0.4 THz at room temperature of 293 K. With decreasing temperature, the resonance frequency continuously blue shifts by 30 GHz at 77 K. On the other hand, with increasing temperature, the resonance frequency gradually red shifts by 80 GHz and saturates at 0.32 THz for 400 K. Furthermore, as the temperature is increased above room temperature, which results in the downward actuation of the microcantilever, a significant resonance line-narrowing with an enhanced quality factor is observed due to tight field confinement in the metamaterial structure. The thermal control of the microcantilever possesses numerous inherent advantages such as enhanced tunable range (∼37.5% in this work compared to previously reported microcantilever metamaterials), continuous tunability, and repeatable operations. The microcantilever metamaterial also shows high robustness to operate at cryogenic conditions and hence opens up the possibility of using meta-devices in harsh environments such as space, polar, and deep sea applications.

Dielectric Properties of DMSO‐Doped‐PEDOT:PSS at THz Frequencies

Poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) is a promising candidate for terahertz (THz) metamaterial devices. Its dielectric properties can be changed by the dopant dimethylsulfoxide (DMSO). In this study, the dielectric properties of DMSO‐doped‐PEDOT:PSS films are investigated using terahertz time‐domain spectroscopy (THz‐TDS) measurements. The values of the carrier density and mobility are also determined by fitting the dielectric permittivity to the Drude–Smith model. A simple THz frequency selective surface (FSS) is designed based on these DMSO‐doped‐PEDOT:SS films. Strong band rejection can be achieved at resonant frequencies. These results may inspire the development of new materials for manipulating THz waves in future studies.

One-step growth of graphene-carbon nanotube trees on 4″ substrate and characteristics of single individual tree

We develop a simple one-step growth method to obtain a novel kind of tree-like carbon nano composite structure on a 4″ stainless steel substrate, which is a nano graphene-carbon nanotube tree (GCT). The structures are synthesized by using a plasma enhanced chemical vapor deposition method. The GCT consists of few-layer graphenes (FLG) epitaxially growing from a carbon nanotube (CNT) and they can grow uniformly on a 4″ substrate. Analysis of chemical bonding between FLG and CNT is given. In addition, electron micrographs are presented to show how a tree grows. Finally, we also develop techniques to characterize both experimentally and theoretically the electrical conductivity, field electron emission, heat conduction and heat radiation of a GCT. The electrical conductivity of a GCT is compatible to high quality CNT, the current performance of field electron emission is significantly high compared to the other individual single nanostructure, and very outstanding heat radiation effect is found. We believe that this large-area GCTs film can find potential applications such as super capacitor, terahertz wave manipulation, heat dissipation and field electron emission.

Terahertz wavefront manipulating by double-layer graphene ribbons metasurface

It was recently presented that the phase gradient metasurface can focus the reflection in terahertz range. However, narrow bandwidth and complex tuning method are still challenges. For instance, the size is difficult to be changed once the device is built. We propose a tunable double-layer graphene ribbons array (DLGRA) metasurface which has great potentials for applications in terahertz wavefront control. By changing the Fermi level of each graphene ribbon independently, the DLGRA separated by a bonding agent and a thin dielectric spacer can achieve nearly 2π phase shift with high reflection efficiency. A reflector which can focus terahertz waves over a broad frequency range is demonstrated numerically by the DLGRA. Intriguingly, through a lateral shift between the nearby graphene ribbons, the variation of coupling induces a shift of focusing frequency. Hence, this approach increases the frequency range to a higher degree than the fixed state. The proposed metasurface provides an effective way for manipulating terahertz waves in a broad frequency range.

Ultra-thin and high-efficiency graphene metasurface for tunable terahertz wave manipulation

We propose a type of ultra-thin metasurfaces composed of rectangular graphene patches supporting orthogonal plasmonic resonances, which can work in transmission or reflection modes for dynamic terahertz wavefront manipulation with high polarization conversion ratio. By controlling the response of each patch via electrical biasing, the phase of the output wave can be tuned in a wide range over 180° while keeping its amplitude high and relatively stable. We demonstrate several functional devices based on such metasurfaces: a linear polarization converter with nearly 100% polarization conversion ratio, a switchable anomalous wave deflection device, and a dual-polarity focusing mirror with the focal spot tunable in both the transversal and longitudinal directions.

3D Printed Terahertz Focusing Grating Couplers

We have designed, constructed and characterized a grating that focuses electromagnetic radiation at specific frequencies out of a dielectric waveguide. A simple theoretical model predicts the focusing behaviour of these chirped gratings, along with numerical results that support our assumptions and improved the grating geometry. The leaky waveguide was 3D printed and characterized at 120 GHz demonstrating its potential for manipulating terahertz waves.

Complementary bilayer metasurfaces for enhanced terahertz wave amplitude and phase manipulation

Manipulation of terahertz wave by metasurfaces has shown tremendous potential in developing compact and functional terahertz optical devices. Here, we propose complementary bilayer metasurfaces for enhanced te-rahertz wave amplitude and phase manipulation. The metasurfaces are composed of one layer of metal cut-wire arrays and one layer of their complementary aperture arrays separated by a dielectric spacer. Through the near-field coupling between transverse magnetic resonances in the metal apertures and electric resonances in the metal cut-wires, the structures can manipulate the cross polarization conversion and phase dispersion of terahertz wave. Particularly, the designed metasurfaces demonstrate a phase delay of 180° between two orthogonal axes with the same transmission amplitude between 0.70 and 1.0 THz, enabling a 45° broadband polarization conversion. When the metal cut-wires are rotated with respect to the apertures or the thickness of the dielectric spacer is changed, the amplitude and phase dispersion of the transmitted terahertz wave can be tuned. Such complementary coupled bilayer metasurfaces offer a new method to control the amplitude and phase dispersion of terahertz wave and promise great potential for applications in terahertz meta-devices.

Electrically Tunable Goos–Hänchen Effect with Graphene in the Terahertz Regime

Goos–Hänchen (G–H) effect is of great interest in the manipulation of optical beams. However, it is still fairly challenging to attain efficient controls of the G–H shift for diverse applications. Here, a mechanism to realize tunable G–H shift in the terahertz regime with electrically controllable graphene is proposed. Taking monolayer graphene covered epsilon‐near‐zero metamaterial as a planar model system, it is found that the G–H shifts for the orthogonal s‐polarized and p‐polarized terahertz beams at oblique incidence are positive and negative, respectively. The G–H shift can be modified substantially by electrically controlling the Fermi energy of the monolayer graphene. Reversely, the Fermi energy dependent G–H effect can also be used as a strategy for measuring the doping level of graphene. In addition, the G–H shifts of the system are of strong frequency‐dependence at oblique angles of incidence, therefore the proposed graphene hybrid system can potentially be used for the generation of terahertz “rainbow,” a flat analog of the dispersive prism in optics. The proposed scheme of hybrid system involving graphene for dynamic control of G–H shift will have potential applications in the manipulation of terahertz waves.