Low-voltage driven graphene-loaded metal wire grating device in total internal reflection geometry for broadband THz modulation

Terahertz modulators with capability of both intensity and phase are essential for THz imaging and communication systems. The low-voltage driven THz modulation technique is crucial for integrating the modulators with electronics components. There is still a lack of broadband devices able to achieve both amplitude and phase modulation with low voltage, due to the underlying physics behind existing approaches. Here, we demonstrate a graphene-loaded metal wire grating THz modulator in the total internal reflection geometry to achieve intensity modulation of 80% and phase modulation of 70 degree within 3 volts gate voltage. Quite different from using the strategy of metamaterials based on the electromagnetic resonance effects, our design has performed a broadband modulation for over 1 THz bandwidth.

All-silicon terahertz metamaterials absorber and pesticides sensing

Perfect absorption based on metamaterials at terahertz frequencies range has attracted a great deal of interest in the field of sensing, imaging, bolometers and stealth technology. This review is focused on presenting several recently developed absorbers based on all-silicon metamaterials, such as single-band, dual-band, multi-band and broadband absorbers. The partial physical mechanisms and optical tunability corresponding to the absorption are also reported. Furthermore, the presented absorbers can be used to detect the concentration of trace pesticides, and a good linear regression coefficient was obtained between the absorption amplitude and the concentration. Notably, the presented all-silicon metamaterials perfect absorbers are compatible with COMS processing which is beneficial to promote the development of terahertz functional devices.

Nonlinear terahertz effects of gold nanofilms

Nonlinear interaction between strong-field terahertz electromagnetic waves and matters will become one of the next hot research frontiers in nonlinear optics. However, the lack of strong terahertz radiation sources and appropriate nonlinear terahertz materials have impeded its progress. Here we systematically have investigated the strong-field terahertz nonlinear effects of gold (Au) nanofilms on different substrates, including SiO2, high-resistivity Si and SiO2-high-resistivity Si hybrid substrates. The strong-field terahertz waves are emitted from lithium niobate crystals via tilted pulse front technique, and obvious nonlinear transmission responses are observed along with varying the incident field strengths for all the Au samples on the three types of the substrates. The nonlinear behavior is enhanced when the gold nanofilm thickness increases, which can be qualitatively understood by introducing the quantum tunneling effect and carrier multiplication theory generated at the Au nano-slits under the illumination of the strong-field terahertz pulses. Our demonstrations not only open a new paradigm for nonlinear terahertz investigations and future high-speed terahertz devices, but also provide an effective platform for exploring extreme terahertz sciences.

Photo-Excited Silicon-Based Spatial Terahertz Modulators

The increasing development of terahertz (THz) technology has led to various potential applications in THz imaging, spectroscopy and communications. These devices capable of actively manipulating the amplitude, phase and frequency of THz waves are thus gaining numerous interests. All-optical silicon-based spatial terahertz modulators (STMs), as a simple, cost-effective, and reconfigurable technique, are standing the focus of research. Beginning with a fundamental concept of THz radiation, this paper systematically summarized the modulation mechanism and theoretical model for this kind of STM, reviewed the recent advancements in THz functional devices implemented by this optical method and yet, discussed the performance-improved measures with an emphasis on the reflection reduction. Despite that, there has been considerable progress in realizing high-performance STMs, and novel design is urgent to realize higher modulation rate and more functionality.

Fano resonances in the corrugated disk resonator and their applications

We have experimentally excited terahertz multipolar Fano resonances in two asymmetrical metal particles: a defective corrugated metallic disk(CMD) structure and a hybrid structure consisted of a C-shaped resonator and a CMD. Furthermore, the Fano resonance modes can also be excited by the interaction between plasmonic waveguide and CMD. Our findings have shed light into the terahertz multipolar Fano resonances in asymmetrical CMD and opened the way to the design of terahertz plasmonic devices.

Ultrafast carrier dynamics in graphene and graphene nanostructures

In this paper we provide a comprehensive view on the ultrafast conduction dynamics in graphene and graphene nanostructures. We show that ultrafast conduction in graphene can be well understood within a simple thermodynamic picture, by taking into account the dynamical interplay between electron heating and cooling, with the driving electric field acting as a supplier of thermal energy to graphene electron population. At the same time, the conductive properties of graphene nanostructures, such as graphene nanoribbons (GNRs) and carbon nanotubes (CNTs), can be well explained within the concept typical for disordered materials, such as e.g. organic semiconductors - the conduction by the free charge experiencing long-range localization.

A review of high-efficiency Pancharatnam–Berry metasurfaces

Manipulating circularly polarized (CP) electromagnetic waves as desired is important for a wide range of applications ranging from chiral-molecule manipulations to optical communication, but conventional natural-materials-based devices suffer from bulky configuration and low efficiencies. Recently, Pancharatnam–Berry (PB) metasurfaces have demonstrated strong capabilities to control CP waves in different frequency domains. In this article, we present a concise review on PB metasurfaces for CP light manipulations, focusing mainly on the research works done by our own group. After briefly introducing the working principles of PB metasurfaces, we separately discuss how to construct high-efficiency PB metasurfaces in reflection and transmission geometries, and how to utilize them to control CP waves in different frequency domains, including meta-lensing, meta-hologram, and surface couplers. Finally, we conclude this review with our perspectives on future developments of PB metasurfaces.

Terahertz photoconductive antenna with all-dielectric nanopillars

Photoconductive antennas (PCAs), as a popular terahertz (THz) radiation source, have been widely used in spectroscopy, material characterization, biological imaging and detection of hazardous materials. However, PCAs have a relatively low energy conversion efficiency from femtosecond laser pulses to THz radiation which often limits the signal-to-noise ratio and bandwidth of THz imaging and spectroscopy systems. To address these limitations, here we report a THz photoconductive antenna emitter with all-dielectric nanopillars integrated on top of the SI-GaAs substrate to increase the generated photocarriers, which achieves a broadband and frequency insensitive THz power enhancement factor around 1.25 at frequencies 0.05 - 1.6 THz. Our results reported here provide a new method for increasing the THz power of PCAs, which paves the way for the subsequent researches of next-generation PCAs.

The Development of broadband millimeter-wave and terahertz gyro-TWAs

The gyrotron travelling wave tube amplifiers (gyro-TWAs) presented in this paper can operate with high efficiency (30%), huge powers and wide bandwidths at high frequencies that no other amplifier can provide. In principle, this is a technology that can be scaled to >1 THz and operate with 20% bandwidths. Resonant coupling of two dispersive waveguide modes in a helically corrugated interaction region (HCIR) can give rise to a non-dispersive eigenwave over a wide frequency band. The synchronism between the ideal wave and an electron cyclotron mode, either fundamental or harmonic, of a large orbit electron beam contributes to the broadband amplification. An electron beam of 55 keV, 1.5 A with a velocity pitch angle of ~1 generated by a thermionic cusp gun is used in our 100 GHz gyro-TWA experiment, which achieves an unsaturated output power of 3.4 kW and gain of 36–38 dB. The design and experimental results of the many components making the gyro-TWA will be presented individually and then the whole system will be introduced. The amplification of a swept signal by the W-band gyro-TWA is demonstrated showing its capabilities in the field of telecommunications. Furthermore, the design studies of a cusp electron gun in the triode configuration and the realization of a 3-fold HCIR operating at 372 GHz will also be displayed.

Channel estimation for intelligent reflecting surface aided multi-user MISO terahertz system

Intelligent reflecting surface (IRS) is considered as a promising application in terahertz (THz) communications since it is able to enhance the THz communication with no additional power consumptions. In this letter, we consider the channel estimation problem for an IRS-aided THz multi-user multi-input single-output (MISO) system with lens antenna array. The main challenge of the problem is that we need to estimate multiple channels and some of the channels are cascaded. To deal with the problem, we propose a two-stage channel estimation scheme, where we set different IRS modes to estimate different channels for each stage. In stage 1, we set the IRS to an absorbing mode and estimate the channel without IRS. Removing the influence of the prior estimated channel, in stage 2, we estimate the channel with IRS by setting the IRS to a perfect reflecting mode. And we decompose the total channel estimation problem into a series of independent problems, where we estimate each independent channel component with a least square method.