Active Broadband Terahertz Research Articles

Active Terahertz Anisotropy and Dispersion Engineering Based on Dual-frequency Liquid Crystal and Dielectric Metasurface

Active broadband terahertz (THz) phase difference and its dispersion engineering based on the combination of dielectric metasurface and dual-frequency liquid crystal (DFLC) has been demonstrated, which can realize broadband achromatic wave plates and active manipulation by tuning the alternating ( AC ) frequency of electric field from 1 to 90 kHz. The artificial anisotropy of the dielectric metasurface is introduced to offset the tunable natural birefringence of the DFLC, so that the effective tuning range of the composite metadevice has been significantly improved, across from 0 to 1.0π, thus realizing the conversion from linear polarization (LP) light to arbitrarily polarized light. Moreover, a negative-dispersion phase difference of the dielectric metasurface can be achieved in the range of 0.69–1.4 THz by designing proper geometric parameters, which can be compensated with the positive-dispersion phase difference of DFLC to realize an achromatic device with broadband zero dispersion. The results show that the device can be tuned from OFF state without polarization conversion to a half-wave plate in 0.97–1.3 THz with the polarization conversion ratio (PCR) of over 97%, or to a quarter-wave plate in 0.67–1.3 THz with the PCR of over 96% by changing the AC frequency of the electric field. The active AC frequency tuning characteristics, anisotropic enhancement and broadband dispersion compensation show its great potential in THz application systems.

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.

Active bidirectional electrically-controlled terahertz device based on dimethyl sulfoxide-doped PEDOT:PSS.

A high-efficiency active bidirectional electrically-controlled terahertz device based on DMSO-doped PEDOT:PSS with low-power photoexcitation is investigated. Under low-power optical excitation of 30 mW (0.5 W/cm2) and under bias voltages ranging from -0.6 V to 0.5 V, spectrally broadband modulation of THz transmission over a range from -54% to 60% is obtained over the frequency range from 0.2 to 2.6 THz in a MEH-PPV/PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO hybrid structure. By considering the combined carrier density characteristics of the proposed device, it is found that the large-scale amplitude modulation can be ascribed to the electrically-controlled carrier density in the silicon layer with the assistance of the p-n junction that consists of the DMSO-doped PEDOT:PSS and silicon. Bidirectional modulation has a larger modulation range and is easier to use in communications applications when compared with unidirectional modulation. These results show great potential for application to the design of active broadband terahertz devices.

Active Optically Controlled Broadband Terahertz Modulator Based on Fe3O4 Nanoparticles

We report an active broadband terahertz (THz) modulator based on an Fe3O4 nanoparticle/silicon (Si) structure, where the interface effects were measured in a homemade THz time-domain spectroscopy system. An approximately 100-nm Fe3O4 nanoparticle thin film on the high-resistance Si substrate was easily attained by spin-coating ferrofluids. In our experiment, a modulation depth as high as 92% was achieved at an external laser irradiance of 3.6 W/cm2. This result can be explained by the accumulation of carriers at the interface of the hybrid structure, which induces intense absorption of the THz transmission. In addition, the limit modulated frequency of the device is ∼12 kHz. The superior performance of this device for THz wave modulation in comparison to other nanomaterial-based THz modulators and the ease of fabrication both illustrate that this is a promising method in the modulation of THz transmission. Furthermore, this modulator could also potentially provide an essential component in a wide variety of technologies, such as THz communications, THz imaging, etc.

Invited Article: An active terahertz polarization converter employing vanadium dioxide and a metal wire grating in total internal reflection geometry

Active broadband terahertz (THz) polarization manipulation devices are challenging to realize, but also of great demand in broadband terahertz systems. Vanadium dioxide (VO2) shows a promising phase transition for active control of THz waves and provides broadband polarization characteristics when integrated within grating-type structures. We creatively combine a VO2-based grating structure with a total internal reflection (TIR) geometry providing a novel interaction mechanism between the electromagnetic waves and the device, to realize a powerful active broadband THz polarization-controlling device. The device is based on a Si-substrate coated with a VO2 layer and a metal grating structure on top, attached to a prism for generating the TIR condition on the Si-VO2-grating interface. The grating is connected to electrodes for electrically switching the VO2 between its insulating and conducting phases. By properly selecting the incident angle of the THz waves, the grating direction, and the incident polarization state, we first achieved a broadband intensity modulator under a fused silica prism with an average modulation depth of 99.75% in the 0.2-1.1 THz region. Additionally, we realized an active ultra-broadband quarter-wave converter under a Si prism that can be switched between a 45° linear rotator and a quarter wave converter in the 0.8-1.5 THz region. This is the first demonstration of an active quarter-wave converter with ultra-broad bandwidth performance. Our work shows a highly flexible and multifunctional polarization-controlling device for broadband THz applications.