Switchable THz Research Articles

Switchable THz wave absorber based on disks and its complement graphene surfaces

A new geometrical vision for a graphene-based THz wave absorber is investigated in this work. A high-performance THz absorber is proposed to exploit complementary conventional periodic arrays of graphene disks. The equivalent circuit model is modified to take account of the complement pattern. The absorber works in two different operational modes based on chemical potential values. One mode shows absorption around 1 THz while the other one expresses perfect absorption at 6 THz and 9 THz. Such a considerable shifting ability via gate stimulations makes the proposed absorber an ideal block for reconfigurable metasurface. The comparison between the circuit model and full-wave simulation verifies an excellent match while ample sensitivity analysis are reported to show the reliability of the proposed switchable THz absorber.

Switchable and tunable bifunctional THz metamaterial absorber

The recent rise of active materials offers new opportunities for dynamically tunable metamaterial devices. However, metamaterial devices with a dynamically switchable function are in high demand. Inspired by previous studies, a dynamically switchable and tunable bifunctional THz (THz) metamaterial absorber based on vanadium dioxide ( V O 2 ) and graphene is proposed and investigated in this paper. Using the phase transition properties of V O 2 , the switchable performance between broadband near-perfect absorption and multiband near-perfect absorption can be achieved. When V O 2 is in the metallic state, the designed metamaterial absorber acts as a broadband near-perfect absorber with more than 90% absorption in the frequency range from 2.6 THz to 7.5 THz. Alternatively, the designed metamaterial absorber with the same geometry can be transformed into a tunable multiband absorber when V O 2 is converted to the insulated state. The performance of the absorber is analyzed by the multiple interference theory (MIT), which is in good agreement with the numerical simulation results. Due to the high symmetry of the structure, the presented absorber exhibits excellent polarization insensitivity and wide incident-angle stability, which opens a direction to design a new type of multifunctional metamaterial devices in the THz regime.

Enhanced performance of a fast GaAs-based terahertz modulator via surface passivation

Surface-modified semiconductors show enormous potential for opto-terahertz (THz) spatial modulation due to their enhanced modulation depth (MD) along with their inherent broad bandwidth. Taking full advantage of the surface modification, a performance-enhanced, all-optical, fast switchable THz modulator was achieved here based on the surface-passivated GaAs wafer. With a decreased surface recombination rate and prolonged carrier lifetime induced by passivation, S-passivated GaAs was demonstrated as a viable candidate to enhance THz modulation performance in MD, especially at low photodoping levels. Despite a degraded modulation rate owing to the longer carrier lifetime, this passivated GaAs modulator simultaneously realizes a fast modulation at a 69-MHz speed and as high an MD as ∼ 94 % in a spectral wideband of 0.2–1.2 THz. The results demonstrated a new strategy to alleviate the tradeoff between high MD and speed in contrast to bare surfaces or heterogeneous films/unusual geometry on semiconductors including Si, Ge, and GaAs.

A bi-tunable switchable polarization-independent dual-band metamaterial terahertz absorber using VO2 and Dirac semimetal

A bi-tunable switchable dual-band THz absorber using vanadium dioxide (VO2) and Dirac semimetal films (DSFs), which is independent on polarization, is proposed. The frequency and intensity of absorptances of the two absorption peaks can be dynamically changed by tuning the Fermi energy of the DSF or the VO2 conductivity. Moreover, the absorber can alternate between an absorption function (with an absorptance of 99.4%) and a reflection function (with a reflectance of 77.4%) at the frequencies of absorption peak B in case of VO2 transitions from metallic state to insulator state. In addition, the proposed absorber exhibits advantages such as polarization-independent nature and wide incident angle tolerance. This study provides references to future studies of THz bi-tunable dual-band or multi-band polarization-independent absorbers.

Switching between the Functions of Half-Wave Plate and Quarter-Wave Plate Simply by Using a Vanadium Dioxide Film in a Terahertz Metamaterial**Supported by the Scientific and Technological Developing Scheme of Jilin Province (Grant No. 20180101281JC), “135” Research Project of Education Bureau of Jilin Province (Grant No. JJKH20190579KJ), and “111” Project of China (Grant No. D17017).

We propose a switchable THz metamaterial that can be switched between two functions of half-wave plate and quarter-wave plate. The two switchable functions can be simply achieved by inserting a VO2 film in the metamaterial design. Finite-difference time-domain (FDTD) simulation results show that the proposed metamaterial can convert x-polarized incident wave to y-polarized reflected wave when VO2 is at metal phase, and convert x-polarized wave to circularly polarized wave when VO2 is at insulator phase. The metamaterial performs well in the two functions, i.e., the same broad working frequency band and near perfect polarization conversion. The switching effect originates from the switchable Fabry–Pérot cavity length induced by the phase change of VO2. We believe that our findings provide a reference in designing switchable metamaterials.

A broadband and switchable VO2-based perfect absorber at the THz frequency

We demonstrate a broadband and switchable THz metamaterial absorber by utilizing the phase transition of VO2, which is a stacked structure composed of VO2 periodic array, dielectric layer, VO2 film, Au periodic array, dielectric layer and Au reflective layer, respectively. The absorption band from 0.76 THz to 0.86 THz at room temperature can be changed into the absorption band from 1.12 THz to 1.25 THz when temperature increases above the phase change temperature of VO2, with the absorptivity in both bands over 90%. Furthermore, the effective medium theory is introduced to explain the perfect absorption mechanism. Such absorber can work well over a wide range of incidence around 40°. Thickness of the absorber is only about one twentieth of the working wavelength. The proposed structure can be applied to absorbers working at other frequencies.

Terahertz Metamaterial Devices Based on Graphene Nanostructures

We propose here the concept and practical design of THz metamaterial devices consisting of subwavelength graphene nanostructures placed on a dielectric substrate and backed by a metal gate. This geometry allows us to introduce the design of a THz metaferrite screen with effective permeability that can be tuned from positive to negative, as a function of graphene's Fermi energy. We then discuss its potential applications to realize tunable and switchable THz and far-infrared artificial magnetic screens, perfect absorbing films, and polarizing filters.

Electrically Tunable Terahertz Notch Filters

In this letter we present a switchable THz notch filter. The filter contains a liquid crystal layer that acts as a half wave retarder in one state and as an isotropic layer in the other state. The device combines three unique properties: it can be switched electrically, it provides a filter depth of 35 dB at 350 GHz and it can be tuned over wide frequency range from 350 GHz to 700 GHz.

Channel-switchable single-/dual-wavelength single-longitudinal-mode laser and THz beat frequency generation up to 3.6 THz

We propose and demonstrate a simple approach to realize channel-switchable single-/dual-wavelength single-longitudinal-mode (SLM) lasers and switchable beat frequency generation in the THz frequency regime. It is a compact fiber-ring laser incorporating a Fabry–Perot etalon, tunable filters, and a piece of unpumped polarization maintaining erbium-doped fiber (PM-EDF) as the saturable absorber. Two parallel-arranged tunable filters are configured inside the ring cavity to support switchable dual-wavelength SLM operation and consequent THz beat frequency generation. Switchable 19-channel 200-GHz ITU-grid-compatible single-wavelength SLM operation is implemented. Moreover, the channel-switchable dual-wavelength SLM laser and switchable THz beat frequency generation ranging from 0.2 to 3.6 THz with a tuning step of 0.2 THz are demonstrated in the experiment.