THz Switch Research Articles

All-dielectric double-layer honeycomb tunable metamaterial absorber with integrated gold nanoparticles

The optical regulation strategy of gold nanoparticles can significantly improve the performance of terahertz devices. We designed an all-dielectric double-layer honeycomb metamaterial absorber (MA) to demonstrate the broadband terahertz absorption characteristics in the presence or absence of gold nanoparticles. When it does not contain gold nanoparticles, MA exhibits a peak absorption efficiency of over 99% within the bandwidth range of ∼486 GHz. In particular, gold nanospheres (AuNPs), gold nanobipyramids (AuNBPs), and gold nanorods (AuNRs) are used to modulate the optical coupling effect of metamaterial absorbers, which improves their modulation performance. In the simulation, the effective medium theory (EMT) was applied to quantitatively calculate the optical response of a metamaterial absorber with an integrated gold nanoparticle equivalent gold layer. The integrated gold nanoparticle equivalent gold layer can achieve modulation enhancement of one order of magnitude. In the experiment, our process is compatible with CMOS technology, which may contribute to the development of terahertz detectors. In addition, the tunability and modulation enhancement characteristics demonstrated are beneficial for creating dynamic functional terahertz devices, such as THz modulators and switches.

Absorption and Reflection of Switchable Multifunctional Metamaterial Absorber Based on Vanadium Dioxide

A dynamically tunable terahertz broadband absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The absorber consists of two patterned VO2 layers and a metal bottom layer separated by two polytetrafluoroethylene (PTFE) dielectric layers. Simulation results show that the absorption exceeds 90% in the frequency range of 2.4–11 THz with a relative bandwidth of 128.4% under normal incidence. When VO2 is in the metal phase, the designed absorber functions as an ideal absorber. The absorption rate can be flexibly adjusted from 2% to 99% as vanadium dioxide transitions from the insulator phase to the metal phase. Therefore, the newly developed broad structure has the capability to seamlessly transition between functioning as an absorber or reflector through modifications in the conductivity of VO2 from the insulator phase to the metal phase. Moreover, further insight into the underlying physical processes can be gained by studying the insensitivity of the proposed absorber to the polarization of incident light and its ability to achieve high absorption across a wide range of incident angles. Impedance matching theory and electric field distribution of the absorber are investigated. The THz absorber has many potential applications in fields such as THz sensors, modulation, and switches.

Molecular Junctions for Terahertz Switches and Detectors.

Molecular electronics targets tiny devices exploiting the electronic properties of the molecular orbitals, which can be tailored and controlled by the chemical structure and configuration of the molecules. Many functional devices have been experimentally demonstrated; however, these devices were operated in the low-frequency domain (mainly dc to MHz). This represents a serious limitation for electronic applications, although molecular devices working in the THz regime have been theoretically predicted. Here, we experimentally demonstrate molecular THz switches at room temperature. The devices consist of self-assembled monolayers of molecules bearing two conjugated moieties coupled through a nonconjugated linker. These devices exhibit clear negative differential conductance behaviors (peaks in the current-voltage curves), as confirmed by ab initio simulations, which were reversibly suppressed under illumination with a 30 THz wave. We analyze how the THz switching behavior depends on the THz wave properties (power and frequency), and we benchmark that these molecular devices would outperform actual THz detectors.

Active dual-control terahertz electromagnetically induced transparency analog in VO2 metasurface

In this paper, an active dual-control electromagnetically induced transparency (EIT) analog is realized by using vanadium dioxide (VO2) metasurface on a sapphire substrate. The unit cell of the metasurface is a composite-split-ring-resonator (CSRR) composed of two resonators, one of which containing gold pattern and VO2 is named as VSRR and the other containing two T-type resonators is called TTR. The resonant frequency of VSRR and that of TTR are located at 0.43 and 0.75 THz, respectively. While, the CSRR have two resonant frequencies at 0.39 and 0.72 THz, and an EIT-like band has a central frequency at 0.56 THz. When the metasurface is electrically stimulated, the insulator-to-metal-transition (IMT) of VO2 can result in the reconstruction of the metasurface. Therefore, the EIT resonance can be controlled by bias voltages. At central frequency of 0.56 THz, a modulation depth of 87.7% and a group delay of 2.7 ps are obtained. The EIT mechanism is further explained by using a coupled Lorentz model, and theoretical calculation and simulation show good agreement with the experiment. Moreover, by mechanically adjusting the incidence angle, the adjustable EIT-like phenomenon is also observed and the modulation depth achieves 73%. This work paves a way for the development of THz modulators, switches, and slow light devices.

Excitation of THz plasmonic eignmode of parallel plain waveguide by using GaAs material

A Terahertz wave incident on waveguide made up of two parallel thin metal plates sandwiching a thin region of GaAs semiconductor, excites a surface plasma wave (SPW), at the metal–semiconductor interface. As the frequency of SPW increases the real part of the propagation vector increases linearly where the imaginary part of the propagation vector decay parabolically. It is also observed that the distortion of the pulse due to the Terahertz wave (THz) has different component of different phase velocity. The proposed methodology entails the generation of terahertz plasmonic eigenmodes within a parallel planar waveguide composed of GaAs material. The areas of focus in this field include THz Waveguiding and Routing, THz Modulators and Switches, THz Sensing and Imaging and THz Integrated Photonics.

High-Q transmission characteristics in terahertz guided-mode magnetic resonance system

Introduction: Guided mode resonance is generated by coupling wave diffractions with the waveguided mode. The guided mode resonances provide narrow-linewidth and resonance intensity for high quality factor (Q-factor) optical resonators.Methods: we demonstrate the high-Q guided mode resonances propagating on a low-loss, terahertz guided-mode magnetic resonance system, which are periodic square lattices of U-shaped split ring resonators (SRRs) on quartz substrates.Results: By choosing a judicious array period, two distinct frequency guided mode resonances and a magnetic dipole resonance with high Q-factor are observed. The interaction of the two resonances at similar frequencies produces a total transmission peak.Discussion: The dependences of the magnetic dipole resonance on the lattice period and structural parameters are investigated and discussed. The frequency difference between these two guided mode resonances widens with increasing Lattice period. The sharp spectral feature of each resonance results in the abrupt degradation of the spectral edge transmission. The proposed scheme is promising for efficient THz sensing, THz switching, and slow-light devices.

A graphene based dual-band metamaterial absorber for TE polarized THz wave

A graphene based tunable metamaterial perfect absorber is designed, which is a classic and simple sandwich structure composed of a non-rotationally symmetric graphene pattern layer, a TOPAS dielectric layer and a thick enough gold ground plane. The dynamically adjustable absorption property of the proposed absorber is polarization sensitive, which is novel compared with the widely studied polarization insensitive metamaterial absorber. For TE polarized incident electromagnetic waves (EMW), the dual-band absorption can be generated at frequencies of 4.268 THz and 6.032 THz with peak absorption coefficients up to 99.99% and 99.79%, respectively. But for TM light, there is only one near unity absorption band located at about 4.148 THz. So it will have potential application value for the polarization state resolution of THz waves. The research mainly concentrates on the dual-band tunable absorption under TE polarized incident light. The factors affecting the absorption properties of the proposed absorber are studied by discussing the dependence of the geometric parameters as well as chemical potential and relaxation time of the graphene on the absorption. The simulation results are verified by using multiple reflection interference theory (MRIT) and impedance matching theory (IMT), and the results show that the theoretical results match well with the simulation results. The absorber can achieve good absorption performance in a wide range of incident angles for TE light. Furthermore, by selecting two different graphene chemical potential, good performance of THz switching can be realized. In addition, the application in the field of refractive sensing is also investigated in detail, and higher sensitivity and better figure of merit (FOM) can be obtained. Thus, the designed graphene-based dual-band THz metamaterial absorber has great potential in polarization resolution, EMW absorbing, THz switching, refractive index sensing and so on.

Clocked molecular quantum-dot cellular automata circuits tolerate unwanted external electric fields

Quantum-dot cellular automata (QCA) may provide low-power, general-purpose computing in the post-CMOS era. A molecular implementation of QCA features nanometer-scale devices and may support ∼THz switching speeds at room-temperature. Here, we explore the ability of molecular QCA circuits to tolerate unwanted applied electric fields, which may come from a variety of sources. One likely source of strong unwanted electric fields may be electrodes recently proposed for the write-in of classical bits to molecular QCA input circuits. Previous models have shown that the input circuits are sensitive to the applied field, and a coupled QCA wire can successfully transfer the input bit to downstream circuits despite strong applied fields. However, the ability of other QCA circuits to tolerate an applied field has not yet been demonstrated. Here, we study the robustness of various QCA circuits by calculating their ground state responses in the presence of an applied field. To do this, a circuit is built from several QCA molecules, each described as a two-state system. A circuit Hamiltonian is formed and diagonalized. All pairwise interactions between cells are considered, along with all correlations. An examination of the ground state shows that these QCA circuits may indeed tolerate strong unwanted electric fields. We also show that circuit immunity to the dominant unwanted field component may be obtained by choosing the orientation of constituent molecules. This suggests that relatively large electrodes used for bit write-in to molecular QCA need not disrupt the operation of nearby QCA circuits. The circuits may tolerate significant electric fields from other sources as well.

High electron mobility effect in band-engineered GaN/quasi-AlGaN based exotic avalanche transit time diode arrays: application as ultra fast THz switches

High electron mobility effect in band-engineered GaN/quasi-AlGaN based exotic avalanche transit time diode arrays: application as ultra fast THz switches

Volatile and Nonvolatile Switching of Phase Change Material Ge2Sb2Te5 Revealed by Time-Resolved Terahertz Spectroscopy.

Phase change materials exhibit unique advantages in reconfigurable photonic devices due to drastic tunability of photoelectric properties. Here, we systematically investigate the thermal equilibrium process and the ultrafast dynamics of Ge2Sb2Te5 (GST) driven by femtosecond (fs) pulses, using time-resolved terahertz spectroscopy. Both fs-pulse-driven crystallization and amorphization are demonstrated, and the threshold of photoinduced crystallization (amorphization) is determined to be 8.4 mJ/cm2 (10.1 mJ/cm2). The ultrafast carrier dynamics reveal that the cumulative photothermal effect plays a crucial role in the ultrafast crystallization, and modulation depth of volatile (nonvolatile) THz has switching limits up to 30% (15%). A distinctive phonon absorption at 1.1 THz is observed, providing fingerprint spectrum evidence of crystalline lattice formation driven by intense fs pulses. Finally, multistate volatile (nonvolatile) THz switching is implemented by tuning optical pump fluence. These results provide insight into the photoinduced phase change of GST and offer benefits for all optical THz functional devices.

Femtosecond Broadband Frequency Switch of Terahertz Three-Dimensional Meta-Atoms

We present the experimental demonstration of a subpicosecond all-optical THz switch based on three-dimensional (3D) terahertz meta-atoms. Combining a special design of 3D meta-devices and the ultrafast dynamics of low temperature grown gallium arsenide, we can modulate the reflectance of the THz microcavities within 2.2 ps. The device enables a 280 GHz switch in resonance frequency within less than 200 fs. The switch back to the original resonance takes 800 fs. Experimental results show that the speed values are strongly convoluted by the THz probing field and, thus, the real switching times are even shorter, in the few 100 fs range.

Bistable Terahertz Switch Designed by Integration of a Graphene Plasmonic Crystal into Fabry-Perot Resonator

An appropriately designed periodic lattice in graphene can cause plasmonic modes, absorbing a specific THz frequency band, minimizing transmission. A suitable Fabry-Perot cavity, on the other hand, can resonate at another desired frequency in the nearby THz band, enhancing the transmittance. Integrating, these two types of structures into a THz switch, in which the peaks of the plasmonic absorption and the Fabry-Perot resonance band play the role of the OFF and ON states, yielding a bistable switch with a very high output due to the resonance and a high extinction (ON/OFF) ratio. By placing a patterned with an array of circular micro-holes with a plasmonic absorption band centered about 4.1 THz in the middle of a Fabry-Perot vertical cavity of resonance band centered about 3.9 THz, we have designed a bistable THz switch with 87% transmission in the ON state and the extinction ratio of 23 dB. The optical bistability, in this switch, is the consequence of graphene nonlinear property invoked by the enhanced optical intensity due to the cavity resonance, adding an intensity-dependent term to the graphene surface conductivity.

Single-shot all-optical switching of magnetization in Tb/Co multilayer-based electrodes

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. In particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards THz frequencies. This work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of Tb/Co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. Toggling of the magnetization of the Tb/Co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm2, for Co-rich compositions of the stack either in isolation or coupled to a CoFeB-electrode/MgO-barrier tunnel-junction stack. Successful switching of the CoFeB-[Tb/Co] electrodes was obtained even after annealing at 250 °C. After integration of the [Tb/Co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. These results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring THz switching speeds.

Thermo Optical Switching and Sensing Applications of an Infrared Metamaterial

Plasmonic induced transparency as an interesting physics behind many novel plasmonic devices is used to design a nano structure composed of graphene and Indium Antimonide (InSb). Due to temperature dependent of graphene and InSb permittivity, the structure is a good candidate for temperature sensor. The sensitivity of the proposed thermal sensor is optimized and the best value for sensitivity is obtained 160(nm/k). The switch application of the proposed structure is also analyzed and a switch with modulation depth of 98% and modulation efficiency of 82% is proposed which has very good values compare to other THz switch reported in literature.

Metamaterial nonlinear and polarization-dependent bi-frequency THz switch

In this work, we propose a novel metamaterial structure with two concentric split-ring resonators. The splits are filled by photoconductive Si. Illuminating the structure using an oblique optical pump with a wavelength of 800 nm, in addition to the normal THz signal wave, excites the structure by varying the Si conductivity. Also, by changing the direction of the incident TE wave by 90°, the transmission frequency is changed. Therefore, the structure operates as a switch at two different windows using two different methods. The response time of the photoconductive switch is less than 3 ps. The substrate was replaced by a more cost-effective and flexible material, polyimide, to achieve even more exciting results in broadening the switching windows. Also, the response time of this switch is less than 1 ps. Finally, for verification of the simulation results, a circuit model of the photoconductive switch is proposed, and the results comply very well with the simulation results. The proposed switch can be used in fast optical systems and networks.

Tunable terahertz metamaterial by using asymmetrical double split-ring resonators (ADSRRs)

Two designs of asymmetrical double split-ring resonators (ADSRRs) with one layer and two reflective symmetric layers are presented, which are composed of Au material on Si substrate. The electromagnetic responses of ADSRRs designs exhibit a single-resonance for g = 0 μm and dual-resonance in the range of g = 2 μm to 10 μm. By changing the gap width of ADSRRs with one layer, the resonant frequencies of two devices are blue-shifted 0.11 THz from 0.67 THz to 0.78 THz at TE mode and blue-shifted 0.07 THz from 0.62 THz to 0.69 THz at TM mode. To improve and perform the active tuning of ADSRRs, two devices are designed to compose of two reflective symmetric layers with a distance between top and bottom ADSRRs. By changing this distance, the tuning range of resonance is enhanced to 0.52 THz. Furthermore, two devices with one layer and two reflective symmetric layers exhibit a switch behavior by changing incident polarization angle. The maximum modulation depth is up to 93%. Therefore, the resonant frequency of devices can be tuned by changing the distance between top and bottom reflective symmetric ADSRRs to be realized as a THz filter and by changing the incident polarization angle to be realized as a THz switch. This study promises applications in THz filters, THz switches, and other controllable metamaterial-based devices.

Actively Tunable Terahertz Switches Based on Subwavelength Graphene Waveguide.

As a new field of optical communication technology, on-chip graphene devices are of great interest due to their active tunability and subwavelength scale. In this paper, we systematically investigate optical switches at frequency of 30 THz, including Y-branch (1 × 2), X-branch (2 × 2), single-input three-output (1 × 3), two-input three-output (2 × 3), and two-input four-output (2 × 4) switches. In these devices, a graphene monolayer is stacked on the top of a PMMA (poly methyl methacrylate methacrylic acid) dielectric layer. The optical response of graphene can be electrically manipulated; therefore, the state of each channel can be switched ON and OFF. Numerical simulations demonstrate that the transmission direction can be well manipulated in these devices. In addition, the proposed devices possess advantages of appropriate ON/OFF ratios, indicating the good performance of graphene in terahertz switching. These devices provide a new route toward terahertz optical switching.

Transparent AlON ceramic combined with VO2 thin film for infrared and terahertz smart window

Transparent AlON ceramics are intriguing window materials with excellent mechanical strength and superb optical transparency from the near ultraviolet to the mid-infrared range. However, previous studies focused their investigations in the visible range; therefore, the application of transparent AlON ceramics to tunable windows has yet to be reported. In this work, a VO2 thin film with a characteristic semiconductor-metal phase transition (SMT) was fabricated on a transparent AlON ceramic, which exhibited remarkable tunable switching properties in the infrared and terahertz ranges. The transparent AlON ceramic was prepared by a two-step method, which included carbothermal reduction and pressureless sintering. The resulting ceramic exhibited high transparency of over 70% in the visible-infrared range and a notable THz transmission of 64.5–73.9% at 0.1–1.5 THz. The VO2 thin film was prepared on a transparent AlON ceramic using the sol-gel method and showed excellent optical and electric switching performance. The square resistance variance was close to four orders of magnitude, and an infrared switching ratio of over 40% was obtained. Furthermore, the combined structure showed an efficient THz switching ratio of approximately 70.9%. This study proposes a composite material combined with a transparent ceramic and a phase transition oxide and provides great insights into their application in infrared and terahertz smart windows as well as in switching devices.

Geometric effect in a vertical stack-up metal-insulator-metal tunnel diode

The geometric effect was investigated in a vertically designed metal-insulator-metal (MIM) tunnel diode for which a narrow tunneling distance can be controlled easily and reliably, with the goal of enhancing rectifying efficiency, based on the angle of a pointed shape electrode and various thicknesses of insulator material. Although MIM tunneling diodes can provide ultra- high working speeds (>THz), the very low contrast ratio between forward and reverse currents results in poor rectifying efficiency. An asymmetric geometry design with two metal electrodes can be an effective approach for enhancing the contrast ratio between the tunneling currents. Using a sharp electrode with a pointed shape, it was determined that the rectifying efficiency and tunneling probability could be increased impressively depending on the angle of the pointed shape of the electrode. Moreover, the selection of insulation material was also important for improving efficiency. Although the band gap of Al2O3 is larger than that of HfO2, the rectifying efficiency was significantly improved by blocking reverse current well. In general, large band gap insulator materials are inappropriate for a tunneling device, due to the low tunneling current. However, in our approach, since the issue of low tunneling probability is compensated by the sharp tip structure, the larger band gap insulator produced better rectifying efficiency with the appropriate current density. The results of this study demonstrated that geometric design could be a possible solution to increase rectifying efficiency. If the geometric effect in the tunneling diode structures will be optimized more, it can improve the applicability of vertical stack-up MIM tunnel diode to THz switching devices, tunneling transistors and ultra-high speed electronics.

History and modern applications of nano-composite materials carrying GA/cm2 current density due to a Bose-Einstein Condensate at room temperature produced by Focused Electron Beam Induced Processing for many extraordinary novel technical applications

The discovery of Focused Electron Beam Induced Processing and early applications of this technology led to the possible use of a novel nanogranular material “Koops-GranMat®” using Pt/C and Au/C material. which carries at room temperature a current density > 50 times the current density which high TC superconductors can carry. The explanation for the characteristics of this novel material is given. This fact allows producing novel products for many applications using Dual Beam system having a gas supply and X.Y.T stream data programming and not using GDSII layout pattern control software. Novel products are possible for energy transportation. -distribution.-switching, photon-detection above 65 meV energy for very efficient energy harvesting, for bright field emission electron sources used for vacuum electronic devices like amplifiers for HF electronics, micro-tubes, 30 GHz to 6 THz switching amplifiers with signal to noise ratio >10(!), THz power sources up to 1 Watt, in combination with miniaturized vacuum pumps, vacuum gauges, IR to THz detectors, EUV- and X-Ray sources. Since focusing electron beam induced deposition works also at low energy, selfcloning multibeam-production machines for field emitter lamps, displays, multi-beam - lithography, - imaging, and - inspection, energy harvesting, and power distribution with switches controlling field-emitter arrays for KA of currents but with < 100 V switching voltage are possible. Finally the replacement of HTC superconductors and its applications by the Koops-GranMat® having Koops-Pairs at room temperature will allow the investigation devices similar to Josephson Junctions and its applications now called QUIDART (Quantum interference devices at Room Temperature). All these possibilities will support a revolution in the optical, electric, power, and electronic technology.