Terahertz Switch Research Articles

Terahertz bifunctional absorber based on a graphene-spacer-vanadium dioxide-spacer-metal configuration

A terahertz bifunctional absorber is presented with broadband and narrowband absorbing properties in a graphene-spacer-vanadium dioxide-spacer-metal configuration. Carrier relaxation time of graphene τ = 1.0ps (τ = 0.1ps) is chosen for narrowband (broadband) absorption. When vanadium dioxide is in the conducting state, the design behaves as a narrowband absorber, and it is composed of a square-shaped graphene, topas spacer, and metallic vanadium dioxide film. There is an absorption band with 100% absorptance at the frequency of 1.37 THz. Narrowband absorption is caused by the localized magnetic resonance. When vanadium dioxide is in the insulating state, the design behaves as a broadband absorber composed of a square-shaped graphene, topas layer, vanadium dioxide film, and metal film. It has a broadband absorption in the frequency range of 1.05-2.35 THz, and the corresponding absorptance is more than 90%. The merging of two resonances with overlapping region ensures broadband performance of the designed absorber. The working bandwidth and intensity of narrowband absorption and broadband absorption can be dynamically adjusted by changing the Fermi energy level of graphene. The influences of structure parameters are discussed on absorption performance. In addition, the designed absorber is not sensitive to incident angle. Because of the simple structure, our design can be applied to many promising fields in intelligent absorption and terahertz switch.

Dual-band terahertz switch based on EIT/Fano effect

Terahertz system requires dynamic manipulation of terahertz wave, however, quite a few terahertz wave devices are static, and their functions are fixed after fabrication. We realize a dual-band terahertz switch based on electromagnetic induced transparency (EIT) and Fano effect through change the operating temperature. It consists of two rectangular strips, VO2-embedded rectangular-split ring, and silicon substrate. The extinction ratios of the dual-band terahertz switch at 0.480 THz and 0.675 THz are 20.5 dB and 12.2 dB, respectively. Our approach can be used to design dynamic multi-wavelength terahertz beam manipulation devices.

Ultracompact and dynamically tunable plasmonic THz switch based on graphene microcavity metamaterial

We propose and discuss a compact and tunable plasmonic terahertz (THz) metamaterial switch based on graphene microcavity. Due to the joint effect of graphene plasmon resonances and Fabry–Perot oscillations, this metamaterial structure can be switched from the ON state to OFF state with the maximum intensity attenuation larger than 95% (ON–OFF ratio larger than 30 dB). The central frequency of the switch could be tuned in a wide frequency range from 3.6 to 9.6 THz by tuning the Fermi energies of the graphene patterns. Moreover, the metamaterial structure is valid for a wide range of incident angles of both transverse electric (TE) and transverse magnetic (TM) polarizations with the highest Q factor of 15.5. This investigation promises the development of high-performance, widely tunable THz switch in chip integrated photonic circuits without reconstructing the physical structures.

Broadband electrically tunable VO2‑Metamaterial terahertz switch with suppressed reflection

AbstractDevices designed to dynamically control the transmission, reflection, and absorption of terahertz (THz) radiation are essential for the development of a broad range of THz technologies. A viable approach utilizes materials which undergo an insulator‐to‐metal transition (IMT), switching from transmissive to reflective upon becoming metallic. However, for many applications, it is undesirable to have spurious reflections that can scatter incident light and induce noise to the system. We present an electrically actuated, broadband THz switch which transitions from a transparent state with low reflectivity, to an absorptive state for which both the reflectivity and transmission are strongly suppressed. Our device consists of a patterned high‐resistivity silicon metamaterial layer that provides broadband reflection suppression by matching the impedance of free space. This is integrated with a VO2 ground plane, which undergoes an IMT by means of a DC bias applied to an interdigitated electrode. THz time domain spectroscopy measurements reveal an active bandwidth of 700 GHz with suppressed reflection and more than 90% transmission amplitude modulation with a low insertion loss. We utilize finite‐difference time domain (FDTD) simulations in order to examine the loss mechanisms of the device, as well as the sensitivity to polarization and incident angle. This device validates a general approach toward suppressing unwanted reflections in THz modulators and switches which can be easily adapted to a broad array of applications through straightforward modifications of the structural parameters.

Photoexcited Blueshift and Redshift Switchable Metamaterial Absorber at Terahertz Frequencies**Supported by the National Key Research and Development Program of China under Grant No. 2017YFA0700202.

We propose a design and numerical study of an optically blueshift and redshift switchable metamaterial (MM) absorber in the terahertz regime. The MM absorber comprises a periodic array of metallic split-ring resonators (SRRs) with semiconductor silicon embedded in the gaps of MM resonators. The absorptive frequencies of the MM can be shifted by applying an external pump power. The simulation results show that, for photoconductivity of silicon ranging between 1 S/m and 4000 S/m, the resonance peak of the absorption spectra shifts to higher frequencies, from 0.67 THz to 1.63 THz, with a resonance tuning range of 59%. As the conductivity of silicon increases, the resonance frequencies of the MM absorber are continuously tuned from 1.60 THz to 1.16 THz, a redshift tuning range of 28%. As the conductivity increases above 30000 S/m, the resonance frequencies tend to be stable while the absorption peak has a merely tiny variation. The optical-tuned absorber has potential applications as a terahertz modulator or switch.

Optically controlled ultrafast terahertz switching in a CdTe nanostructure thin film.

The speed of optical modulation on a terahertz (THz) pulse is mainly dominated by the optical response of the photocarrier. In order to achieve ultrafast THz modulation, the effective method is to reduce the lifetime of the photocarrier by introducing defects that can trap the photocarriers efficiently. In this paper, we reported the ultrafast optical modulation of THz switching in a 10 nm CdTe nanostructure film. After photoexcitation at 800/400 nm, the THz response of the film is extremely fast with a lifetime of ${\sim}{1.3}\;{\rm ps}$∼1.3ps. Further, the ultrafast transient THz transmission shows almost temperature independence down to 100 K. On the other hand, the transient absorption spectroscopy reveals that the lifetime of photocarriers in CdTe nanostructure film lasts as long as several ns. The 1.3 ps THz photoconductivity response is due to the substantial decrease of photocarrier mobility in a CdTe nanostructure, which comes from the increase of the photocarrier scattering between the photocarrier and the surface states of CdTe nanostructural film. Our experimental results provide a new method to design optically driven ultrafast THz response devices, such as THz switch and THz modulator.

Terahertz resonance switch induced by the polarization conversion of liquid crystal in compound metasurface.

We experimentally demonstrate an active terahertz (THz) resonance switch induced by the polarization conversion in a compound metasurface, which is a LC layer sandwiched by a metallic wire grating and resonance metamaterial (LCGM). Here, the liquid crystal (LC) plays the role of polarization conversion, which can induce the TE resonance. Moreover, there exists a localized resonance between metallic grating and metamaterial layers, and then the excited resonance will be greatly enhanced. The results show that the high extinction ratio of the resonance switch exceeds 30dB at 0.82THz. This work will bring new ideas for the research in developing THz phase, polarization, and switch devices with LC and metasurface.

Tunable ultra-narrowband terahertz perfect absorber by using metal-insulator-metal microstructures

Abstract An ultra-narrowband terahertz (THz) perfect absorber with metal-insulator-metal (PA-MIM) microstructures is presented. MIM microstructures is composed of Al disk on SiO2/Al/Si substrate. This MIM configuration will induce dipole-image interaction and cause an electromagnetic confinement between MIM microstructures to eliminate the reflection. Therefore, the design of MIM can absorb most of THz wave to realize the THz PA. The absorption intensity of proposed device could be reached 99% with an ultra-high Q-factor of 143. By modifying the gap between top and bottom Al disks of MIM, the electromagnetic response can be tuned from single-band to dual-band resonance. These resonances can be actively controlled the corresponding absorption intensities to possess tunability in application of THz switch and THz variable optical attenuator (VOA). This tunable ultra-narrowband THz PA-MIM paves a way to be an important role in THz applications, such as sensors, detectors, solar cells, coherent thermal emitters and so on.

Ultra-compact Spatial Terahertz Switch Based on Graphene Plasmonic-Coupled Waveguide

We are proposing graphene (G)-based multilayered plasmonic spatial switch, operating at 10 THz. It is composed of hBN/Ag/hBN/G/hBN/G/hBN/SiO2/p+-Si multilayers. When a 10-THz transverse magnetic (TM)-polarized signal is normally incident upon the structure top surface, the nanoaperture devised in the Ag nanolayer, acting as a grating, excites surface plasmons at the top graphene micro-ribbons/hBN interface. These surface plasmons depending on the graphenes chemical potentials can be coupled to the lower-right or left graphene micro-ribbons and continue to propagate laterally towards the corresponding output port. Numerical simulations show that a change of ∆VG ≈ ± 2.7 V in the voltage, applied to the gated micro-ribbons, can modulate their chemical potentials sufficiently to switch the right (left) output port from ON (OFF) to OFF(ON) and vice versa. Besides its low power consumption, the switch ultra-small dimensions make it a potential spatial router suitable for THz-integrated circuit applications.

Investigation of electromagnetic response of terahertz metamaterial by using split-disk resonator

A terahertz (THz) metamaterial split-disk resonator (SDR) is proposed to possess multifunctionality. This SDR structure is composed of Al disk with a split on Si substrate. It exhibits a large resonance shift in transmission spectrum under reconfiguration of geometrical dimensions. The resonance shift is 0.60 THz. The distinct characteristics at TE and TM modes are investigated and discussed. Such SDR design provides a high-efficiency resonator in THz wave applications and can realize polarization-dependent for THz switch. Furthermore, the design of SDR can resist the variation of ambient refraction index, which provides the capability for the use in various environments.

Switching Terahertz Waves using Exceptional Points

In quantum mechanics, $e\phantom{\rule{0}{0ex}}x\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $p\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}s$ are non-Hermitian degeneracies with intriguing properties, and the non-Hermiticity gives an extra degree of freedom that allows for innovative applications. Exploiting the unique topological structure of energy surfaces near the exceptional point, the authors propose a terahertz switch in which the system's transmission can be tuned by changing the chemical potential of monolayer graphene. Their idea can in principle be applied to design a switch for other frequency ranges, and manipulating the energy surfaces of non-Hermitian systems can serve as a useful paradigm to design photonic devices with further functionalities.

Broadband tunable absorber for terahertz waves based on isotropic silicon metasurfaces

A broadband tunable absorber is studied based on the silicon metasurface at terahertz frequencies. By tuning the conductivity of silicon, absorptance is more than 90% from 0.497 THz to 1.045 THz with central frequency of 0.771 THz when the conductivity is equal to 2500S/m. Simulated results show that absorptance peak can be modulated from 1% to 100% when the conductivity continuously changes from 0S/m to2500S/m. The central-symmetry structure leads to polarization independence. Our design may be used as optoelectronic modulator, tunable detector, and terahertz switch.

Wide Band Terahertz Switch of Undulated Waveguide With VO2 Film Coated Inner Wall

We propose a wide band terahertz (THz) switch of undulated waveguide with vanadium dioxide (VO2 ) film coated inner wall. The forbidden bands formed by periodically undulated walls effectively restrain THz transmission when VO2 is in the insulator phase. An unexpected wide band appears in the transmission spectrum, connecting two forbidden bands, where metal phase VO2 covers the undulations and eliminates band gaps. Simulations confirm that the switch could be realized by varying the temperature in the frequency range 0.71–1.07 THz, and hence such a switch could be applied in THz systems.

An active metamaterials controlled by structured light illumination

The paper reports a novel optical active metamaterials controlled by structured light. In such metamaterials, multiple Au brackets are integrated in its unit-cell; the brackets produce different resonance modes; the metamaterials’ properties (bandwidth or frequency) can be controlled by selectively activating one single resonance mode or multiple modes. This control only depends on the spatial distribution of light illumination. We expound the structured light controlling process and the design method of the metamaterials. Based on the method, a bandwidth and frequency tunable terahertz switch and a tunable absorber are designed and evaluated numerically.

Graphene-based nonvolatile terahertz switch with asymmetric electrodes

We propose a nonvolatile terahertz (THz) switch which is able to perform the switching with transient stimulus. The device utilizes graphene as its floating-gate layer, which changes the transmissivity of THz signal by trapping the tunneling charges. The conventional top-down electrode configuration is replaced by a left-right electrode configuration, so THz signals could transmit through this device with the transmissivity being controlled by voltage pulses. The two electrodes are made of metals with different work functions. The resultant asymmetrical energy band structure ensures that both electrical programming and erasing are viable. With the aid of localized surface plasmon resonances in graphene ribbon arrays, the modulation depth is 89% provided that the Femi level of graphene is tuned between 0 and 0.2 eV by proper voltage pulses.

Photo-excited multi-frequency terahertz switch based on a composite metamaterial structure

We propose a photo-excited tunable multi-frequency metamaterial (MM) switch that can be used in the terahertz region. This metamaterial switch is composed of a polyimide substrate and a hybrid metal–semiconductor square split-ring resonator (SRR) with two gaps, with various semiconductors placed in critical regions of the metallic resonator. By changing the incident pump power, we were able to tune the conductivity of the diverse semiconductors filling the gaps of the SRR, and by using an external exciting beam, we were able to modulate the resonant absorption properties of the composite metamaterial structure. We demonstrated the tunable multi-frequency metamaterial switch by irradiating the composite metamaterial structure with a pump laser. In addition, we proposed a tunable metamaterial switch based on a circular metallic split-ring resonator.

Switching terahertz wave with grating-coupled Kretschmann configuration.

We present a terahertz wave switch utilizing Kretschmann configuration which consists of high-refractive-index prism-liquid crystal-periodically grooved metal grating. The switching mechanism of the terahertz switch is based on spoof surface plasmon polariton (SSPP) excitation in the attenuated total reflection regime by changing the liquid crystal refractive index. The results highlighted the fact that the feasibility to "tune" the attenuated total reflection terahertz wave intensity by using the external applied bias voltage. The extinction ratio of the terahertz switch reaches 31.48dB. The terahertz switch has good control ability and flexibility, and can be used in potential terahertz free space device systems.

Dynamic control of coherent pulses via destructive interference in graphene under Landau quantization

We analyze the destructive interference in monolayer graphene under Landau quantization in a time-dependent way by using the Bloch-Maxwell formalism. Based on this analysis, we investigate the dynamics control of an infrared probe and a terahertz (THz) switch pulses in graphene. In presence of the THz switch pulse, the destructive interference take places and can be optimized so that the monolayer graphene is completely transparent to the infrared probe pulse. In absence of the THz switch pulse, however, the infrared probe pulse is absorbed due to such a interference does not take place. Furthermore, we provide a clear physics insight of this destructive interference by using the classical dressed-state theory. Conversely, the present model may be rendered either absorbing or transparent to the THz switch pulse. By choosing appropriate wave form of the probe and switch pulses, we show that both infrared probe and THz switch pulses exhibit the steplike transitions between absorption and transparency. Such steplike transitions can be used to devise a versatile quantum interference-based solid-state optical switching with distinct wave-lengths for optical communication devices.

Phase controllable terahertz switch in a Landau-quantized graphene nanostructure

The optical properties of a weak probe light by applying coupling fields in Landau-quantized graphene nanostructure is investigated. In this structure the electromagnetic field of terahertz-infrared radiations interfere with the electromagnetic field of the short-wavelength probe field and this effect changes the absorption and dispersion characteristics of the probe field. The linear dynamical properties of the graphene by means of perturbation theory and density matrix method are discussed. We show that the group velocity of a light pulse can be switched from superluminal to subluminal or vice versa by controlling the coupling field’s intensities and relative phase of the applied fields. Therefore, this model can be used as an all-optical switch which is suitable for next generation of future all optical quantum communicational system and networks.

All-optical THz wave switching based on CH3NH3PbI3 perovskites.

Hybrid structures of silicon with organic–inorganic perovskites are proposed for optically controllable switching of terahertz (THz) waves over a broad spectral range from 0.2 to 2THz. A 532-nm external laser was utilized to generate photoexcited free carriers at the devices and consequentially to control the terahertz amplitude modulation, obtaining a depth of up to 68% at a laser irradiance of 1.5 W/cm2. In addition, we compared the performances from three types of perovskite devices fabricated via different solution processing methods and suggested a stable and highly efficient THz switch based on a one-step processing. By this we demonstrated the possibility of perovskites as THz wave switching devices in addition to photovoltaics.