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The transmission properties of terahertz wave through subwavelength metallic slits have been investigated using a quasianalytical modal expansion method. The effects of slit widths, periodic length and incident angle on the transmission spectrum and scattering phase are given and dicussed. The results demonstrate that the transmittance increases with the increasing of the ratio of width to slit period. As the incident angle increases, the transmittance increases and shows more peaks.

We have analyzed the spatio-temporal patterns of current self-oscillation under DC bias and terahertz frequency-dependent nonlinear dynamics in semiconducting single-walled zigzag carbon nanotubes (CNTs). It is found that different transport states, including periodic and chaotic, appear with the influence of an external AC signal. The global transitions between periodic and chaotic states are clearly resolved from Poincaré bifurcation diagram. When the driving frequency is fixed at inverse golden mean ratio times natural frequency, the nonlinear CNT system exhibits a more complex transition behavior than at other driving frequencies.

We have calculated the fifth-order nonlinear optical response at experimentally relevant field strengths within the model of massless Dirac fermions by coupling the massless Dirac fermions to the time-dependent electric field quantum mechanically. It demonstrates that the fifth-order nonlinear optical response plays an important role in the contribution to the optical conductivity of pristine single-layer graphene in the low frequency part of the terahertz regime. The nonlinear effect can enhance the optical activity of single-layer graphene in the terahertz regime and significantly decreases the transmittance of graphene in the regime of frequencies from 0.1 to 0.5 THz. These properties of graphene may be used for photonic and optoelectronic device in the terahertz regime.

On the basis of transfer matrix method (TMM), the effective medium theories (EMT) which can be applied to the n-layer unit cell structures have been developed. By using this method, we have systematically investigated the influences of the structure parameters, operation frequency, and material gain on the dispersion properties and transmission of the metallic-dielectric stratified structure (MDSS) in the terahertz (THz) regime. The n-unit cell structures show much more freedom to study the MDSS. The results show that the dispersion relationship acquired from the EMT agrees well with those from the TMM when the value of k_x is small. With the increase of the frequency, the n-layer unit cell MDSS can show more than one dip, and the dip positions can be changed by altering the doping concentration of the InSb layer. As the material gain increase, the transmission increases, and the full-width-half-maximum of the transmission spectrum decrease. This proposed method can also be applied to other spectral regions, such as microwave and infrared regimes. The results are beneficial to improve the performance of MDSSs devices and have great potential for many applications in fields of sub-wavelength image and superlens design.

The characteristics of terahertz (THz) generation based on cascaded difference frequency generation (DFG) process in periodically inverted gallium arsenide (GaAs) and gallium phosphide (GaP) is calculated from coupled wave equations, in which the output enhancement factors are 5.4 and 3.9 in the two crystals, respectively, compared with DFG without cascading. The optimal interaction length, influence of crystal absorption, wave vector mismatch and pump intensity are analyzed. A short discussion on wavelength tuning is also given. The calculated optimal operating parameters and conclusions can provide good directions for the experimental design.

We propose the design of a broadband planar metamaterial absorber (MA) at terahertz frequencies. The unit cell of the MA is composed of four dual-band sub-cells with different dimensions in a coplanar. The four dual-band sub-cell structures resonate at several neighboring frequencies. The absorber consists of two metallic layers separated by a dielectric spacer. Simulation results show that the metamaterial absorption at normal incidence is above 90% in the frequency of 6.56–8.10 THz. This design provides an effective way to construct broadband absorber. The multiple-reflection theory was used to explain the absorption mechanism of our investigated structures. The coupling of adjacent four dual-band sub-cells can introduce additional capacitance to affect the performance of absorber.

The terahertz nonlinear optical response in Kronig–Penney (KP) type graphene superlattice is demonstrated. The single-, triple- and quintuple-frequencies of the fifth-order nonlinear responses are investigated for different frequencies and temperatures with the angle φ along the periodicity of the superlattice toward the external field tuning from 0 to π/2. The results show that the fifth-order nonlinear optical conductance of graphene superlattice is enhanced in the terahertz regime when φ = 0, i.e. an external field is applied along the periodicity of the superlattice. The fifth-order nonlinear optical conductances at φ = 0 for different frequencies and temperatures are calculated. The results show that the nonlinear optical conductance is enhanced in low frequency and low temperature. Our results suggest that KP type graphene superlattices are preferred structures for developing graphene-based nonlinear photonics and optoelectronics devices.

The asymmetrical nested metamaterial, composed of two split-ring resonators (SRRs) and two embedded gallium arsenide (GaAs) islands placed in the two SRRs, has been elaborately designed on quartz substrate. Its tunable and switchable resonances at terahertz (THz) frequencies are numerically demonstrated here based on different conductivities of GaAs, which can be transformed from semiconductor to metallic state through appropriate optical excitation. Without photoexcitation, our designed metamaterial has three resonance peaks in the range of monitored frequency range, and they are located at 0.813, 1.269 and 1.722 THz, respectively. As the conductivity of the two GaAs islands increases, different new resonances appear and constantly strengthen. Finally, four new resonant points are generated, at 0.432, 0.948, 1.578 and 1.875 THz, respectively. At the same time, the metamaterial structure is changed from the original nested mode to a new integral mode. Applying reversible changing conductivity of semiconductor to push the conversion of resonance, this asymmetrical nested design provides a new instance in application and development of additional THz devices.

This study investigates the sensing applications of metamaterial (MTM) structures in the terahertz (THz) region and is based on a broadside-coupled diamond and square-ring resonator (DSRR) structures. The resonators are designed and simulated as sensors in detail. Compared with single-sided sensors, the sensing capability of double-sided sensors provide an enhancement with respect to the sensitivity. To analyze the structure as sensor, the changes in the transmission resonance are investigated as a function of the permittivity and thickness of overlayer for the single- and double-sided MTM. The results demonstrate that this design can provide good sensitivity when sensing the chemical or biological agents that are resonant in the terahertz region of the electromagnetic spectrum. These types of designs can be employed in the many sensing applications that are of interest in the THz region.

In this paper, using a cross-shaped complementary Indium Tin Oxide (ITO)-based metasurface design, the transmission of THz radiation is shown to be filtered within the 3 dB level from maximum in the frequency range of interest ($\sim$333 GHz). Various metasurface structures primarily composed of cross-shaped openings with a 400 micron unit cell size are patterned on top of 1750 micron thick fused silica substrates. They are patterned using UV lithography methods after the films were grown using DC sputtering. The fabricated structures were characterized using Terahertz Time Domain Spectroscopy (THz-TDS) measurement technique. The measured transmission agrees well with the simulation of the structure for four different samples with different geometries. These results suggest that metasurface and/or metamaterial patterns based on ITO in visibly transparent media can be utilized for filtering of frequencies in the long wavelength spectrum. These types of filters can be very useful in the near future for THz communication and security applications.

A high-sensitivity sensor based on the resonant transmission characteristics of terahertz (THz) metamaterials was investigated, with the proposal and fabrication of rectangular bar arrays of THz metamaterials exhibiting a period of 180 $\mu \mathrm{\text{m}}$ on a 25 $\mu \mathrm{\text{m}}$ thick flexible polyimide. Varying the size of the metamaterial structure revealed that the length of the rectangular unit modulated the resonant frequency, which was verified by both experiment and simulation. The sensing characteristics upon varying the surrounding media in the sample were tested by simulation and experiment. Changing the surrounding medium from that of air to that of alcohol or oil produced resonant frequency redshifts of 80 GHz or 150 GHz, respectively, which indicates that the sensor possessed a high sensitivity of 667 GHz per unit of refractive index. Finally, the influence of the sample substrate thickness on the sensor sensitivity was investigated by simulation. It may be a reference for future sensor design.

An ultra-broadband absorber is achieved by constructing novel island-shape structures on a single-layered broadband absorber. The single-layered absorber contains four same sized metallic split rings with different heights of the base. Its absorption goes beyond 98% from 1.82 THz to 3.70 THz and the full width at half maximum (FWHM) is 102.3% (from 1.47 THz to 4.55 THz), much better than these achieved by assembling multiple nested resonators. Then, by stacking novel island-shape structures over the single-layered absorber, we can excite 1-order magnetic response to precisely add a new peak near the broadband and enhance the bandwidth. The multi-layered absorber covers a bandwidth of 3.51 THz (the absorption over 90% from 1.23 THz to 4.74 THz) and the FWHM is up to 139.7%, far larger than previous reported results. It also performs well at considerably large oblique incident angles. Moreover, the island-shape structures have the potential to be applied in some other single-layered absorbers to widen their bandwidth.

A novel chiral metasurface (CMS) based on T-shaped resonators is proposed and investigated numerically. The CMS is composed of a periodic array of bi-layered conjugated fourfold T-shaped structure. The retrieved effective electromagnetic (EM) parameters indicate that the multi-band negative refractive index associated with strong optical activity and circular dichroism (CD) effect is realized in terahertz (THz) region. The physical mechanism is illustrated by analyzing the current density distributions. Due to its exotic chiral optical properties, the proposed CMS is useful for the development of THz devices.

Terahertz (THz) gyrotrons can operate with a lower applied magnetic field in harmonic operation, but the weakened harmonic interactions in harmonic gyrotrons can introduce serious challenges when mode competition occurs. The use of an axis-encircling electron beam can greatly alleviate mode competition in a harmonic gyrotron. In this paper, we study axial modes for third-harmonic ${\mathrm{\text{TE}}}_{3,7}$-mode large-orbit gyrotrons. Simulation results reveal that the minimum current for oscillation to begin in each axial mode in the gyrotron regime is associated with a specific range of applied magnetic field. To avoid mode competition, tapered applied magnetic fields and waveguide radii are employed to enhance the high-order axial modes and suppress the low-order axial modes. Furthermore, spurious transverse modes in a THz gyrotron are discussed below. A stable third-harmonic ${\mathrm{\text{TE}}}_{3,7}$-mode large-orbit gyrotron at the third-order axial mode is predicted to yield peak output power of 6.5 kW at 768.1 GHz with an efficiency of 10% for a 75-kV, 0.85-A electron beam with an axial velocity spread of 3%.

This paper presents a numerical simulation of a Wurtzite-GaN-based IMPATT diode operating at the low-end frequency of terahertz range. Conventional classical drift–diffusion model is independent of the energy relaxation effect at high electric field. However, in this paper, a hydrodynamic carrier transport model including a new energy-based impact ionization model is used to investigate the dc and high-frequency characteristics of an IMPATT diode with a traditional drift–diffusion model as comparison. Simulation results show that the maximum rf power density and the dc-to-rf conversion efficiency are larger for conventional drift–diffusion model because it overestimates the impact ionization rate. Through hydrodynamic simulation we revealed that the impact ionization rates are seriously affected by the high and rapidly varied electric field and the electron energy relaxation effect, which lead to the rf output power density and the dc-to-rf conversion efficiency falls gradually, and a wider operation frequency band is obtained compared with the drift–diffusion model simulation at frequencies over 310 GHz.

Two simple polarization-independent electromagnetically-induced transparency (EIT) metamaterials are numerically and experimentally demonstrated at the terahertz region. The first structure is composed of two metal concentric rings on a substrate, while the second one is composed of one metal ring with a cross in it. The bright–bright coupling behavior appears in the two symmetric polarization-independent EIT structures while it is generally observed in asymmetrically structures. In addition, the large group index is extracted to verify the slow-light effect of the two EIT structures. These simple EIT structures may have potential applications in certain areas, including sensing, slow-light and filtering devices.

We present a three-dimensional (3D) perfect metamaterial absorber (PMA) for temperature sensing application in terahertz region. The PMA consists of a 3D metal resonator structure array and a continuous metal film separated by an indium antimonide (InSb) layer. The numerical simulations demonstrate that the PMA can achieve perfect absorption (about 99.9%) with the high $Q$-factor of about 18.8 at 2.323 THz when the temperature is 300 K (room temperature). Further simulation results indicate that this terahertz PMA is polarization-insensitive and wide-angle for both transverse electric (TE) and transverse magnetic (TM) waves. The electric field and surface current distributions of the unit-cell structure indicate that the perfect absorption is originated from the excitation of the fundamental magnetic and electric dipole resonance mode. Since the permittivity of the InSb is sensitive to the external temperature, the resonance absorption frequency of the PMA can be dynamically adjusted. The temperature sensitivity of the PMA is about 15.24 GHz/K, which may have potential prospects in temperature sensing and detection.

Multiple-band terahertz filter device consisting of two different-sized metallic split rings with the nested design method is proposed and investigated in this paper. Five separated filtering resonant dips having different resonance amplitudes and quality factors are gained, which are mainly attributed to the hybrid coupling effect between the two nested split-ring resonators. More importantly, the resonance features of the five filtering dips show a significant dependence on the designed parameters, especially the gap between two nested split rings. The multiple-band filtering resonance device given here can open up new avenues to control terahertz waves in many technology-related fields.

We theoretically and numerically demonstrate a tunable and wide-angle terahertz absorber, which is composed of multilayer graphene-dielectric grating and bottom metal substrate. Numerical simulation shows that the proposed absorber has the advantage of dynamically tunable range from 1.015 THz to 1.165 THz when the chemical potential of graphene increases from 10 meV to 150 meV. The absorption efficiency can reach a high value of 99%. To show the working mechanism of absorption, the near field distributions of magnetic components are presented at the absorption wavelength. We also demonstrate that the tunable range of absorption can be engineered by designing the geometry parameters. In addition, it is shown that the designed absorber can maintain the good performance of absorption over a wide incident angle from $0^{\circ }$ to $60^{\circ }$ under TM-polarization.

In this study, a design for the high-efficiency transmissive terahertz polarization beam splitter is proposed. Based on the metal–insulator–metal waveguide array structure, it is found that the phase change between the transverse-electric (TE) and transverse-magnetic (TM) modes of terahertz wave transmission depends greatly on the medium width. According to this phenomenon, our designed devices can achieve polarization splitting of TE and TM modes in the frequency range 0.8–2.4 THz, and the transmittance can be maintained above 85%. In addition, through judicious design, polarization splittings with 93% transmittance at 1 THz and 95% transmittance at 1.5 THz are obtained, and polarization splitting at different angles is achieved according to variable periods. Compared with the traditional polarization beam splitter, this design has the advantages of adjustable frequency, high efficiency, and easy integration, thus having potential application in terahertz optical systems.