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Electrically tunable metasurfaces with pixel-level phase control are good candidates for dynamic manipulation of the beam direction in the terahertz band, which is a critical and highly desired technique for next generation communications. Yet the complex biasing, limited aperture and cross talk hinders its progress. In this study, we propose a magnetic-field-globally controlled metasurface for beam switching. The metasurface is composed of a bi-layer silicon metagrating on a magneto-optical substrate InSb. The beam direction is governed by two physical processes: the magnetically-controlled polarization and the polarization-controlled diffraction. By utilizing the circular dichroism and Faraday rotation effects in InSb, the metasurface can operate at two frequency bands. The beam is switched among four directions by properly choosing the magnetic field biasing and the frequency. This study offers a promising solution for beam management using magnetic field biasing and in a global manner. [ABSTRACT FROM AUTHOR]

Abstract: The gyromagnetic properties of ferrite materials and the nonreciprocal property of a silicon-ferrite photonic crystal cavity are investigated in the terahertz region. Through the structure optimization and analysis of defect mode coupling, we design a magnetically tunable circulator, of which central operating frequency can be tuned from 180 to 205GHz and the maximum isolation is 65.2dB. Moreover, the further study shows that the gyrotropy, dispersion, and ferromagnetic loss of ferrite materials under the different external magnetic fields greatly affect the transmission and isolation property of this device. This circulator is flexible to realize functions of controllable splitting, routing, filtering and isolation by changing the external magnetic field for the THz applications. [Copyright &y& Elsevier]

The training of neural networks using the extended Kalman filter (EKF) algorithm is plagued by the drawback of high computational complexity and storage requirement that may become prohibitive even for networks of moderate size. In this paper, we present a local EKF training and pruning approach that can solve this problem. In particular, the by-products obtained along with the local EKF training can be utilized to measure the importance of the network weights. Comparing with the original global approach, the proposed local EKF training and pruning approach results in a much lower computational complexity and storage requirement. Hence, it is more practical in solving real world problems. The performance of the proposed algorithm is demonstrated on one medium- and one large-scale problems, namely, sunspot data prediction and handwritten digit recognition. [ABSTRACT FROM AUTHOR]

Manipulating terahertz (THz) polarization in an efficient and broadband manner is of great significance to facilitating THz applications, including communications, imaging, defense, and homeland security. In this work, a dispersion compensation scheme is proposed for high‐efficiency and ultra‐broadband THz polarization manipulation using an anisotropic dielectric‐metal hybrid metadevice. The operating bandwidth is broadened by dispersion compensation, where the dielectric grating provides an artificial birefringence with a phase dispersion of positive slope and the metallic grating provides a phase dispersion of negative slope. Experimental results show that the device achieves two ultra‐broadband dispersion compensation, corresponding to the achromatic quarter‐wave plate in the lower frequency band (QWP: 0.5–2.0 THz, PCR >0.95) and the achromatic half‐wave plate in the higher frequency band (HWP: 1.0–2.1 THz, PCR >0.9), respectively. Theoretically, the bandwidth of QWP can be further improved to 2.6 THz by optimizing the grating dispersion, which brings huge application space to cover most of the THz radiation. This hybrid metadevice configuration offers a versatile platform for engineering electromagnetic waves, and the strategy of phase compensation can be generalized to extend the bandwidth of the metadevice in imaging and communications. [ABSTRACT FROM AUTHOR]

The dynamic tuning of terahertz (THz) electromagnetically induced transparency (EIT) offers significant potential in rapidly changing THz regulation fields, such as the next generation wireless communication, switching, and sensing. In this work, we demonstrate a THz EIT metamaterial by combining a circular and a rectangular split-ring resonator (CSRR and RSRR) based on the bright-bright mode coupling. The introduction of graphene into the EIT structure enables active tuning, and reveals two distinct regulation mechanisms in the metal-graphene hybrid EIT metamaterial: 1. the internal coupling effect diminishes with the increase of the Fermi level in the CSRR-graphene integration; 2. the surface electric field undergoes redistribution in the RSRR-graphene integration. These two dynamic regulation mechanisms are well confirmed by the simulated electric field distribution. As a result, the transmittance within the EIT window of the hybrid metamaterial can be precisely modulated, exhibiting a significant modulation depth of >85%. Notably, the integration with graphene in the RSRR allows for simultaneous modulation of resonances on either side of the EIT peaks, which expands more THz modulation channels. This work enriches the design and implementation methods for active THz EIT and shows potential applications in THz dynamic application scenarios. • A THz hybrid EIT metamaterial incorporates monolayer graphene is proposed. • This work achieves simultaneous modulation of the EIT-window and two adjacent bright modes. • The proposed structure has the advantages of active control, more modulation channels, and higher modulation depth. [ABSTRACT FROM AUTHOR]

Generally, fuzzy approximation models require some human knowledge and experience. Operator's experience is involved in the mathematics of fuzzy theory as a collection of heuristic rules. The main goal of this paper is to present a new method for identifying unknown nonlinear dynamics such as Rössler system without any human knowledge. Instead of heuristic rules, the presented method uses the input-output data pairs to identify the Rössler chaotic system. The training algorithm is a modified Levenberg-Marquardt (L-M) method, which can adjust the parameters of each linear polynomial and fuzzy membership functions on line, and do not rely on experts' experience excessively. Finally, it is applied to training Rössler chaotic system fuzzy identification. Comparing this method with the standard L-M method, the convergence speed is accelerated. The simulation results demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The Faraday rotation effect of both the La: yttrium iron garnet (YIG) film and the YIG metasurface were experimentally and numerically investigated in the terahertz (THz) region. A THz magneto-optic polarization measurement system was used to observe the transmission, resonance, and magneto-optical effect of the La:YIG film and YIG metasurface. The THz artificial chirality and resonance localization of the helical metasurface generate the superchiral THz field, which enhances the THz magneto-optical effect of the YIG film. The results show that the Faraday effect of the YIG metasurface is about three times that of the pure YIG film, whose differential rotation angle increases from 8° to over 24°. This work achieves more sensitively active polarization control of THz waves, which is of great significance for THz polarization conversion, sensing, and non-reciprocal transmission. [ABSTRACT FROM AUTHOR]

The terahertz (THz) resonance, chirality, and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method. The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface, which leads to a strong polarization conversion effect. Moreover, compared with the traditional THz transmission resonance sensing for film thickness, the polarization sensing characterized by polarization elliptical angle (PEA) and polarization rotation angle (PRA) shows a better Q factor and figure of merit (FoM). The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20 μm, while the Q factor of THz resonance sensing is only 10.6. And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM, over 4 times than that of resonance sensing. This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation, and open a window to the high sensitive sensing by using THz polarization spectroscopy. [ABSTRACT FROM AUTHOR]

Dual-frequency liquid crystals (DFLCs), the mixture of positive and negative liquid crystal (LC) molecules, exhibit unique alternating (AC) frequency dependent anisotropic properties. Carbon nanotube (CNT), as a novel nanomaterial with strong anisotropy, has attracted much attention in recent years. Herein, we investigate the tunable terahertz birefringence and phase shift characteristics of the DFLC doped with CNT (CNT-LC) by using the terahertz time-domain polarization spectroscopy. The results show that the CNT-LC (1.5 wt%) can reach 0.5π at 0.793 THz, which can be used as a tunable THz phase shifter that is 0.12π higher than the pure DFLCs. Furthermore, through measuring the output polarization state, it is confirmed that the dielectric anisotropy enhancement mechanism of CNT-LC originates from the surface interaction between CNTs and LC molecules. Therefore, a tunable quarter-wave plate with the active polarization conversion from linearly polarized (LP) to LP or LP to circularly polarized (CP) can be realized at 0.925 THz in the CNT-LC, while the pure DFLC cannot be achieved at the same frequency. The dielectric anisotropy enhancement of CNT-LC shows its utility in the improvement of various tunable terahertz LC phase shifter and wave plate. Image 1 [ABSTRACT FROM AUTHOR]

While the terahertz regime has proved to be a promising solution for wireless big-data transmission, the lack of available functional devices working in this band seriously constrain the wide engineering applications. In this paper, the silver nanorods in polyvinyl alcohol (PVA) film are developed for enhancing and accelerating THz all-optical modulation. The modulation depth can reach 80% under 0.6W/cm2, and the speed can reach 3kHz which is much faster than our previous research. It has been attributed to the plasmonic hot carriers' effect of the silver nanorods, which makes the photoexcited electrons more easily jump out of the barrier and assemble in the heterostructure between PVA and silicon. Moreover, the photoelectronic response is proved to be related to the plasmonic absorption of the pumping light, which means the modulation speed is decided by the color of the pump light. This experiment phenomenon provides a way to build color-selective terahertz all-optical modulators. Considering its capability for broadband, efficient, and fast modulation of THz waves, this low-cost and conveniently fabricated device could be broadly applied in polymer related THz devices. [ABSTRACT FROM AUTHOR]

Abstract Carbon nanotube (CNT), as a recently emerged newfangled nanomaterial, has been applied in terahertz (THz) polarizer due to its anisotropic hyper-ordered orientation. Herein, we simply adhere two orthogonal CNT sheets to the both sides of dielectric subwavelength grating to form a compound structure (i.e. CNT@Grating). Due to the subwavelength integration and cavity modes, the CNT@Grating presents local resonances between two CNT sheets, which greatly enhances the polarization rotation and expands the bandwidth. Compared with the large phase shift dispersion of single subwavelength grating, its phase shift dispersion and impedance can be manipulated by adjusting the layer number of CNT, and finally a broadband close zero dispersion from 0.4 to 0.95 THz has been obtained, which leads to a broadband THz polarization conversion. This work provides a new design idea towards practical applications for THz broadband polarization conversion and dispersion control. Graphical abstract We proposed a broadband THz linear polarization convertor, which integrated with the dielectric grating and CNT (CNT@Grating). Here, CNT plays a role of polarizer like the commercial metal wire grid and dielectric grating has a large birefringence that can employ as artificial wave-plate. Image 1 [ABSTRACT FROM AUTHOR]

In this paper, a novel flight control scheme is proposed by using the variable domain fuzzy control method. The proposed control strategy can improve control performance of near-space hypersonic vehicles (NHV) despite uncertainties and external disturbances. The control accuracy and anti-interference velocity are improved, and the calculated quantity is decreased by the change of fuzzy domain. To overcome the chattering phenomenon, the dynamic adaptive terminal slidingmode control (TSMC) strategy is raised. The control law is designed by an intermediate auxiliary systems, step-by-step. Additionally, the Lyapunov method is used to prove that the tracking errors are converged in finite time. [ABSTRACT FROM AUTHOR]

Coupled dielectric-metal gratings are investigated for broadband terahertz (THz) wave polarization conversion and asymmetric transmission by the experiments and numerical simulations, which are composed of the subwavelength Si grating and metallic wire grating layers. The dielectric grating layer with a large artificial birefringence and low dispersion is employed as a phase engineered waveplate, and the metal wire grating arranged with a 45° angle to the dielectric grating is utilized as a high-efficiency polarizer. Due to the subwavelength integration, this coupled grating presents a local resonance coupling mechanism between dielectric and metal gratings, which greatly enhances the polarization rotation and expands the bandwidth, not a simple combination with dielectric and metallic gratings. The results demonstrate that a broadband asymmetric transmission with an extinction ratio of 30dB from 0.2 to 1.2 THz is achieved and the highest transmission of 90% can be obtained. It provides a simple way towards practical applications for THz artificial dispersion materials, polarization control and asymmetric transmission. [ABSTRACT FROM AUTHOR]

An active terahertz (THz) liquid crystal (LC) metamaterial has been experimentally investigated for THz wave modulation. Some interesting phenomena of resonance shifting, tunable electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) have been observed in the same device structure under different DC bias directions and different incident wave polarization directions by the THz time domain spectroscopy. Further theoretical studies indicate that these effects originate from interference and coupling between bright and dark mode components of elliptically polarized modes in the LC metamaterial, which are induced by the optical activity of LC alignment controllable by the electric field as well as the changes of LC refractive index. The LC layer is indeed a phase retarder and polarization converter that is controlled by the DC bias. The THz modulation depth of the analogs of EIT and EIA effects are 18.3 dB and 10.5 dB in their frequency band, respectively. Electrical control, large modulation depth and feasible integration of this LC device make it an ideal candidate for THz tunable filter, intensity modulator and spatial light modulator. [ABSTRACT FROM AUTHOR]

In this paper, a T-S fuzzy model of the NSV (Nearspace Vehicle) kinematic model is established based on fuzzy approximation theory, and a new fuzzy robust tracking control law is designed in reference to the feedforward control of the linear system. In order to account for a case in which no augmented matrix is introduced, the control law is designed as a compound form of feedback and feedforward, and the gains of feedback and feedforward are solved by LMI (Linear Matrix Inequalities). The strategy is applied to the anti-interference control of NSV attitudes, and the convergence of tracking errors is analyzed according to the Lyapunov method. Simulation results based on the NSV demonstrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

A phase transition photonic crystals (PhC) based silicon column arrays with VO 2 coating have been proposed and fabricated, which can be actively converted between PhC directional coupler and metallic PhC waveguide by the thermal means to form a switchable directional coupler. The bandgap and transmission properties of this device have been investigated by the numerical simulation. With the changes of temperature, this device exhibits three different modulation and coupling effects. The direct output port shows the two opposite intensity modulation processes, of which transmissions decrease from dielectric PhC coupling state to the loss state and then increase from the loss to the metallic PhC waveguide state; the coupling port realizes another intensity modulation from the coupling state to non-coupling state. This device can realize switching, modulation, actively splitting and routing functions in the future's THz applications. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we design a novel Y-junction power splitter which is optimized according to coupled mode theory. Two rods are added in the junction. One rod is a silicon rod and another one is a ferrite rod, the refractive index of which can be varied by adjusting the external magnetic field. So, it not only can split the power, but also controls the output energy of the two branches automatically. The transmission spectrum is calculated by finite difference time domain (FDTD) method, which shows that the output energy of the two branches is closely related to the frequency of the incident terahertz (THz) wave and the refractive index of the ferrite rod. The device is a good candidate for integrated optical circuit, the tuning rate of which is about a few microseconds. [Copyright &y& Elsevier]

Abstract: Two highly birefringent polymer terahertz (THz) fibers were proposed in this paper, which were formed with honeycomb cladding and some elliptical air holes in the fiber core. The losses and mode birefringence of two different fibers are investigated by finite-different time-domain method. The results show that fiber 2 can achieve both high birefringence (larger than 0.022) and low confinement loss (0.01dB/m) in a wide THz frequency range. Moreover, compared with a round solid-core fiber, guiding loss of the THz fiber caused by polymer material absorption can be reduced effectively as a part of the mode power is trapped in the air holes. [Copyright &y& Elsevier]

Subwavelength dielectric gratings are widely applied in the phase and polarization manipulation of light. However, the dispersion of the normal dielectric gratings is not flat while their birefringences are not enough in the THz regime. In this paper, we have fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations. The transmission and dispersion characteristics are also improved since the gradient grids break the periodicity of grating lattices as a chirp feature. From 0.6-1.4 THz, a broadband birefringence reaches 0.35 with a low dispersion and good linearity of phase shift, and the maximum phase shift is 1.4π. Furthermore, these gradient gratings are applied as half-wave plates and realize a linear polarization conversion with a conversion rate over 99%, also much higher than the equal-grid gratings. These gradient gratings show great advantages compared to the periodic gratings and provide a new way in the designing of artificial birefringence material. [ABSTRACT FROM AUTHOR]

We investigated terahertz (THz) magneto-optical properties of a ferrofluid and a ferrofluid-filled photonic crystal (FFPC) by using the THz time-domain spectroscopy. A magnetoplasmon resonance splitting and an induced THz transparency phenomenon were demonstrated in the FFPC. The further investigation reveals that the induced transparency originates from the interference between magnetoplasmon modes in the hybrid magneto-optical system of FFPC, and the THz modulation with a 40% intensity modulation depth can be realized in this induced transparency frequency band. This device structure and its tunabilty scheme will have great potential applications in THz filtering, modulation and sensing. [ABSTRACT FROM AUTHOR]

The microfluidic sensing based on the photonic crystal (PC) pillar array was investigated in the terahertz (THz) region. We fabricated the silicon PC sensors, and experimentally and theoretically demonstrated their resonances by using the THz time-domain spectroscopy system. According to the corresponding changes of the resonances dependent on the different microfluidics on the PC sensor, the qualitative monitoring can be achieved. By establishing the relation between the experimental data and simulations, the amount of microfluidic at each moment on the different PC sensors can be exactly determined. These PC sensors have great promising potential in the real-time quantitative sensing. [ABSTRACT FROM AUTHOR]