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Because of the wealth of fascinating physical phenomena observed in topological insulators (TIs), their exciting properties have been intensively investigated for electronic and optoelectronic appl...

The extraordinary proliferation of digital coding metasurfaces turns the real-time manipulation of electromagnetic (EM) waves into reality and promotes the programmable operation of multifunctional equipment. However, current studies are mainly involved in the modulation of the transmission process, and little attention has been given to the control of EM wave generation, especially in the terahertz (THz) band. Here, we conceptually propose and experimentally demonstrate coded terahertz emission, which integrates the efficient generation and control of THz waves across a wide frequency band. For validation, two types of stripe-patterned ferromagnetic heterostructures with opposite spin Hall angles were utilized as coding units. The two distinct states in each coding unit (with two polarization or phase states of 0° and 180°) can be characterized as "0" and "1" digits, which can be switched by manipulating the optical field distribution of the pump beam. Such an ability to realize simultaneous terahertz coding and terahertz emission is essential for meeting the increasingly demanding requirements of integration and miniaturization. Our work endows ferromagnetic heterostructures with controllable spatial characteristics and benefits their applications in wireless communications and holographic imaging.

The structure defect of the traditional grey Verhulst model is a key factor leading to its unstable performance. A new-structure grey Verhulst model (N_Verhulst) was proposed by introducing a new non-homogeneous exponential function. The N_Verhulst model has a better structure and stronger modeling ability; Meanwhile, it overcomes the shortcomings of the traditional Verhulst model including parameter dislocation and unreasonable selection of initial values. Four typical cases were used to test the performance of N_Verhulst, and the simulation/prediction results of the new model were compared with the traditional Verhulst model and the Pearl model. Results showed that the N_Verhulst model had evidently superior performance to the traditional grey Verhulst model, which confirms that the structure development of the new model is reasonable and effective.

MnBi2Te4 (MBT) is the first intrinsic magnetic topological insulator with the interaction of spin-momentum locked surface electrons and intrinsic magnetism, and it exhibits novel magnetic and topological phenomena. Recent studies suggested that the interaction of electrons and magnetism can be affected by the Mn-doped Bi2Te3 phase at the surface due to inevitable structural defects. Here we report an observation of nonreciprocal transport, i.e. current-direction-dependent resistance, in a bilayer composed of antiferromagnetic MBT and nonmagnetic Pt. The emergence of the nonreciprocal response below the N\'eel temperature confirms a correlation between nonreciprocity and intrinsic magnetism in the surface state of MBT. The angular dependence of the nonreciprocal transport indicates that nonreciprocal response originates from the asymmetry scattering of electrons at the surface of MBT mediated by magnon. Our work provides an insight into nonreciprocity arising from the correlation between magnetism and Dirac surface electrons in intrinsic magnetic topological insulators.

MnBi

Generating terahertz waves using thin-layered materials holds great potential for the realization of integrated terahertz devices. However, previous studies have been limited by restricted radiation intensity and finite efficiency. Exploiting materials with higher efficiency for terahertz emission has attracted increasing interest worldwide. Herein, with visible-light excitation, a thin-layered GaTe film is demonstrated to be a promising emitter of terahertz radiation induced by the shift-current photovoltaic effect. Through theoretical calculations, a transient charge-transfer process resulting from the asymmetric structure of GaTe is shown to be the origin of an ultrafast shift current. Furthermore, it was found that the amplitude of the resulting terahertz signals can be manipulated by both the fluence of the pump laser and the orientation of the sample. Such high emission efficiency from the shift current indicates that the layered material (GaTe) is an excellent candidate for photovoltaics and terahertz emitters.

Miniaturized ultrafast switchable optical components with high efficiency and broadband response are in high demand to the development of optical imaging, sensing, and high-speed communication. Sharp Fano-type resonance switched by active materials is one of the key concepts that underpins the control of light in metaoptics with high sensitivity. However, actuating such metasurfaces exhibits a long-standing trade-off between modulation depth and operational bandwidth. Here, the limitations are circumvented by theoretical analysis, numerical simulation, and experimental realization of an achromatic Fano metasurface so that a high contrast of tunability with ultrafast switching rate over a broad range of frequency is achieved. By developing the physics of inter-mode coupling, the Fano metasurface is designed according to a complete phase diagram derived from coupled mode theory. Unlike conventional Fano metasurfaces, the cross-polarized inter-metaatoms coupling is discovered as a superior ability of high-efficiency broadband achromatic polarization conversion. To prove the ultrasensitive nature, a metadevice is constructed by incorporating a thin amorphous Ge layer with a weak photoconductivity perturbation. Transmission modulation over broadband frequency range from 0.6 to 1.1 THz is thus successfully realized, featuring its merits of modulation depth over 90% and On-Off-On switching cycle less than 10 ps.

Advances in tunable metamaterials/metasurfaces facilitates their utilization in novel optical components, and lead to many breakthroughs in light tailoring by giving birth to diverse spatiotemporal dynamics. In the ascendant field of terahertz (THz) photonics, the ultrafast modulation is the fundamental process of technological advancements in high-speed wireless communications, sensing, and imaging. However, the current research efforts have been mainly devoted to studies of single functionality under the control of one stimulus, which has plateaued in terms of innovative new features. Here, building on the incident angle-induced C2 symmetry breaking of split ring pairs, we experimentally demonstrate extremely versatile, ultrafast THz switching behaviors at continuously alterable resonant states. The direction-controlled resonance hybridization provides another excellent degree of routing freedom, owing to its robustness, simplicity, and wide tunability. By leveraging such virtues, single LC mode and EIT-like resonance under normal and oblique incidence conditions are both effectively switched-off by means of photon injection. Considering the ultrashort lifetime of free carriers in MoSe2 crystal, the corresponding transient dynamics show an ultrafast recovery time within 700 ps. The strategy proposed here is a viable pathway for multidimensional THz wave manipulation, which gears up a crucial step for diversified functionalities in deployable metaphotonic devices.

${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ has been proved to be a semimetal with topological surface states (referred to as a topological semimetal, TSM) in the presence of spin-orbit coupling (SOC), and predicted to be a type-II nodal line semimetal (NSM) in the absence of SOC. It is possible to tune the effective SOC in ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by substituting the heavy Bi atom with lighter atoms, such as Sb or As, which results in topological phase transitions. Here in our work we investigate the evolution of the transport properties and band structures of ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by doping different concentrations of Sb through molecular beam epitaxy. Our results demonstrate the existence of phase transitions of TSM to trivial semimetal (SM) and SM to trivial insulator, with Sb concentration $x$ at ${x}_{1}\ensuremath{\approx}0.44--0.64$ and ${x}_{2}\ensuremath{\approx}1.39$, respectively. Further theoretical calculations give values of ${x}_{1}\ensuremath{\approx}0.523$ and ${x}_{2}\ensuremath{\approx}1.10$, which agrees well with our experimental results. And all these results are concluded into a ${\text{Mg}}_{3}{\text{Bi}}_{2\ensuremath{-}x}{\text{Sb}}_{x}$ phase diagram. Our work will stimulate more explorations to the possibilities of the realization of type-II NSM in ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by various elements substitution.

Emergent phenomena such as unconventional superconductivity, Mott-like insulators, and the peculiar quantum Hall effect in graphene-based heterostructures are proposed to stem from the superlattice-induced renormalization of (moiré) Dirac fermions at the graphene Brillouin zone corners. Understanding the corresponding band structure commonly demands photoemission spectroscopy with both sub-meV resolution and large-momentum coverage, beyond the capability of the current state-of-the-art. Here the realization of moiré Dirac cones around the Brillouin zone center in monolayer In

Data envelopment analysis (DEA) has been extended to cross-efficiency to provide better discrimination and ranking of decision-making units (DMUs). Current researches about cross-efficiency mainly focus on the non-uniqueness of optimal solution of linear programming and information aggregation. As a common distance metric, standardized Euclidean distance is introduced to define the discrimination power between two vectors and the deviation degree for measuring the difference between the individual preference and group ideal preference. Based on above definitions, an alternative method is presented to compare multiple optimal solutions, and further, a universal weighted cross-efficiency model considering both dynamic adjustment of weights and preference formulation is constructed for evaluation and ranking. Two numerical examples are given to illustrate the effectiveness of the comparison method for multiple optimal solutions and weights determination method of DMUs, respectively. At last, a practical application aimed at evaluating environmental treatment efficiency in western area of China is given. Comparative analysis shows that our model could be more moderate, flexible, and general than some available models and methods, which can extend the theoretical research of cross-efficiency evaluation.

The enhancement of terahertz (THz) radiation is of extreme significance for the realization of the THz probe and imaging. However, present THz technologies are far from being enough to realize high-performance and room-temperature THz sources. Fortunately, topological insulators (TIs), with spin-momentum-locked Dirac surface states, are expected to exhibit a high terahertz emission efficiency. In this work, the novel concept of a Rashba-state-enhanced spintronic THz emitter is demonstrated on the basis of ferromagnet/heavy metal/topological insulator (FM/HM/TI) heterostructure. We find that the THz emission intensity changes as a function of HM interlayer thickness, and a 1.98 times higher intensity compared to that of FM/TI can be achieved when a meticulously designed thickness of the HM layer is inserted. The improvement of terahertz radiation is ascribed to the additive effect of Rashba splitting and topological surface states at the HM/TI interface. These results offer new possibilities for realizing spintronic THz emitters in TI-based magnetic heterostructures.

Carbon dioxide emission is an important environmental issue, and it has also become an important reference factor for governments to formulate social and economic policies. The objective and accurate prediction of carbon dioxide emissions can provide reference and early warning for the implementation of the government’s environmental strategy. The change of the original data of carbon dioxide emissions is S-type, but not saturated S-type. The grey Verhulst model is mainly used to describe the process with saturation state, which is suitable for modeling S-type data series. However, it is found that there are inherent errors and limitations in this model. In this paper, the grey action of the grey Verhulst model is improved, a new action Verhulst model is obtained, and its properties are studied. Finally, the new model is used to predict the carbon dioxide emissions of China and Russia, and ARIMA model is added for comparison. The results show that compared with the original Verhulst model, the simulation and prediction accuracy of the optimized Verhulst model are improved by more than 10%, and the ARIMA model underestimates the carbon dioxide emissions. From the result analysis, China and Russia need to formulate strong energy conservation and emission reduction policies, vigorously develop clean energy industry, and promote green production and lifestyle.

In article number of 2000799, Tian Jiang and co‐workers propose a novel functionality of pump‐colorselective ultrafast switching speed in terahertz metadevices by hybridizing meta‐atoms with judiciously engineered bilayer semiconductors. The proof‐of‐concept demonstration breaks the long‐standing challenge that all‐optical switching dynamics is hardly tunable in metaphotonic devices. This strategy could inspire more opportunities for designing ultrafast, multifunctional, all‐optical switchable photonic devices. [Image: see text]

The nodal-line semimetal is a new type of topological state of matter in which the crossing of two energy bands forms a nodal loop. In the absence of spin-orbit coupling, Mg3Bi2 is predicted as a type II nodal-line semimetal, which may evolve to a topological insulator with a small energy gap of ∼35 meV in the presence of spin-orbit coupling. However, the transport evidence is still lacking. Here, we measure the magneto-transport in Mg3Bi2. At low temperatures, the magnetoconductivity exhibits a weak antilocalization behavior. We fit the experimental data with a magnetoconductivity formula for the weak antilocalization effect of three-dimensional nodal-line semimetals as well as the well-known Hikami-Larkin-Nagaoka formula for two-dimensional weak (anti)localization effects. By comparing the fitting results of these two theories, we demonstrate that the weak antilocalization in Mg3Bi2 is better described by the theory for nodal-line semimetals. Our work will inspire more explorations to use the new weak localization theory to identify a large spectrum of nodal-line semimetals.

Incorporating active materials into metamaterials is expected to yield exciting advancements in the unprecedented versatility of dynamically controlling optical properties, which sheds new light on the future optoelectronics. The exploration of emerging semiconductors into terahertz (THz) meta‐atoms potentially allows achieving ultrafast nanodevices driven by various applications, such as biomedical sensing/imaging, ultrawide‐band communications and security scanners. However, ultrafast optical switching of THz radiation is currently limited to a single level of tuning speed, which is a main hurdle to achieve multifunctionalities. Here, a hybrid metadevice which can realize the pump‐wavelength controlled ultrafast switching response by the functionalization of double photoactive layers is demonstrated experimentally. A whole cycle of electromagnetically induced transparency switching with a half‐recovery state changes from 0.78 ns to 8.8 ps as pump wavelength varies from near infrared to near ultraviolet regions. The observed pump‐color selective switching speed changing from nanosecond scale to picosecond scale is ascribed to the wavelength‐dependent penetration length of Ge and the contrasting defect states between noncrystalline Ge and epitaxial Si layers. It is believed that the schemes regarding pump‐color controllable ultrafast switching behavior introduced here can inspire more innovations across the field of ultrafast photonics and can boost the reconfigurable metamaterial applications., All‐optical terahertz modulators with pump‐color selective ultrafast switching speed are realized by combining double layers of semiconductors with electromagnetically induced transparency meta‐atoms. The transmission/group delay of terahertz wave is modulated at contrasting switching speeds from sub‐nanoseconds to picoseconds when pumped by the near‐infrared and near‐ultraviolet beams. This work opens up new avenues for manipulating the dynamics of ultrafast metaphotonic devices.

Image filtering can change or enhance an image by emphasizing or removing certain features of the image. An image is a system in which some information is known and some information is unknown. Grey system theory is an important method for dealing with this kind of system, and grey correlation analysis and grey prediction modeling are important components of this method. In this paper, a fractional grey prediction model based on a filtering algorithm by combining a grey correlation model and a fractional prediction model is proposed. In this model, first, noise points are identified by comparing the grey correlation and the threshold value of each pixel in the filter window, and then, through the resolution coefficient of the important factor in image processing, a variety of grey correlation methods are compared. Second, the image noise points are used as the original sequence by the filter pane. The grey level of the middle point is predicted by the values of the surrounding pixel points combined with the fractional prediction model, replacing the original noise value to effectively eliminate the noise. Finally, an empirical analysis shows that the PSNR and MSE of the new model are approximately 27 and 140, respectively; these values are better than those of the comparison models and achieve good processing effects.

Controllable synthesis of metal–organic framework (MOF) materials with tunable morphologies, sizes, compositions and pore structures is critically important for MOF materials and their pyrolysis derivatives’ applications in environmental and energy fields. Here we report the synthesis of Co-MOF crystals with controllable morphologies, sizes and S/N ratios in water/NaOH and ethylene glycol/NaOH systems using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (Bpy) as S, N dual organic ligands by a “pillar-layer” assembly method. Water and ethylene glycol with different viscosities result in various crystallization processes of [Co(Tdc)(Bpy)]n crystals in the corresponding reaction system, thus respectively obtaining three-dimensional (3D) Co-MOF ([Co(Tdc)(Bpy)]n) bulk and cuboid structures in water/NaOH and ethylene glycol/NaOH reaction systems. The as-prepared Co-MOF crystals in two different reaction systems were further pyrolytically treated at 800 °C in a N2 atmosphere to obtain Co9S8@S,N-doped carbon materials with different surface areas, pore distributions and S/N doping ratios. As electrocatalysts, the Co9S8@S,N-doped carbon cuboid (Co9S8@SNCC) obtained in the ethylene glycol/NaOH system demonstrates superior bifunctional electrocatalytic activities toward both oxygen reduction and evolution reactions resulting from Co9S8 and S, N doping in the carbon structure providing catalytic active sites, better than that of Co9S8@S,N-doped carbon bulk (Co9S8@SNCB) obtained in the water/NaOH system and comparable to commercial Pt/C and RuO2 catalysts. Owing to its high surface area and porous structure, Co9S8@SNCC also exhibits great potential as the electrode material for application in supercapacitors, with high performance and recycling stability.

Due to their controllable morphologies, tunable porous structures, diverse compositions and easy fabrication, metal–organic frameworks (MOFs) are an ideal class of precursor material to develop high performance carbon-based materials for energy applications. In this work, two-dimensional (2D) Co/Ni MOFs nanosheets with a molar ratio of Co2+ to Ni2+ of 1 : 1 were first synthesized at room temperature using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (4,4′-Bpy) as organic linkers. As a precursor material, the as-synthesized 2D Co/Ni MOFs nanosheets were further pyrolized at 550 °C in N2 atmosphere to incorporate 2D CoNi alloy nanoparticles into S, N-doped carbon nanosheets (CoNi@SNC) with a surface area of 224 m2 g−1, a porous structure, and good conductivity. Interestingly, it was found that the 2D Co/Ni MOFs nanosheets can be directly used as electrode materials for supercapacitors, delivering a specific capacitance of 312 F g−1 at 1 A g−1, whereas CoNi@SNC derived from its MOFs precursor as an electrode material for supercapacitors exhibits a much higher specific capacitance (1970, 1897 and 1730 F g−1 at 1, 2, 5 A g−1, respectively) with long cycling life (retaining 95.1% of the value at 10 A g−1 after 3000 cycles) and excellent rate capability at a high charge/discharge current. Further, an asymmetric supercapacitor device was constructed with CoNi@SNC as the positive electrode and active carbon as the negative electrode, exhibiting an energy density of 55.7 W h kg−1 at a power density of 0.8 kW kg−1 with lifetime stability up to 4000 charge–discharge cycles (capacitance retention of ∼90.6%). The results demonstrate that electrochemical activation-generated CoNi oxides/oxyhydroxides on the surface of the CoNi alloy nanoparticles in alkaline electrolyte during electrochemical measurements are the electrochemical active species of the CoNi@SNC-constructed supercapacitor. Additionally, the high performance of the CoNi@SNC-constructed supercapacitor can be collectively attributed to its relatively high surface area, which is favourable for the exposure of electrochemical active sites; its porous structure, which promotes redox-related mass transport; and the combination of CoNi alloy nanoparticles with graphitic carbon, which functions as an electron collector to improve electron transfer.

Identification and control of topological phases in topological thin films offer great opportunity for fundamental research and the fabrication of topology-based devices. Here, combining molecular beam epitaxy, angle-resolved photoemission spectroscopy and ab-initio calculations, we investigate the electronic structure evolution in (Bi1-xInx)2Se3 films with thickness from 2 to 13 quintuple layers. We identify several phases with their characteristic topological nature and evolution between them, i.e., dimensional crossover from a three-dimensional topological insulator with gapless surface state to its two-dimensional counterpart with gapped surface state, and topological phase transition from topological insulator to a normal semiconductor with increasing In concentration x. Furthermore, by introducing In alloying as an external knob of band gap engineering, we experimentally demonstrated the trivial topological nature of Bi2Se3 thin films (below 6 quintuple layers) as two-dimensional gapped systems, in consistent with our theoretical calculations. Our results provide not only a comprehensive phase diagram of (Bi1-xInx)2Se3 and a route to control its phase evolution, but also a practical way to experimentally determine the topological properties of a gapped compound by topological phase transition and band gap engineering., Comment: 24 pages, 4 figures

Emergent phenomena such as unconventional superconductivity, Mott-like insulators, and the peculiar quantum Hall effect in graphene-based heterostructures are proposed to stem from the superlattice-induced renormalization of (moire) Dirac fermions at the graphene Brillouin zone corners. Understanding the corresponding band structure commonly demands photoemission spectroscopy with both sub-meV resolution and large-momentum coverage, beyond the capability of the current state-of-the-art. Here the realization of moire Dirac cones around the Brillouin zone center in monolayer In2 Se3 /bilayer graphene heterostructure is reported. The renormalization is evidenced by reduced Fermi velocity (≈23%) of the moire Dirac cones and the reshaped Dirac point at the Γ point where they intersect. While there have been many theoretical predictions and much indirect experimental evidence, the findings here are the first direct observation of Fermi velocity reduction of the moire Dirac cones. These features suggest strong In2 Se3 /graphene interlayer coupling, which is comparable with that in twisted bilayer graphene. The strategy expands the choice of materials in the heterostructure design and stimulates subsequent broad investigations of emergent physics at the sub-meV energy scale.

Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements, such as quantum communications, on-chip communication processing and storage. Here, we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg3Bi2 by using photon polarization states. The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects, and the helicity-independent photocurrent that is attributed to linear photogalvanic effect. Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.

The adaptability of the traditional GM (1, 1) model is poor because it is a rigorous homogenous exponent model with a single fixed structure. To improve the adaptability of the traditional grey model, a self-adaptive intelligence grey predictive model with an alterable structure is proposed in this paper. The proposed model has the advantages of adjustable parameters and is characterised by its variable structure as a homogenous/non-homogenous exponent model or as a single-variable linear-auto-regression model. It can be used to automatically compute the relative optimal modelling parameters and adaptively choose a more reasonable model structure based on the real data characteristics of a modelling sequence. Hence, this novel model outperforms traditional grey models with a single fixed structure. To verify its efficiency and applicability, the proposed model was used to simulate China's electricity consumption from 2001 to 2013 and to forecast it in 2014 using real data; the results indicate that the novel model has better simulative and predictive accuracy than the GM (1, 1) and DGM (1, 1) models.

Metal oxides are a class of promising electrode materials for supercapacitors because of their high theoretical energy density; however, the low electrical conductivity and instability of metal oxides limit their large-scale practical applications. Here we report a facile and scalable method to synthesize ultrafine nickel–cobalt alloy nanoparticles (5–10 nm) embedded into three-dimensional porous graphitic carbon (3D Ni–Co@PGC) using NaCl as the template to create a porous structure and glucose as the carbon source by pyrolysis treatment at 800 °C under N2 atmosphere. As an electrode material for supercapacitors, the ultrafine Ni–Co alloy nanoparticles of 3D Ni–Co@PGC not only serve as current collectors, but also their surfaces convert to corresponding metal oxides when exposed to an alkaline electrolyte, responsible for redox reactions in pseudocapacitors, exhibiting high supercapacitor performance. The results demonstrate that the supercapacitor assembled with 3D Ni–Co@PGC electrodes shows high energy density (1091, 1064 and 1041 F g−1 at 1, 2 and 4 A g−1, respectively), long cycling life and excellent rate capability at a high charge/discharge current. Furthermore, an asymmetric supercapacitor assembled by using 3D Ni–Co@PGC as the positive electrode and active carbon as the negative electrode shows a high energy density of 33.7 W h kg−1 and remarkable cycling stability (98% capacitance retention over 4000 cycles). The superior performance of the 3D Ni–Co@PGC constructed supercapacitor can be ascribed to its high surface area (265 m2 g−1), porous structure and excellent electrical conductivity, favourable for the exposure of reaction active sites, redox-related mass transport and electron transfer, respectively.

Dielectric properties and their evolutions are fundamental to reveal the optical responses of topological insulators and to guide the design of related devices. Here, we systematically investigate the layer-dependent dielectric permittivity of Bi2Se3 over an ultra-broad spectral range (0.73–6.43 eV) by combining the spectroscopic ellipsometry, energy-loss function (ELF), critical point (CP) analysis, and density function theory. Results demonstrate that the epsilon-near-zero point of the real permittivity and the ELF peak exhibit blue shifts, indicating the resonance frequencies of surface plasmon modes move towards the higher energy, due to the enhanced hybridization between the opposite surfaces. The joint density of states dominates the layer-dependent increase in imaginary permittivity as the thickness increases. We also find that the CP transition positions in band structures are layer-independent, and the CP center energies exhibit red shifts resulted by the reduced surface state gap with the thickness increasing.

The emergence of larger-scaled two-dimensional (2D) layered materials has attracted intense research efforts and significant progress in the area of photonics and optoelectronics recently. As a remarkable representative of 2D materials, transition metal dichalcogenides (TMDCs) have demonstrated an astonishing photoconductivity and the ultrafast charge carrier dynamics, which provides excellent potentials for ultrafast optical modulators. Herein, by simply transferring a high-quality CVD-grown WSe2 multilayer on plasmon-induced transparency (PIT) metasurfaces, we demonstrate that the transmission amplitude modulation is as high as 43% and the slow light switching is up to 6 ps in the THz regime. Under photoexcitation, both functionalities are dynamically controlled within ~8 ps owing to a merit of ultrafast free carriers’ relaxation in the WSe2 multilayer. Moreover, a theoretical model consisting of two coupled harmonic oscillators and the near-field distributions are simultaneously employed to verify the strong dependence of the active PIT switching behavior on the suppression of the bright mode of split ring resonators. Our proposed versatile active WSe2-functionalized metasurface with a low cost and simple manufacturing will give researchers new insights into the ultrafast switchable metaphotonic devices.

In this work, a soliton mode-locked erbium-doped fiber laser (EDFL) with a high-quality molecular beam epitaxy (MBE)-grown topological insulator (TI) Bi2Se3 saturable absorber (SA) is reported. To fabricate the SA device, a 16-layer Bi2Se3 film was grown successfully on a 100 μm thick SiO2 substrate and sandwiched directly between two fiber ferrules. The TI-SA had a saturable absorption of 1.12% and a saturable influence of 160 MW/cm2. After inserting the TI-SA into the unidirectional ring-cavity EDFL, self-starting mode-locked soliton pulse trains were obtained at a fundamental repetition rate of 19.352 MHz. The output central wavelength, pulse energy, pulse duration, and signal to noise ratio of the radio frequency spectrum were 1530 nm,18.5 pJ, 1.08 ps, and 60 dBm, respectively. These results demonstrate that the MBE technique could provide a controllable and repeatable method for the fabrication of identical high-quality TI-SAs, which is critically important for ultra-fast pulse generation.

This paper describes the dynamic analysis method of a three degree-of-freedom electromagnetic spherical actuator considering the mutual effect among X-, Y- and Z-axis employing the 3-D finite element method, in which the magnetic field equation is fully combined with equations of electric circuit and motion. The validity of the analysis is confirmed by comparing with the measured results on a prototype.

Recently, multidimensional actuators have been attracting considerable interest as a solution for increases in size and weight. In this paper, we propose a position feedback control method using optical image sensors for an actuator with three degrees of freedom. The validity of this control method is verified on the basis of the measurement results. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 181(2): 47–54, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21290

This paper describes the dynamic analysis of a three-degree-of-freedom electromagnetic spherical actuator employing 3-D FEM. In this analysis, the magnetic field equation is combined with the electric circuit equation and motion equations of three axes. The influence of one axis operation on other axes is evaluated. The validity of the analysis is confirmed by carrying out the measurement on a prototype.