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© 1963-2012 IEEE. Based on transformation optics, a simple and efficient strategy to design 3-D electromagnetic and optical devices is proposed. Using this new method, the materials required are isotropic and free of any singularity or zero value as long as the transformed domains in the virtual and physical spaces are finite. Three electromagnetic devices are designed and illustrated to verify this method. In order to avoid using a magnetic material which is a significant barrier in fabrication, the results are also tested when absorbing the magnetic permeability of the transformed material inside the permittivity. This is the first time to guarantee the obtained materials are all isotropic by transformation optics in 3-D problems. Therefore, the proposed method offers great potential in designing novel 3-D electromagnetic and optical devices, which will make these designed 3-D devices easier to fabricate in practical implementations.

Based on the theory of transformation optics (TO), a new and simple method for generating multimodal orbital angular momentum (OAM) carrying beams in 3-D space is proposed in this letter. The proposed 3-D arbitrary-way multimodal OAM generator only requires an electric dipole as the source. It consists of a four-way plane waves converter and four multimodal OAM generators. Since this method only requires the operation of radial scaling map, different topological charge modes can be easily obtained by appropriately setting the sizes of the physical geometries.

The multilayer surface integral equations (SIEs) for electromagnetic scattering by infinitely long magnetodielectric cylinders with an arbitrary number of layers are derived and solved by the spectral integral method (SIM). Singularity subtraction for the 2-D Green’s function is used to enhance the computation accuracy and achieve exponential convergence. The final matrix equation after discretization is formed in the Fourier spectral domain rather than the spatial domain, which greatly expedites the SIM solution by accelerating the convolution via the fast fourier transform (FFT) algorithm. A recursive method is proposed to solve the spectral integral equations instead of using an iterative method to lower the computation complexity. Numerical examples for ordinary multilayer cylinders and invisibility cloak cylinders are presented to validate the SIM results by comparing the total fields, scattered fields, and radar cross section (RCS) to analytical solutions or finite-element simulations. They verify that the recursive solution has a complexity of $O(MN\log N)$ for an $M$ -layer cylinder with $N$ discrete points on each interface. Meanwhile, the SIM outperforms the analytical method because only the 0th-order and 1st-order special functions (Bessel functions and Hankel functions) are used in the SIM but higher-order functions are necessary for the analytical method to maintain the accuracy.

A new method to implement transformation optics is proposed. The method is simple but very effective. As long as the transformed regions in the virtual and physical spaces are finite, the material parameters obtained are free of zero or infinite values and can be realized by index-only media. It is the first time that the absence of zero values is guaranteed by the transformation scheme itself. The range for the refractive index is easy to control since the proposed scheme only requires the operation of stretching transformation. The designs of multibeam antennas, which convert the cylindrical waves to plane waves, are illustrated in this paper. The proposed method can be used to achieve many other electromagnetic and optical devices.

A newly designed grounded electrical-source airborne transient electromagnetics (GREATEM) system was introduced recently. Detailed data preprocessing techniques to acquire the high-precision measured magnetic field are discussed here. Different from the previous work in which the reconstruction of the underground structure is performed in 1-D, we interpret the GREATEM data in 3-D by the volume integral equation (VIE) method in the frequency domain. Therefore, the VIE in the forward electromagnetic scattering model is formulated in the low-frequency regime. It is solved by using the stabilized biconjugate gradient fast Fourier transform (BCGS-FFT) method. In the nonlinear inversion, the Born iterative method (BIM) and the conjugate gradient method are adopted to minimize the cost function. A synthetic model of GREATEM survey is used to validate the proposed 3-D forward and inversion algorithms. Then, the field data from two GREATEM surveys are used to test the effectiveness and accuracy of the proposed inversion algorithm. The reconstructed conductivity structures are consistent with geological drilling results, confirming the potential of our method for solving the 3-D GREATEM inversion problems in geophysical engineering applications. This paper represents the first application of the BCGS-FFT and BIM algorithms to a GREATEM system.

© 1963-2012 IEEE. An iterative spatiotemporal Fourier transform (STFT) method is presented to efficiently design finite-impulse-response (FIR) filter coefficients for generating a desired frequency-invariant (FI) beam pattern of an antenna array. In this method, by introducing the concepts of normalized temporal angular frequency and spatial angular frequency, the broadband pattern is treated as a spatiotemporal spectral distribution. The relationship between the FIR coefficient distribution and the spatiotemporal spectral distribution can be built as an STFT, and consequently the 2-D fast Fourier transform (2D-FFT) and inverse 2D-FFT (2D-IFFT) can be utilized to efficiently accomplish the transformation between the FIR coefficient distribution and the spatiotemporal spectral distribution. Thus, the proposed synthesis method starts from an initial spatiotemporal spectral distribution and then adopt an iterative modification-and-transformation strategy to successively update the obtained spatiotemporal spectral distribution and the corresponding FIR coefficients. Two kinds of pattern modification techniques including the mainlobe FI modification and broadband sidelobe control are adopted in each iteration. Several examples for synthesizing different FI patterns are conducted. Synthesis results show that the proposed method can obtain much better FI pattern performance in terms of both mainlobe FI property and sidelobe control than the original FT method whilst costing less CPU time than the convex optimization method especially for the case of large FI arrays.

Understanding the photoinduced ultrafast structural transitions and electronic dynamics in single-walled carbon nanotubes (SWCNTs) is important for the development of SWCNT-based optoelectronic devices. In this study, we conducted femtosecond-resolved electron diffraction and electron energy-loss spectroscopy (EELS) measurements on SWCNTs using ultrafast transmission electron microscopy. The experimental results demonstrated that dominant time constants of the dynamic processes were ∼1.4 ps for electron-driven lattice expansion, ∼17.4 ps for thermal phonon-driven lattice expansion associated with electron–phonon coupling. The time-resolved EELS measurements clearly revealed a notable red shift of plasmon peaks by ∼100 meV upon femtosecond laser excitation. Different features of charge carrier excitation and relaxation were carefully discussed in correlation with the lattice dynamics and photoinduced absorption signals of SWCNTs. Our results provide a comprehensive understanding of the ultrafast dynamics in SWCNTs and powerful techniques to characterize the dynamics of low-dimensional structures.

We report bidirectional red-light passively Q -switched all-fiber lasers based on a ring-cavity configuration using single-wall carbon nanotubes (SWNTs) as saturable absorber (SA). By manipulating the cavity design and intra-cavity polarization state, two sets of output pulses with different features could be achieved from the clockwise (C.W.) and counterclockwise (C.C.W.) directions, respectively. In the C.W. direction, the lasers generate the wavelength of 635.4 nm, the pulse trains with minimum pulse duration of 1.35 μs (1.51 μs) and the repetition rate ranging from 65.2 to 143 kHz (67.8 to 96.5 kHz). In the C.C.W. direction, the lasers, also emitting at 635.4 nm, deliver the pulse trains with minimum pulse duration of 0.81 μs (3.59 μs), as well as the repetition rate range of 57.5–98.2 kHz (37.2–58.9 kHz). This is, to the best of our knowledge, the first demonstration of bidirectional red-light passively Q -switched all-fiber ring lasers based on SWNT-SA.

A new iterative inversion algorithm is proposed to reconstruct the electrical conductivity profile in a stratified underground medium for the grounded electrical source airborne transient electromagnetic (GREATEM) system. In forward modeling, we simplify the mathematical expressions of the magnetic fields generated by a finite line source in the layered ground to semianalytical forms in order to save the computation time. The Frechet derivative is derived for the electromagnetic response at the receivers due to a small perturbation of the conductivity in a certain layer underground. The initial expression of the Frechet derivative has an expensive triple integral and contains the Bessel function in the integrand. It is simplified by partially eliminating the integration along the source line and deriving the analytical expression for the integration in the vertical direction inside the perturbed layer. In the inverse solution, we use the distorted Born iterative method (DBIM). This is the first time that the DBIM is applied to data measured by the GREATEM system. Besides, the forward and inverse procedures are carried out in the frequency domain and based on the Frechet derivative of a line source. We demonstrate the validity of our forward model, Frechet derivative, inverse model, and the precision as well as robustness of the inversion algorithm through numerical computation and comparisons. Finally, we apply the inversion algorithm to the measured data and compare the retrieved conductivity to the actual drilling data.

Non-equilibrium electrons induced by ultrafast laser excitation in a correlated electron material can disturb the Fermi energy as well as optical nonlinearity. Here, non-equilibrium electrons translate a semiconductor TiS2 material into a plasma to generate broad band nonlinear optical saturable absorption with a sub-picosecond recovery time of ∼768 fs (corresponding to modulation frequencies over 1.3 THz) and a modulation response up to ∼145%. Based on this optical nonlinear modulator, a stable femtosecond mode-locked pulse with a pulse duration of ∼402 fs and a pulse train with a period of ∼175.5 ns is observed in the all-optical system. The findings indicate that non-equilibrium electrons can promote a TiS2-based saturable absorber to be an ultrafast switch for a femtosecond pulse output.

The Chinese government is actively promoting urbanization to stimulate its economic growth while facing increasingly prominent environmental concerns. The main objective of this research is to assess whether the Chinese government is making efforts to promote cleaner energy demand while pushing for urbanization. This study employs system GMM models to empirically investigate the causal relationship between urbanization and natural gas demand by using a sample of 30 provinces in China over the period 2005–2018. The estimates of the preferred specifications show that government-led urbanization has a positive impact on natural gas demand conditional on total energy use. By attaching natural gas facilities to new structures through the use of administrative power, the government induces natural gas demand while promoting urbanization. Robustness checks indicate that adding more potentially influential factors will not qualitatively change the results from the baseline. A constrained two-step static panel data estimation is used to estimate the depreciation rates of natural gas and of all fuel appliances, suggesting that the promotion of natural gas demand provides a relatively economical way to balance the trade-off between economic growth and environmental protection. The empirical results also show that the dynamic model outperforms its static counterpart in predictions. Based on the results, policy recommendations are made towards the goals of the Fourteenth Five-Year Plan for the National Economic and Social Development of China.

One fundamental difficulty in low-frequency subsurface electromagnetic exploration is the low-frequency breakdown phenomenon in numerical computation. It makes the discretized linear system very poorly conditioned and thus difficult to solve. This issue is present in both integral equation and partial differential equation solution methods, and thus has attracted many researchers who have proposed various methods to overcome this difficulty. In this paper, we propose a new mixed spectral element method (mixed SEM) to eliminate this low-frequency breakdown problem and apply this method to solve the subsurface electromagnetic exploration problem. Since Gauss’ law is now explicitly enforced in the mixed SEM to make the system matrix well-conditioned even at extremely low frequency, we can solve the linear system from dc to high frequencies. With the proposed method, we study the surface-to-borehole electromagnetic system for hydrocarbon exploration. Numerical examples show that the mixed SEM is accurate and efficient, and has significant advantages over conventional methods.

© 2002-2011 IEEE. This letter develops an extended generalized matrix pencil method (EGMPM) for the synthesis of multiple-pattern frequency-invariant linear arrays. In this method, all the pattern data sampled from different frequencies and different patterns are organized into a Hankel matrix. The problem of estimating the best common excitations and locations is converted into solving a singular value decomposition and some least-square problems. The previously proposed generalized matrix pencil method and extended matrix pencil method are both special cases of the EGMPM. A set of numerical experiments on Chebyshev pattern, shaped pattern, Taylor pattern, pencil-beam pattern, and flat-beam pattern is shown to validate the proposed method.

Structural dynamics and changes in electronic structures driven by photoexcited carriers are critical issues in both semiconducting and optoelectronic nanodevices. Herein, a phase diagram for the transient states and relevant dynamic processes in multiwalled boron nitride nanotubes (BNNTs) has been extensively studied for a full reversible cycle after a fs-laser excitation in ultrafast TEMs, and the significant structural features and evolution of electronic natures have been investigated using pulsed electron diffraction and femtosecond-resolved electron energy-loss spectroscopy (EELS). It is revealed that nonthermal anisotropic alterations of the lattice apparently precede the phonon-driven thermal transients along the radial and axial directions. Ab initio calculations support these findings and show that electrons excited from the π to π* orbitals in the BN nanotubes weaken the intralayer bonds while strengthening the interlayer bonds along the radial direction. Importantly, time-resolved EELS measurements show contraction of the energy bandgap after fs-laser excitation associated with nonthermal structural transients. This fact verifies that laser-induced bandgap renormalization in semiconductors can essentially be correlated with both the rapid processes of excited carriers and nonthermal lattice evolution.

The interaction of laser pulses with magnetic materials has become a major topic in modern magnetism, with ultrashort pulses providing unique windows to view ultrafast phenomena. It is still a challenge, though, to extend spatial resolution, to study magnetic nanostructures and complex spin textures. Here the high spatiotemporal resolution of Lorentz ultrafast transmission electron microscopy (Lorentz UTEM) reveals remarkable features and key details of the magnetic transient states in Mn-Ni-Ga after a femtosecond laser pulse. This work showcases Lorentz UTEM as a means of directly imaging ultrafast magnetization phenomena at the nanoscale.

A new design scheme for ultrafast transmission electron microscopy (UTEM) has been developed based on a Schottky-type field emission gun (FEG) at the Institute of Physics, Chinese Academy of Sciences (IOP CAS). In this UTEM setup, electron pulse emission is achieved by integrating a laser port between the electron gun and the column and the resulting microscope can operate in either continuous or pulsed mode. In pulsed mode, the optimized electron beam properties are an energy width of ~0.65 eV, micrometer-scale coherence lengths and sub-picosecond pulse durations. The potential applications of this UTEM, which include electron diffraction, high-resolution imaging, electron energy loss spectroscopy, and photon-induced near-field electron microscopy, are demonstrated using ultrafast electron pulses. Furthermore, we use a nanosecond laser (~10 ns) to show that the laser-driven FEG can support high-quality TEM imaging and electron holography when using a stroboscopic configuration. Our results also indicate that FEG-based ultrafast electron sources may enable high-performance analytical UTEM.

Low-frequency subsurface electromagnetic measurements are important tools for characterizing natural resources and environmental wastes. Rapid simulations of low-frequency subsurface electromagnetic measurements are still a challenge because of the large computational domain and low-frequency breakdown phenomenon. We develop an effective method to simulate these low-frequency subsurface electromagnetic measurements by using the spectral element method together with a domain decomposition method (DDM). A specific mesh has been designed based on the traveling wave nature in the air and the diffusion field nature in the underground space to greatly reduce the number of unknowns. The frequency-domain version of the Riemann solver (upwind flux) is used as an effective transmission condition to simulate the interactions between neighboring subdomains in DDM. Several numerical examples demonstrate the efficiency of the proposed approach in low-frequency subsurface electromagnetics simulations.

A new mixed finite-element method is proposed for computing the anisotropic, lossy, and open waveguide modes. By incorporating Gauss' law into the vectorial wave equation, the variational formulation is completely free of spurious modes. It utilizes the curl-conforming linear tangential and quadratic normal (LT/QN) edge elements to expand the tangential component of the electric field, and the modified nodal-based scalar basis functions to expand the longitudinal component. A smaller scale generalized eigenvalue problem is obtained by employing the mass lumping technique. Numerical examples verify that the mixed finite-element method with mass lumping is free of any spurious eigenmode and has high efficiency.

Passive Q-switching or mode-locking by placing a saturable absorber inside the laser cavity is one of the most effective and popular techniques for pulse generation. However, most of the current saturable absorbers cannot work well in the visible spectral region, which seriously impedes the progress of passively Q-switched/mode-locked visible pulsed fibre lasers. Here, we report a kind of visible saturable absorber-two-dimensional transition-metal dichalcogenides (TMDs, e.g. WS2, MoS2, MoSe2), and successfully demonstrate compact red-light Q-switched praseodymium (Pr(3+))-doped all-fibre lasers. The passive Q-switching operation at 635 nm generates stable laser pulses with ∼200 ns pulse duration, 28.7 nJ pulse energy and repetition rate from 232 to 512 kHz. This achievement is attributed to the ultrafast saturable absorption of these layered TMDs in the visible region, as well as the compact and all-fibre laser-cavity design by coating a dielectric mirror on the fibre end facet. This work may open a new route for next-generation high-performance pulsed laser sources in the visible (even ultraviolet) range.

It is shown in this paper that the problem of reducing the number of elements for multiple- pattern arrays can be solved by a sequence of reweighted � 1 optimizations under multiple linear constraints. To do so, conjugate symmetric excitations are assumed so that the upper and lower bounds for each pattern can be formulated as linear inequality constraints. In addition, we introduce an auxiliary variable for each element to define the common upper bound of both the real and imaginary parts of multiple excitations for different patterns, so that only linear inequality constraints are required. The objective function minimizes the reweighted � 1-norm of these auxiliary variables for all elements. Thus, the proposed method can be efficiently implemented by the iterative linear programming. For multiple desired patterns, the proposed method can select the common elements with multiple set of optimized amplitudes and phases, consequently reducing the number of elements. The radiation characteristics for each pattern, such as the mainlobe shape, response ripple, sidelobe level and nulling region, can be accurately controlled. Several synthesis examples for linear array, rectangular/triangular-grid and randomly spaced planar arrays are presented to validate the effectiveness of the proposed method in the reduction of the number of elements.

We numerically demonstrate a switchable broadband terahertz spatial modulator composed of ginkgo-leaf-patterned graphene and transition material vanadium dioxide (VO2). The phase transition property of VO2 is used to switch the spatial modulator from absorption mode to transmission mode, and the graphene behaves as dynamically adjustable material for a large scale of absorption and transmittance modulation. When VO2 is in the metallic state and the Fermi energy of graphene is set as 0.8 eV, the proposed modulator behaves as a broadband absorber with the absorbance over 85% from 1.33 to 2.83 THz. By adjusting the graphene Fermi level from 0 to 0.8 eV, the peak absorbance can be continuously tuned from 24.3% to near 100% under the absorption mode, and the transmittance at 2.5 THz can be continuously tuned from 87% to 35.5% under the transmission mode. To further increase the bandwidth, a three-layer-patterned-graphene is introduced into a new modulator design, which achieves a wide bandwidth of 3.13 THz for the absorbance over 85%. By the combination of the tunability of graphene and VO2, the proposed modulators not only can flexibly switch between dual-functional modulation modes of absorption and transmission but also possess deep modulation depth. Benefitting from the excellent modulation performance, the proposed switchable dual-functional spatial modulators may offer significant potential applications in various terahertz smart optoelectronic devices.

© 2018 Wiley Periodicals, Inc. A novel controllable wideband filtering power divider (FPD) with sharp roll-off skirt and good isolation is presented in this article. By introducing a pair of coupled lines and several uniformed quarter-wavelength microstrip lines to the power divider, a wide flat passband with good selectivity is achieved. A resistor is then arranged between the 2 coupled line structures to obtain good isolation. Even-mode and odd-mode approach is applied to deduce the design equations of the proposed FPD. The center frequency can be adjusted flexibly by simply controlling the length of the coupled lines and the other microstrip lines. Eventually, the proposed power divider centered at 2.3 GHz with a fractional bandwidth of 75% is fabricated and measured for verification. The measured minimum in-band insertion loss is 0.3 dB, while the isolation is better than 20 dB. Good agreement is shown between the simulated and measured results.

Ultrafast laser excitation of electronic crystals reveals hidden quantum states with new polaron cluster ordering., The hidden (H) quantum state in 1T-TaS2 has sparked considerable interest in the field of correlated electron systems. Here, we investigate ultrafast switches to stable H charge density wave (H-CDW) states observed in 1T-TaS2−xSex, with x = 0 and 0.5 crystals, upon excitation with a single femtosecond laser pulse. In situ cooling transmission electron microscopy observations, initiated by a single femtosecond laser pumping with a low fluence, reveal a clear transition from a commensurate CDW phase (qC) to a new CDW order with qH = (1 − δ)qC for the H-CDW state (δ = 1/9) accompanied by an evident phase separation. H-CDW domain relaxation then occurs and yields a stable metallic phase under a high-fluence excitation. Furthermore, electrical resistivity measurements show that the notable drop in x = 0 and 0.5 samples associated with the appearance of H-CDW states depend on laser fluence and temperature. These results potentially provide a new perspective on the photodoping mechanism for the emergence of H-CDW states in the 1T-TaS2−xSex family.

In this Letter, the transient nonlinear absorption of three-dimensional (3D) topological Dirac semimetal Cd3As2 thin film was characterized in the near-infrared band. By performing broadband pump–probe measurements, we experimentally proved that molecular beam epitaxy (MBE) grown Cd3As2 exhibits strong and tunable saturable absorption effects across 1–2 μm. By further inserting the Cd3As2 film into the cavities of Tm- and Er-doped fiber lasers, we obtained stable mode-locked operations at 1.96 and 1.56 μm. Our results experimentally establish that Cd3As2 is a promising broadband saturable absorber (SA) for pulsed lasers in the infrared range.

Pulsed lasers operating in the mid-infrared (3-25 µm) are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy, biomedical research, sensing, imaging, and communication. Up to now, one of the factors limiting the mid-infrared pulsed lasers is the lack of optical switch with a capability of pulse generation, especially for those with wideband response. Here, a semiconductor material of bismuth oxyselenide (Bi2 O2 Se) with a facile processibility, constituting an ultrabroadband saturable absorber for the mid-infrared (actually from the near-infrared to mid-infrared: 0.8-5.0 µm) is exhibited. Significantly, it is found that the optical response is associated with a strong nonlinear character, showing picosecond response time and response amplitude up to ≈330.1% at 5.0 µm. Combined with facile processibility and low cost, these solution-processed Bi2 O2 Se materials may offer a scalable and printable mid-infrared optical switch to open up the long-sought parameter space which is crucial for the exploitation of compact and high-performance mid-infrared pulsed laser sources.

Due to their strong light-matter interaction, monolayer transition metal dichalcogenides (TMDs) have proven to be promising candidates for nonlinear optics and optoelectronics. Here, we characterize the nonlinear absorption of chemical vapour deposition (CVD)-grown monolayer MoSe2 in the 720–810 nm wavelength range. Surprisingly, despite the presence of strong exciton resonances, monolayer MoSe2 exhibits a uniform modulation depth of ∼80 ± 3% and a saturation intensity of ∼2.5 ± 0.4 MW/cm2. In addition, pump-probe spectroscopy is performed to confirm the saturable absorption and reveal the photocarrier relaxation dynamics over hundreds of picoseconds. Our results unravel the unique broadband nonlinear absorptive behavior of monolayer MoSe2 under ultrafast excitation and highlight the potential of using monolayer TMDs as broadband ultrafast optical switches with customizable saturable absorption characteristics.

Single-wall carbon nanotubes (SWNTs) have been extensively explored as an ultrafast nonlinear optical material. However, due to the numerous electronic and morphological arrangements, a simple and self-contained physical model that can unambiguously account for the rich photocarrier dynamics in SWNTs is still absent. Here, by performing broadband degenerate and non-degenerate pump-probe experiments on SWNTs of different chiralities and morphologies, we reveal strong evidences for the existence of bandgap renormalization in SWNTs. In particularly, it is found that the broadband transient response of SWNTs can be well explained by the combined effects of Pauli blocking and bandgap renormalization, and the distinct dynamics is further influenced by the different sensitivity of degenerate and non-degenerate measurements to these two concurrent effects. Furthermore, we attribute optical-phonon bath thermalization as an underlying mechanism for the observed bandgap renormalization. Our findings provide new guidelines for interpreting the broadband optical response of carbon nanotubes.

© The Institution of Engineering and Technology. Antenna arrays with shaped patterns have drawn significant attention for their wide applications in radar, sonar and communication systems. The combination of genetic algorithm (GA) and fast Fourier transform (FFT) has been used to synthesise shaped pattern of antenna array's factor without considering coupling effects. In this work, the GA-FFT method is generalised by integrating with a new virtual active element pattern (AEP) expansion method which approximates each AEP as the radiation pattern by exciting several equally spaced virtual elements surrounding the real element position. The generalised GA-FFT can be applied for the shaped pattern synthesis of unequally spaced linear arrays including mutual coupling and platform effect. Several synthesis examples with different pattern shapes and different antenna structures are given to demonstrate the effectiveness, accuracy and robustness of the proposed method.

We report that MBE-grown three-dimensional (3D) topological Dirac semimetal Cd 3 As 2 thin-film exhibits remarkable saturable absorption effects at 1, 1.5 and 2 μm. A mode-locked Tm fiber laser is demonstrated using such a SESAM-like material.

We report the first photocarrier dynamics investigation on type-II Weyl semimetal WTe 2 . A transient feature where an initial photo-bleaching signal is found followed by a persistent photoinduced absorption over the 1.8–2.5 μm range is revealed.

An efficient and exact spectral integral method (SIM) for the general problems of scattering by a circular cylinder is presented, both for acoustic and electromagnetic cases. Fast Fourier transform (FFT) and convolution theorem help us to execute the algorithm efficiently with O(Nlog(N)) computational complexity. The Fourier coefficients of the integral kernels are computed in a semianalytical way to accomplish a fast convergence rate with spectral accuracy. Compared with the method not using the semianalytical forms, without the increase of the computing and memory abundance, it saves more than 90% sampling points to achieve about the same accuracy, and with the same number of sampling points, several to more than ten orders of magnitude more accuracy can be achieved. For example, for the tested large wavenumber case with 2 × 106 wavelengths on the boundary, only two sampling points per wavelength are required to achieve a relative error of less than 0.001%. More than 90% computational time is saved compared with the normal summation formula of harmonic cylinder expansions. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Invisibility cloaks designed from the coordinate transformation method have attracted increasing interest recently. Conformal transformation optics scheme leads to cloaks that possess isotopic media, thus provides a prospective way to facilitate easier realization. Reducing the maximum value of the refractive index required by the cloaks is very important in practical imple- mentation. This letter studies on how the parameters in the logarithm conformal mapping control the cloaking effect. The optimized invisibility cloaks are designed. The maximum values of the refractive index required from the first kind and the second kind of logarithm conformal mappings are reduced to 9.779 and 12.936, respectively.

Graphene has proved to be an excellent broadband saturable absorber for mode-locked operation of ultrafast lasers. However, for the mid-infrared (mid-IR) range where broadly tunable sources are in great needs, graphene-based broadly tunable ultrafast mid-IR lasers have not been demonstrated so far. Here, we report on passive mode-locking of a mid-IR Cr:ZnS laser by utilizing a transmission-type monolayer graphene saturable absorber and broad spectral tunability between 2120 nm and 2408 nm, which is the broadest tuning bandwidth ever reported for graphene mode-locked mid-IR solid-state lasers. The recovery time of the saturable absorber is measured to be ~2.4 ps by pump-probe technique at a wavelength of 2350 nm. Stably mode-locked Cr:ZnS laser delivers Fourier transform-limited 220-fs pulses with a pulse energy of up to 7.8 nJ.

In this paper, an active output feedback fault-tolerant control design for submarine vertical movements is proposed. An unknown input disturbance observer is given first, and then an observer is designed to diagnose all of actuator faults by decoupling external disturbance from actuator faults. Then a fault parameter tracking law is designed to estimate the fault parameters of bow and stern planes. Finally, an adaptive H ∞ dynamic output feedback fault-tolerant control scheme is designed by using the estimated fault parameters of the bow and stern planes. Simulation results show that the proposed control scheme is effective in the both pitch and depth control when the bow and stern planes suffer from unknown faults.

Pulsed lasers operating in the mid-infrared (3–20 μm) are important for a wide range of applications in sensing, spectroscopy, imaging and communications. Despite recent advances with mid-infrared gain platforms, the lack of a capable pulse generation mechanism remains a significant technological challenge. Here we show that bulk Dirac fermions in molecular beam epitaxy grown crystalline Cd3As2, a three-dimensional topological Dirac semimetal, constitutes an exceptional ultrafast optical switching mechanism for the mid-infrared. Significantly, we show robust and effective tuning of the scattering channels of Dirac fermions via an element doping approach, where photocarrier relaxation times are found flexibly controlled over an order of magnitude (from 8 ps to 800 fs at 4.5 μm). Our findings reveal the strong impact of Cr doping on ultrafast optical properties in Cd3As2 and open up the long sought parameter space crucial for the development of compact and high-performance mid-infrared ultrafast sources., Mid-infrared pulsed sources are technologically important for sensing and spectroscopy but their implementation is challenging due to the lack of a tuneable optical switch. Here, the authors address this limitation by engineering the band structure of an emerging Dirac semimetal, Cd3As2.

Understanding of the fundamental photoresponse of carbon nanotubes has broad implications for various photonic and optoelectronic devices. Here, Z-scan and pump-probe spectroscopy performed across 600-2400 nm were combined to give a broadband 'degenerate' mapping of the nonlinear absorption properties of single-wall carbon nanotubes (SWNTs). In contrast to the views obtained from non-degenerate techniques, sizable saturable absorption is observed from the visible to the near-infrared range, including the spectral regions between semiconducting excitonic peaks and metallic tube transitions. In addition, the broadband mapping unambiguously reveals a photobleaching to photoinduced absorption transition feature within the first semiconducting excitonic band ∼2100 nm, quantitatively marking the long-wavelength cut-off for saturable absorption of the SWNTs investigated. Our findings present a much clearer physical picture of SWNTs' nonlinear absorption characteristics, and help provide updated design guidelines for SWNT based nonlinear optical devices.

An efficient and accurate algorithm for solving Helmholtz equation in 1-D is presented in this paper. The key point of this work is to derive the analytic form for the convolution of the Green’s function and a complex exponential function with a finite support domain. A linear subtraction skill is introduced to improve the sampling efficiency. Therefore, the convolution of any function with the Green’s function is given in a semi-analytic form, which can be computed in a fast way with the help of FFT or NUFFT.

2–5 μm saturable absorbers are highly desirable for constructing convenient mid-infrared ultrafast sources. We reveal that three-dimensional Dirac semimetal Cd 3 As 2 constitutes an ideal mid-infrared saturable absorber, with broadband operation and flexible parameter control.

A new method for the synthesis of wideband frequency-invariant pencil-beam patterns with nonuniformly spaced linear arrays is proposed. This scheme is designed under the framework of fast multitask compressive sensing. The minimum spacing between antenna elements can be controlled to make sure that the obtained arrays are physically realizable. The results achieved are comparable to those obtained by the state-of-the-art techniques. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Bi2O2Se is emerging as a high mobility functional material for optoelectronics, but its fundamental optical properties remain less well studied. Here, ultrafast photocarrier dynamics in single crystal Bi2O2Se is investigated by pump fluence-dependent, broadband ultrafast spectroscopy. Our results reveal that bimolecular recombination plays an important role in the photocarrier relaxation process, and a room-temperature bimolecular recombination constant of (1.29 ± 0.42) × 10−9 cm−3 s−1 is obtained for Bi2O2Se. Such a level of the recombination constant combined with a high mobility (∼1006 cm2 V−1 s−1 at 200 K for Bi2O2Se) suggests that Bi2O2Se can be a promising material for photovoltaic applications.

WTe2 is a unique material in the family of transition metal dichalcogenides and it has been proposed as a candidate for type-II Weyl semimetals. However, thus far, studies on the optical properties of this emerging material have been significantly hindered by the lack of large-area, high-quality WTe2 materials. Here, we grow a centimeter-scale, highly crystalline WTe2 ultrathin film (∼35 nm) by a pulsed laser deposition technique. Broadband pump-probe spectroscopy (1.2–2.5 μm) reveals a peculiar ultrafast optical response where an initial photo-bleaching signal (lasting ∼3 ps) is followed by a long-lived photoinduced absorption signature. Nonlinear absorption characterization using femtosecond pulses confirms the saturable absorption response of the WTe2 ultrathin films, and we further demonstrated a mode-locked Thulium fiber laser using a WTe2 absorber. Our work provides important insights into linear and nonlinear optical responses of WTe2 thin films.

The hot-carrier relaxation dynamics of Cd3As2 thin films has been investigated by using femtosecond pump-probe spectroscopy in a transmission geometry. A comparative study of degenerate and non-degenerate experiments reveals that hot-carrier distribution in Cd3As2 is established with a time constant of ∼400 fs. Significantly, the broadband measurements allow the extraction of the time evolution of electron temperature and the carrier-phonon coupling factor g = 5.3 × 1015 W m−3 K−1 is deduced by a semiclassical two-temperature model. These results provide fundamental insights into the hot-carrier dynamics of Cd3As2.

We experimentally demonstrated a compact single-wall carbon-nanotube (SWNT)-based deep-red passively Q-switched Prsup3+/sup-doped ZBLAN all-fiber laser operating at 716 nm. A free-standing SWNT/polyvinyl alcohol composite film embedded between a pair of fiber connectors was employed as a saturable absorber (SA). The deep-red Q-switched operation is attributed to the combination of implementing a pair of fiber end-facet mirrors to achieve the linear laser resonator and incorporating a SWNT-SA into the cavity as a Q-switcher. Stable short-pulse generation with a duration of 2.3 μs was realized. When gradually increasing the incident pump power, the pulse repetition rate can be linearly tuned from 32.6 to 86.5 kHz, corresponding to a maximum average output power of 1.5 mW and the highest single-pulse energy of 18.3 nJ. To the best of our knowledge, this is the first demonstration of SWNT-based SA for a Q-switched laser at a deep-red wavelength ∼716 nm.

We observed a novel phenomenon where single-wall carbon nanotube flocculations can be photo-actuated in liquids by either sunlight or laser irradiation. The light-actuation phenomenon is found highly repeatable.