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A new type of reconfigurable negative group delay (NGD) circuits based on active transversal filter topology is introduced in this work. By incorporating varactors with the NGD circuit at the input and output ports of active transversal filter based on distributed amplifiers (DAs), one can notably enhance the tunability for both amplitude and phase responses, thereby enabling NGD characteristic synthesis with a wider reconfigurable range. In this article, the proposed reconfigurable NGD circuits are applied to create both tunable non-Foster elements and tunable squint-free beamforming networks, which are verified by experiment. Furthermore, a general N-stage transfer function for the proposed reconfigurable NGD circuit is derived, in which a two-stage particular case is given as an illustrative proof-of-concept example. Based on the reconfigurable NGD circuits, a prototype of a tunable negative capacitor is fabricated and measured, of which the capacitance can be tuned from -1.0 to -4.0 pF over a bandwidth of 100 MHz at 1 GHz or around 10% relative bandwidth. In addition, such an NGD circuit is utilized to create a low-dispersion superluminal transmission line, which enables a reconfigurable series antenna feedline for squint-free beam steering. Experimental results show that the main beam of the NGD antenna array has a tuning range from -15° to 15° within a squint-free bandwidth of 100 MHz at 2 GHz or within a relative bandwidth of around 5%.

A new kind of negative group delay (NGD)/non-Foster circuit using monolithic microwave integrated circuit (MMIC) technology is presented. The proposed design is based on an active transversal filter topology using a distributed amplifier (DA) with a cascode configuration at each stage of gain cells, realized by using WIN's 0.1-um GaAs pseudomorphic high electron mobility transistor (pHEMT) process. Furthermore, a forward-biased Schottky barrier diode (SBD) is utilized as a phase shifter to adjust the absolute phase of S21. By properly applying the gate bias voltage and controlling the phase shifter, the proposed NGD circuit can synthesize a negative capacitance of −0.3 pF with a bandwidth of 500 MHz at around 5 GHz while ensuring unconditional stability. The fabricated MMIC NGD/non-Foster circuit has a footprint of 3. 05×3. 15mm2 with a power consumption of 125 mW. Measurement and simulation results show great agreement with each other.

A nonreciprocal composite right/left-handed (CRLH) leaky wave antenna (LWA) based on a distributed mixer (DM) is proposed to achieve simultaneous transmit and receive (STAR). The DM structure consists of field-effect transistors (FETs) as mixing devices with a CRLH transmission line (TL) as the drain line and a microstrip line as the gate line. In the receive mode, the carrier LO signal in the gate line and the RF signal in the drain CRLH TL are along the same direction, while in the transmit mode, the RF signal is injected from the drain CRLH line at the opposite direction of the carrier LO signal from the gate line. Owing to distributed mixing principle, it will be shown that the reverse conversion gain for the down-converted IF signal can provide isolation between the transmit and receive operation, i.e. RF signals of opposite directions, thereby enabling the simultaneous transmit and receive characteristic.

Recent development of reconfigurable negative group delay (NGD) circuits based on distributed amplifiers (DAs) has been reviewed. It is shown that reconfigurable DA-based NGD circuits can be used as tunable non-Foster components, such as negative capacitor/inductor/T-networks. Such NGD circuits also have been used to reduce the squint effect of an antenna array, while steering the main beam direction. The squint-free bandwidth can be increased if one applies more stages in the DA-based design. Furthermore, in this paper, we demonstrate a new method to increase the squint-free bandwidth, which incorporates a tunable squint-free series type antenna array feed line with each element controlled by a two-stage DA-based NGD circuit. The enhanced bandwidth is realized by slightly shifting the center frequency of the NGD circuit at each antenna element, therby increasing the bandwidth of the NGD response. Such schematic has advantages of reduced tuning complexity, compact size, as well as good gain performance.

A dispersion-engineered artificial transmission line (TL) enabled by negative group delay (NGD) is proposed to exhibit squint-free dominant mode leaky-wave radiation. The unit cell of the proposed artificial TL is based on a multi-section directional coupler, which is used to generate the desired NGD, followed by a composite right/left-handed transmission line (CRLH-TL) structure. An amplifier is incorporated within each unit cell to compensate for the power loss from the NGD coupler. By incorporating the NGD response, the resulting artificial TL can demonstrate a dispersionless fast wave characteristic within the NGD frequency band. For proof-of-concept, prototypes of a single unit cell and a three-unit cell NGD artificial TL are fabricated and tested to verify the dispersion and radiation characteristics. The measured radiation patterns of the proposed NGD artificial TL indicate that the main beam angle is located at -35 degrees in the backward direction, with a squint-free bandwidth from 2.55 GHz to 2.75 GHz.

A recent development of reconfigurable negative group delay (NGD) circuit design based on distributed amplifiers (DA) is introduced. By incorporating varactors at the input and output port of the DA-based NGD circuit, tunable non-Foster elements - negative capacitors and non-Foster T-networks, can be realized. The negative capacitor can be tuned to realize an effective capacitance between −0.5 pF and −1.0 pF within a bandwidth of 100 MHz. For the non-Foster T-network, the tuning range is from 0.0 pF to −1.5 pF for the shunt capacitor, and 0 nH to −1.875 nH for the series inductor. Furthermore, the DA-based reconfigurable NGD circuit can also be utilized as antenna feedlines to realize squint free beam steering for antenna arrays.

A new type of tunable series feed network for squint-free antenna beamforming using microwave distributed amplifier (DA)-based negative group delay (NGD) circuits is proposed. Utilizing the DA-based NGD circuits as delay lines between elements in an antenna array, squint-free beamforming with reconfigurable radiation main beams can be achieved. The tuning mechanism is realized by changing the transistor's transconductance. To achieve this, the transistors placed at each stage of the DA are tuned separately. For the proof of concept, a prototype of a two-stage DA-based tunable squint-free series feed network built and tested. The measurement results show that, the main-beam direction of the antenna array can be successfully tuned while maintaining squint-free within a bandwidth of 100 MHz.

An approximate three-wave model is suggested for describing the modal field inside the high-index dielectric rod of a hybrid plasmonic waveguide. An evanescent wave, an uniform wave and a propagating wave are considered along the direction perpendicular to the metal surface. The superposition of these three waves forms the modal field inside the high-index rod. Through numerical tests, we find that this model is highly valid for a large range of waveguide sizes.

A novel tunable non-Foster T-network loaded with transmission lines (TLs) using a two-stage distributed amplifier (DA)-based negative group delay (NGD) circuit is proposed. The tunability comes from the NGD response synthesized by adjusting the ratio of the transconductance of the transistors, and the varactors at both the input and output port. Due to the distributed topology, the proposed T-network not only works within a relatively broad band (several hundreds of Mega-Hertz), but also has good input and output matching characteristics. A prototype of the proposed non-Forster T-network is built and tested. By properly amending the in-band magnitude and phase characteristics, the NGD circuit can realize the targeted non-Foster T-network with unconditional stability. The experimental results show that the tunable equivalent T-network has a 0 pF to − 1.5 pF shunt capacitance and a 0 nH to −1.875 nH inductance.

A novel tunable non-Foster negative capacitor with unconditional stability using a two-stage microwave distributed amplifier (DA) based negative group delay (NGD) is presented. The tuning mechanism is realized by changing both the transistor's transconductance and input/output phase responses. To achieve this, the transistors placed at each stage of the DA are tuned separately, and two identical varactors placed at the input port and output port are tuned by one single voltage. As proof-of-concept, a prototype of the proposed DA-NGD circuit based tunable negative capacitor is built and tested. The measurement results show that, the negative capacitor can be successfully tuned between −0.5 pF to −1.0 pF.

An angular trapezoidal phase mask used for a wideband coronagraph is proposed. The azimuthal phase of the mask is double-periodic and has both trapezoidal and constant parts in each period. This kind of continuous phase distribution can be employed to avoid the abrupt phase variation of the 6-level phase distribution we proposed previously. Numerical calculations show that this more practical phase mask can still keep its superior performance in terms of starlight elimination, small inner working angle, and good achromatism. It is of great importance that there is no singularity in this kind of mask except for a singularity at the center. This mask design is close to real manufacturing conditions, and the process technology is superior.

We introduce the technique of perturbation of boundary condition to the problem of terahertz two-wire metallic waveguides with different radii. Based on the quasi-TEM analytical mode fields derived by use of Mobius transformation, a concise expression for the complex effective index is obtained analytically. The expression is in good agreement with the simulation result. Further, the dispersion and attenuation are obtained from the expression. In addition, we find a zero value point of the group velocity dispersion around 1.268 THz. The results show that the technique of perturbation of boundary condition is helpful in the analysis and design of terahertz metal waveguide.

This is the reply to the comment by Chavez-Cerda and Pu [J. Opt. Soc. Am. A32, 1209 (2015)JOAOD61084-752910.1364/JOSAA.32.001209] on our recent work about the 50,000λ long needle-like field [J. Opt. Soc. Am. A31, 500 (2014)JOAOD60740-323210.1364/JOSAA.31.000500]. First, they employed an incorrect boundary condition as the fundament of their argument. In fact, it is not the electric field but its tangential component that must be zero at the surface of the perfect metal. Our result is completely consistent with the correct boundary condition. Second, a constant phase factor in the incident radially polarized beam, exp(jπ/4), for instance, has no influence on the result. Accordingly, our initial condition is proper.

An achromatic six-region phase mask coronagraph, used for the detection of exoplanets, is proposed. The mask has six regions in angular direction and could work in wideband. Furthermore, a six-level phase mask, as an example of the six-region phase mask, is theoretically investigated. According to numerical simulations, this specific mask has a deep elimination of starlight, good performance of achromatism and small inner working angle. As a single phase mask, the ratio of the remaining starlight of the six-level phase mask to the total incident starlight is less than 0.001 when the wavelength is between 500 nm and 600 nm.

It is a great challenge to create a needle-like field with properties of long beam length, narrow lateral width, uniformity, and high optical efficiency. Here we show a method that can realize these properties all at once. The key element is a 90° apex-angle concave conical mirror. By using this condenser along with a radially polarized incident beam of a specific field distribution, we numerically created a super slim, uniform, pure needle-like axially polarized field. This axially polarized field has a length of 50,000λ along the optical axis, and its lateral width still maintains a minimum 0.36λ size.

Dual-racetrack resonator structures are experimentally investigated. A single spectral is observed in each free-spectral range and the spectral phase variation indicates sufficient phase dynamic range. Electro-optic tuning of the structure produces rich coherent behaviors.