Sajjad Taravati

Mixer-Duplexer-Antenna Leaky-Wave System Based on Periodic Space-Time Modulation

We present a mixer-duplexer-antenna leaky-wave system based on periodic space-time modulation. This system operates as a full transceiver, where the upconversion and downconversion mixing operations are accomplished via space-time transitions; the duplexing operation is induced by the nonreciprocal nature of the structure, and the radiation operation is provided by the leaky-wave nature of the wave. A rigorous electromagnetic solution is derived for the dispersion relation and field distributions. The system is implemented in the form of a spatio-temporally modulated microstrip leaky-wave structure incorporating an array of subwavelengthly spaced varactors modulated by a harmonic wave. In addition to the overall mixer-duplexer-antenna operation, frequency beam scanning at fixed input frequency is demonstrated as one of the interesting features of the system. A prototype is realized and demonstrated by full-wave and experimental results.

Nonreciprocal Nongyrotropic Magnetless Metasurface

We introduce a nonreciprocal nongyrotropic magnetless metasurface. In contrast to previous nonreciprocal structures, this metasurface does not require a biasing magnet, and is therefore lightweight and amenable to integrated circuit fabrication. Moreover, it does not induce Faraday rotation, and hence does not alter the polarization of waves, which is a desirable feature in many nonreciprocal devices. The metasurface is designed according to a Surface-Circuit-Surface architecture and leverages the inherent unidirectionality of transistors for breaking time reversal symmetry. Interesting features include transmission gain as well as broad operating bandwidth and angular sector operation. It is finally shown that the metasurface is bianisotropic in nature, with nonreciprocity due to the electric-magnetic coupling parameters, and structurally equivalent to a moving uniaxial metasurface.

Nonreciprocal electromagnetic scattering from a periodically space-time modulated slab and application to a quasisonic isolator

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies.

Optical isolation based on space-time engineered asymmetric photonic band gaps

Nonreciprocal electromagnetic devices play an important role in modern optical and microwave technologies. Conventional methods for realizing such systems are incompatible with integrated circuits. With recent advances in integrated photonics, the need for efficient on-chip magnetless nonreciprocal devices is more urgent than ever. This paper leverages space-time engineered asymmetric photonic bandgaps to generate optical isolation. It is shown that a properly designed space-time modulated slab is highly reflective/transparent for opposite directions of propagation. The proposed method requires a low modulation frequency, is magnetless and can achieve very high isolation levels. Experimental proof of concept at microwave frequencies is provided.

Space-time modulated nonreciprocal mixing, amplifying and scanning leaky-wave antenna system

A space-time modulated leaky-wave antenna system simultaneously performing mixing (up/down frequency conversion), amplification and beam scanning is presented and demonstrated by full-wave simulations. This system is expected to pave the way for a diversity of novel radiative systems exploiting exotic space-time transitions in the dispersion diagram.

Enhanced Bandwidth and Diversity in Real-Time Analog Signal Processing (R-ASP) Using Nonuniform C-Section Phasers

We show that a continuously nonuniform coupled-line C-section phaser, as the limiting case of the step-discontinuous coupled-line multisection commensurate and non-commensurate phasers, provides enhanced bandwidth and diversity in real-time analog signal processing (R-ASP). The phenomenology of the component is explained in comparison with the step-discontinuous using multiple-reflection theory and a simple synthesis procedure is provided. The bandwidth enhancement results from the suppression of spurious group delay harmonics or quasi-harmonics, while the diversity enhancement results from the greater level of freedom provided by the continuous nature of the nonuniform profile of the phaser. These statements are supported by theoretical and experimental results.

Electromagnetic nonreciprocity and perfect mixing in space-time engineered asymmetric bandgaps

This paper leverages space-time engineered asymmetric electromagnetic bandgaps for generating magnetless electromagnetic nonreciprocity and perfect mixing. It is theoretically and experimentally demonstrated that a properly designed space-time modulated slab is highly reflective/transparent for opposite directions of propagation. The corresponding design is magnet-less, accommodates low modulation frequencies, and can achieve very high isolation levels. Moreover, the wave fully reflected from the space-time modulated slab exhibits frequency shifting, without any undesirable inter-modulation effects.

A guided tour in metasurface land: Discontinuity conditions, design and applications

The paper presents an overview of the recent research on metasurfaces performed in the authors group. This covers the introduction of bianistropic Sheet Transition Conditions (STCs), the subsequent elaboration of a general synthesis technique based on surface susceptibility tensors, the complementary elaboration of Finite Difference (FD) and Finite Element (FE) computational schemes for analysis, and, based upon this solid foundation, the development of a host of novel applications across the whole microwave to optical range of the electromagnetic spectrum.