arXiv subjects

Sajjad Taravati

Publications and source records attributed to Sajjad Taravati.

At least 19 recordsLinked to original sources

arXiv2026

On-Chip Nonreciprocal Superconducting Frequency Multiplier for Cryogenic Millimeter-Wave Multiplexed Control

Space-time-periodic modulation of superconducting circuits offers a compact, on-chip route to nonreciprocity, frequency conversion, and parametric gain, without the bulk of conventional ferrite-based components. This paper presents an on-chip nonreciprocal space-time-periodic Josephson frequency multiplier that up-converts a single low-frequency input tone into a comb of high-order harmonics while enforcing directional signal propagation along the bus. Full-wave time-domain simulations show that the harmonic content and bandwidth of the generated comb can be controlled through the applied DC and RF flux bias, and that individual output channels can be selectively addressed by tuning the bus operating point. We characterize the resulting nonreciprocal isolation between channels and discuss the associated design trade-offs, including the influence of modulation depth on harmonic-generation efficiency and back-reflection. Because the device delivers multiple mutually isolated frequency channels from a single low-frequency source, it is well suited to applications requiring multiplexed delivery of tones to spatially distributed, frequency-addressable loads; we discuss multiplexed control of superconducting qubits as one such potential application area. The proposed architecture offers a compact, integrated route toward on-chip frequency multiplexing and directional isolation for cryogenic microwave and millimeter-wave systems.

quant-ph↗

arXiv2026

Photon-Blockade Analogue Nonreciprocal Absorption in Spatiotemporal Metasurfaces

Controlling the flow of electromagnetic energy is essential for advancing quantum technologies. We introduce a spatiotemporally modulated superconducting metasurface that exhibits photon-blockade-analogue nonreciprocal absorption. In this system, the frequency of incident radiation is matched to the modulation frequency of the metasurface, enabling one-way directional absorption. Forward-traveling waves undergo resonant coupling to higher-order Floquet harmonics and are absorbed within the slab, while backward-traveling waves transmit freely without interaction. This behavior arises from classical wave interference and harmonic conversion in a space-time periodic medium---a classical analogue of quantum photon blockade. We present a design based on a superconductor-semiconductor metasurface incorporating cascaded Josephson field-effect transistors (JoFETs) for millikelvin-temperature operation. Starting from the microscopic Hamiltonian of a single gate-tunable JoFET cell, we derive the system's classical circuit relations, effective space-time-periodic permeability, Floquet band structure, and isofrequency diagrams from first principles, and validate the resulting nonreciprocal absorption with full-wave simulations. These findings establish a pathway toward compact, nonreciprocal superconducting devices for quantum information processing and microwave photonics.

quant-ph↗

arXiv2026

The Bragg Frequency Convertor: A Meeting Between Spatial and Temporal Periodicities For Selective Parametric Frequency Translation

This study introduces the Bragg Frequency Converter, a spatiotemporal-periodic grating concept that extends conventional Bragg gratings into the dynamic domain for pure parametric frequency conversion. By selectively time-modulating either the high-index or low-index layers of a quarter-wave stack, the structure achieves directional frequency conversion: high-index modulation yields efficient down-conversion, while low-index modulation produces up-conversion. This layer selectivity stems from the asymmetric Bloch mode distribution and associated phase-matching conditions. One practical realization, based on a silicon rib waveguide with periodic sidewall corrugations and selective doping of the high-index segments, is presented and analyzed. A coupled-mode theory is developed to explain the mechanism and validated through full-wave simulations. An experimental setup using optical pumping is also proposed for practical implementation. The theoretical and numerical results establish temporal Bragg gratings as a versatile, reconfigurable platform for spurious-free frequency conversion with applications in optical signal processing and integrated photonics.

physics.optics↗

arXiv2026

Temporal Bragg Gratings: Broadband Reconfigurable Parametric Amplifiers

This paper introduces temporal Bragg gratings as a new class of broadband, reconfigurable parametric amplifiers. We present a comprehensive investigation of power amplification in temporal Bragg gratings, spatially periodic structures with refractive index modulated near the Bragg frequency. Through systematic numerical simulations, we explore the effects of modulation location (high-index vs. low-index layers), frequency, and amplitude on gain spectra and field dynamics. Both layer types yield significant parametric amplification, with high-index modulation providing higher gain for comparable depths. Amplification is frequency-agile, with gain peaks tunable across a broad range, and exhibits strong asymmetry: the sub-Bragg regime requires substantially stronger modulation than supra-Bragg for comparable gain. In the extreme sub-Bragg limit, the system transitions from discrete sidebands to a broadband gain continuum via multi-phase-matching. These results establish a unified framework for designing reconfigurable optical amplifiers, tunable frequency converters, and broadband light sources using temporally modulated photonic crystals.

physics.optics↗

arXiv2026

Frequency Conversion Characteristics of Spatiotemporal Josephson Metasurfaces for Quantum Applications

This presentation explores the various characteristics of a nonreciprocal, frequency-converting Josephson metasurface operating at millikelvin temperatures. Leveraging the unique properties of Josephson junctions, which support supercurrent flow without resistance, this metasurface enables efficient manipulation of nonlinear wave interactions, facilitating both frequency conversion and amplification of incident photons.

physics.optics↗

arXiv2025

Designing Space-Time Metamaterials: The Central Role of Dispersion Engineering

Space-time metamaterials are redefining wave engineering by enabling fully dynamic four-dimensional control of electromagnetic fields, allowing simultaneous manipulation of frequency, amplitude, momentum, and propagation direction. This unified functionality moves well beyond reciprocity-breaking mechanisms, marking a fundamental transition from static media to polychromatic, energy-efficient wave processors. This article establishes dispersion engineering as the core design paradigm for these dynamic systems. We show that the dispersion relation, linking frequency and wavenumber, serves as a master blueprint governing exotic wave phenomena such as nonreciprocity, beam splitting, asymmetric frequency conversion, amplification, spatial decomposition, and momentum bandgaps. By analyzing analytical dispersion surfaces and isofrequency contours in subluminal, luminal, and superluminal modulation regimes, we reveal how tailored spatiotemporal modulation orchestrates controlled energy flow among harmonic modes. We further demonstrate how this framework directly informs practical device operation, highlighting advanced implementations including angular-frequency beam multiplexing in superconducting Josephson junction arrays. Combining insights from wave theory, numerical modeling, and experimental realization, this work provides a comprehensive roadmap for leveraging dispersion engineering to design next-generation metamaterials for wireless communication, quantum technologies, and integrated photonics.

physics.optics↗

arXiv2025

Space-Time-Coupled Qubits for Enhanced Superconducting Quantum Computing

The pursuit of scalable and robust quantum computing necessitates innovative approaches to overcome the inherent challenges of qubit connectivity, decoherence, and susceptibility to noise and crosstalk. Conventional monochromatic qubit coupling architectures, constrained by nearest-neighbor interactions and limited algorithmic flexibility, exacerbate these issues, hindering the realization of practical large-scale quantum processors. In this work, we introduce a paradigm leveraging a space-time-modulated cryogenic-compatible Josephson metasurface to enable polychromatic qubit coupling. This metasurface facilitates frequency-selective interactions, transforming nearest-neighbor connectivity into all-to-all qubit interactions, while significantly enhancing coherence, noise robustness, and entanglement fidelity. Our proposed approach capitalizes on the unique capabilities of space-time-modulated Josephson metasurfaces, including dynamic four-dimensional wave manipulation, nonreciprocal state transmission, and state-frequency conversion, to mediate multi-frequency qubit interactions. By isolating qubit couplings into distinct spectral channels, the cryogenic-compatible metasurface mitigates crosstalk and environmental decoherence, extending coherence times and preserving quantum state fidelity. Full-wave simulations and quantum performance analyses demonstrate a significant enhancement in the operational efficiency of a superconducting qubit array, showcasing improved connectivity, robustness, and entanglement stability. This study establishes the potential of space-time-modulated cryogenic-compatible Josephson metasurfaces as a transformative platform for next-generation quantum computing, addressing critical bottlenecks and paving the way for scalable, high-performance quantum processors.

quant-ph↗

arXiv2025

Light Interaction With a Space-Time-Modulated Josephson Junction Array and Application to Angular-Frequency Beam Multiplexing

Josephson junctions, as pivotal components of modern technologies such as superconducting quantum computing, owe their prominence to their unique nonlinear properties at low temperatures. Despite their extensive use in static configurations, the study of dynamic Josephson junctions, particularly under space-time modulation, remains largely unexplored. This study investigates the interaction and transmission of electromagnetic waves through arrays of space-time-modulated Josephson junctions. A comprehensive mathematical framework is presented to model the propagation of electric and magnetic fields within and beyond these structures. We demonstrate how such dynamic arrays enable groundbreaking four-dimensional light manipulation, achieving angular-frequency beam multiplexing through a seamless integration of frequency conversion and beam-splitting functionalities. These advancements open new horizons for electromagnetic field engineering, with far-reaching implications for superconducting quantum technologies, next-generation wireless communications, biomedical sensing, and radar systems.

cond-mat.supr-con↗

arXiv2024

Finite-Difference Time-Domain Simulation of Wave Transmission Through Space-Time-Varying Media

A comprehensive study on the Finite Difference Time Domain (FDTD) numerical modelling of space- and time-varying media is presented. We investigate the dynamic behavior of oblique incidence of both TM and TE electromagnetic fields on space-timemodulated gratings. In their general form, these media have electrical permittivity, magnetic permeability, and electrical conductivity modulated across both space and time. We provide the FDTDschemes, the Courant-Friedrichs-Lewy (CFL) stability condition and equations for both TM and TE wave illuminations of space-time-varying slabs, along with the boundary conditions for the FDTD scheme. Furthermore, with engaging illustrative examples, we showcase the versatility and applications of space-time-varying media.

physics.optics↗

arXiv2024

Enhancement of the Environmental Stability of Perovskite Thin Films via PMMA and AZ5214-Photoresist Coatings

We introduce a pioneering strategy to enhance the environmental stability of perovskite thin films, a critical step forward in advancing their application in optoelectronics. Through the innovative application of matrix encapsulation techniques, we focus on the stabilization of methylammonium lead iodide (MAPbI3) and methylammonium lead bromide (MAPbBr3) films. These films are meticulously prepared via a two-step solution deposition method under controlled ambient conditions. Our approach involves the application of poly(methyl methacrylate) (PMMA) and AZ5214 photoresist layers through spin-coating, aimed at singularly encapsulating the perovskite films. This encapsulation acts as a robust hydrophobic barrier, significantly mitigating moisture ingress and simultaneously addressing the challenge of pinhole presence within the perovskite structure. Through a series of detailed characterizations-spanning scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy-we demonstrate that, despite the thicker nature of the AZ5214 photoresist compared to the PMMA layer, it exhibits markedly enhanced stability. Notably, the integrity and optical properties of the perovskite films are preserved for extended periods of up to 960 hours under environmental exposure. This breakthrough highlights the superior performance of AZ5214 photoresist over PMMA in prolonging the operational life of perovskite thin films, thereby offering a promising avenue for their deployment in a wide range of optoelectronic devices.

physics.app-ph↗

arXiv2022

Magnet-Free Nonreciprocal Phase-Shifter Based on Time Modulation

Recently, nonreciprocal phase shifters have attracted a surge of interest thanks to the advent of nonreciprocal electromagnetic systems, such as nonreciprocal metasurfaces, nonreciprocal-beam antennas, and invisibility cloaks. To overcome the limitations associated with conventional technologies for realizing nonreciprocal phase shifters and gyrators, here we propose a low-noise, lightweight, low-profile, and linear magnetless nonreciprocal phase shifter formed by two temporal loops. The proposed temporal apparatus operates based on the generation of time-harmonic signals and destructive and constructive interferences for the undesired and desired time harmonics, respectively, at different locations of the structure. An external time-harmonic modulation signal injects an effective electronic angular momentum to the system to control the phase and frequency of the two loops. Such a temporal nonreciprocal phase shifter offers low insertion loss, and a large return loss (input matching) of greater than 28.1 dB. Additionally, this nonreciprocal phase shifter possesses a reconfigurable architecture and can be directly embedded in integrated circuit (IC) technology to create high power handling and linear IC-based nonreciprocal phase shifters.

physics.app-ph↗

arXiv2021

Full-duplex Reflective Beamsteering Metasurface Featuring Magnetless Nonreciprocal Amplification

Nonreciprocal radiation refers to electromagnetic wave radiation in which a structure provides different responses under the change of the direction of the incident field. Modern wireless telecommunication systems demand versatile apparatuses which are capable of full-duplex nonreciprocal wave processing and amplification, especially in the reflective state. To realize such a unique, extraordinary and versatile functionality, we propose an architecture in which a chain of series cascaded radiating patches are integrated with nonreciprocal phase shifters, providing an original and efficient apparatus for full-duplex reflective beamsteering. Such an ultrathin reflective metasurface can provide directive and diverse radiation beams, large wave amplification, steerable beams by simply changing the bias of the gradient active nonmagnetic nonreciprocal phase shifters, and is immune to undesired time harmonics. Having accomplished all these functionalities in the reflective state, the metasurface represents a conspicuous apparatus for efficient, controllable and programmable wave engineering.

physics.app-ph↗

arXiv2021

Lightweight Low-Noise Linear Isolator Integrating Phase-Engineered Temporal Loops

The quest for efficient and versatile microwave and optical isolators has recently spawned several novel space-time-modulated isolator structures. However, such space-time isolators suffer from a large profile and complex architecture caused by progressive nonreciprocal space-time coupling properties. To overcome these limitations, we propose a nonmagnetic phase-engineered temporal-loop-based isolator featuring large isolation levels, weak undesired time harmonics, and a low profile. The proposed isolator is composed of two temporal loops that provide desired constructive and destructive interferences of different time harmonics. Furthermore, these two loops are designed in a way to assure that the circulation and reflection of different time harmonics strengthen a low insertion loss unidirectional signal transmission. An experimental demonstration of the proposed time-modulated isolator at microwave frequencies is provided, featuring strong unidirectional wave transmission through the isolator with more than 27 dB contrast between the forward and backward waves across a fractional bandwidth of 14.3%. The proposed isolator outperforms the nonlinear-based and transistor-based isolators by featuring a highly linear response with OP1dB of higher than 31 dBm, high power rating of more than 47 dBm, and a low noise figure of 3.4 dB.

physics.app-ph↗

arXiv2021

Highly Linear Nonmagnetic Circulator Enabled By A Temporal Nonreciprocal Phase Shifter

Conventional circulators are made of magnetic ferrites and suffer from a cumbersome architecture, incompatibility with integrated circuit technology and inability for high frequency applications. To overcome these limitations, here we propose a lightweight low-profile non-magnetic circulator comprising a nonreciprocal time-varying phase shifter. This circulator is composing a nonreciprocal temporal phase shifter and two reciprocal delay-line-based phase shifters. The proposed nonreciprocal temporal phase shifter is based on the generation of time-harmonic signals, enforcing destructive interference for undesired time harmonics and constructive interference for desired time harmonics at different locations of the structure. Such a unique task is accomplished through two phase-engineered temporal loops. The phase and frequency of these two loops are governed by external signals with different phases, imparting an effective electronic angular momentum to the system. We observe large isolation level of greater than 32 dB, a P1dB of +31.7 dBm and IIP3 of +42.4 dBm. Furthermore, this circulator is endowed with a reconfigurable architecture and can be directly embedded in a conventional integrated circuit (IC) technology to realize a class of high power handling and linear IC circulators.

physics.app-ph↗

arXiv2021

Intelligent-Metasurface-Assisted Full-Duplex Wireless Communications

The limited radio-frequency spectrum is a fundamental factor in the design of wireless communication systems. The ever increasing number of wireless devices and systems has led to a crowded spectrum and increased the demand for versatile and multi-functional full-duplex wireless apparatuses. Recently, dynamic and intelligent metasurfaces are explored as a prominent technological solution to the current paradigm of spectrum scarcity by opportunistically sharing the spectrum with various users. In general, intelligent metasurfaces are dynamic, ultra-compact, multi-functional and programmable structures which are capable of both reciprocal and nonreciprocal signal wave transmissions in a full-duplex manner. The controllability and programmability of such metasurfaces are governed through the dc bias and occasionally a radio-frequency (RF) modulation applied to the the active components of the unit cells of the metasurface, e.g., diodes and transistors. This article shows that such intelligent metasurfaces can enhance the performance of wireless communications systems thanks to their unique features such as real-time signal coding, nonreciprocal-beam radiation, nonreciprocal beamsteering amplification, and advanced pattern-coding multiple access communication.

eess.SP↗

arXiv2021

Nonreciprocal-Beam Phased-Array Antennas

This study presents a nonreciprocal-beam phased-array antenna constituted of phase-gradient patch radiators integrated with transistor-based nonreciprocal phase shifters. Such an antenna exhibits different beams for transmission and reception states. The proposed phased-array antenna provides power amplification for both transmission and reception states, which is of paramount importance in most practical applications. In addition, in contrast to the recently proposed time-modulated antennas, the proposed nonreciprocal-beam phased-array antenna introduces no undesired time harmonics and unwanted frequency conversion, requires no radio frequency bias signal. Furthermore, the nonreciprocal phased-array antenna is lightweight and is amenable to integrated circuit fabrication. The transmission and reception beam angles, the beam shapes, and the power amplification level may be easily tuned by changing the direct current (dc) bias of the transistors and phase of the passive phase shifters. Such a nonreciprocal-beam phased-array antenna is expected to find military and commercial applications.

physics.app-ph↗

arXiv2021

Pure and Linear Frequency Converter Temporal Metasurface

Metasurfaces are ultrathin structures which are constituted by an array of subwavelength scatterers with designable scattering responses. They have opened up unprecedented exciting opportunities for extraordinary wave engineering processes. On the other hand, frequency converters have drawn wide attention due to their vital applications in telecommunication systems, health care devices, radio astronomy, military radars and biological sensing systems. Here, we show that a spurious-free and linear frequency converter metasurface can be realized by leveraging unique properties of engineered transmissive temporal supercells. Such a metasurface is formed by time-modulated supercells; themselves are composed of temporal and static patch resonators and phase shifters. This represents the first frequency converter metasurface possessing large frequency conversion ratio with controllable frequency bands and transmission magnitude. In contrast to conventional nonlinear mixers, the proposed temporal frequency converter offers a linear response. In addition, by taking advantage of the proposed surface-interconnector-phaser-surface (SIPS) architecture, a spurious-free and linear frequency conversion is achievable, where all undesired mixing products are strongly suppressed. The proposed metasurface may be digitally controlled and programmed through a field programmable gate array. This makes the spurious-free and linear frequency converter metasurface a prominent solution for wireless and satellite telecommunication systems, as well as invisibility cloaks and radars. This study opens a way to realize more complicated and enhanced-efficiency spectrum-changing metasurface.

physics.app-ph↗

arXiv2021

Programmable Nonreciprocal Metaprism

Optical prisms are made of glass and map temporal frequencies into spatial frequencies by decomposing incident white light into its constituent colors and refract them into different directions. Conventional prisms suffer from their volumetric bulky and heavy structure and their material parameters are dictated by the Lorentz reciprocity theorem. Considering various applications of prisms in wave engineering and their growing applications in the invisible spectrum and antenna applications, there is a demand for compact apparatuses that are capable of providing prism functionality in a reconfigurable manner, with a nonreciprocal/reciprocal response. Here, we propose a nonreciprocal metasurface-based prism constituted of an array of phase- and amplitude-gradient frequency-dependent spatially variant radiating super-cells. In conventional optical prisms, nonreciprocal devices and metamaterials, the spatial decomposition and nonreciprocity functions are fixed and noneditable. Here, we present a programmable metasurface integrated with amplifiers to realize controllable nonreciprocal spatial decomposition, where each frequency component of the incident polychromatic wave can be transmitted under an arbitrary and programmable angle of transmission with a desired transmission gain. Such a polychromatic metasurface prism is constituted of frequency-dependent spatially variant transistor-based phase shifters and amplifiers for the spatial decomposition of the wave components. Interesting features include three-dimensional prism functionality with programmable angles of refraction, power amplification, and directive and diverse radiation beams. Furthermore, the metasurface prism can be digitally controlled via a field-programmable gate array (FPGA), making the metasurface a suitable solution for radars, holography applications, and wireless telecommunication systems.

physics.app-ph↗