arXiv subjects

S. Ali Hassani Gangaraj

Publications and source records attributed to S. Ali Hassani Gangaraj.

At least 19 recordsLinked to original sources

arXiv2026

Surface Plasmon Polaritons: Creation Dynamics and Interference of Slow and Fast Propagating SPPs at a Temporal Boundary

We establish the theoretical framework for a material system that supports surface plasmon polaritions (SPPs) excited by a dipole excitation, where the media configuration suddenly changes at a temporal boundary. We employ three-dimensional Green's function analysis in the Laplace transform domain. We use this framework to demonstrate dynamic SPP formation and time-boundary-induced interference of slow and fast propagating SPPs. This analysis provides insight into how SPPs are formed in time and how they interfere at a temporal boundary.

physics.optics↗

arXiv2025

Entanglement mediated by DC current induced nonreciprocal graphene plasmonics

We investigate entanglement mediated by DC current induced nonreciprocal graphene plasmon polaritons. Nonreciprocal systems are ideal for the enhancement, control, and preservation of entanglement due to the potential for unidirectional beam-like wave propagation, i.e., efficiently transporting photons from one emitter to another. Using a quantum master equation and three-dimensional Green's function analysis, we investigate a system consisting of two two-level emitters dominantly interacting via electric current induced nonreciprocal plasmonic modes of a graphene waveguide. We use concurrence as a measure of entanglement. We show that nonreciprocal graphene plasmon polaritons are a promising candidate to generate and mediate concurrence, where it is shown that there is good enhancement and control of entanglement over vacuum, which is beneficial for the broad applications of entanglement as a quantum resource. We believe our findings contribute to the development of quantum devices, enabling efficient and tunable entanglement between two-level systems, which is a central goal in quantum technologies.

quant-ph↗

arXiv2025

Roadmap on Nonlocality in Photonic Materials and Metamaterials

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally made in optical material modeling. On one end, the growing interest in plasmonic, polaritonic and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials--metamaterials and metasurfaces--can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In the case of metasurfaces, in particular, nonlocality engineering has become a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward--toward new scientific discoveries and technological advancements.

cond-mat.mes-hall↗

arXiv2025

Simulating quantum emitters in arbitrary photonic environments using FDTD: beyond the semi-classical regime

We propose a numerical algorithm that integrates quantum two-level systems (TLSs) into the finite-difference time-domain (FDTD) framework for simulating quantum emitters in arbitrary 3D photonic environments. Conventional methods struggle with these systems due to their semi-classical nature and spurious self-interactions that arise when a TLS is driven by its own radiation field. We address these issues by determining the correct electric field for driving the TLS, as well as the current source used in FDTD for modeling photon emission. Our method, focusing on single-excitation states, employs a total field-incident field (TF-IF) technique to eliminate self-interactions, enabling precise simulations of photon emission and scattering. The algorithm also successfully models complex phenomena such as resonant energy transfer, superradiance, and vacuum Rabi splitting. This powerful computational tool is expected to substantially advance research in nanophotonics, quantum physics, and beyond.

quant-ph↗

arXiv2024

Quantum Photonic Gates with Two-Dimensional Random Walkers

Quantum gates are crucial for processing quantum information, but implementing them in a photonic platform poses unique challenges due to the peculiar way photons propagate and interfere. Here, we examine quantum photonic gates that utilize continuous time two-dimensional random walking photons. These gates can be implemented using the inverse design method, where photons randomly walk in a two-dimensional silicon host medium embedded with silicon dioxide scatterers. We propose a C-NOT gate as a multiqubit gate and an X-gate as a single qubit gate. In addition, we provide studying the non-trivial spatial correlations of random walking photons by utilizing the quantum correlation function. The results demonstrate high-fidelity probabilistic quantum gates. Further work is required to address error-correction. This work advances the practical implementation photonic elements in linear optics quantum computation schemes.

quant-ph↗

arXiv2024

Dynamical Casimir Effects: The Need for Nonlocality in Time-Varying Dispersive Nanophotonics

Both real and virtual photons can be involved in light-matter interactions. A famous example of the observable implications of virtual photons -- vacuum fluctuations of the quantum electromagnetic field -- is the Casimir effect. Since quantum vacuum effects are weak, various mechanisms have been proposed to enhance and engineer them, ranging from static, e.g., strong optical resonances, to dynamic, e.g., systems with moving boundaries or time-varying optical properties, or a combination of them. In this Letter, we discuss the role of material nonlocality (spatial dispersion) in dynamical Casimir effects in time-varying frequency-dispersive nanophotonic systems. We first show that local models may lead to nonphysical predictions, such as diverging emission rates of entangled polariton pairs. We then theoretically demonstrate that nonlocality regularizes this behavior by correcting the asymptotic response of the system for large wavevectors and reveals physical effects missed by local models, including a significant broadening of the emission rate distribution, which are relevant for future experimental observations. Our work sheds light on the importance of nonlocal effects in this new frontier of nanophotonics.

quant-ph↗

arXiv2023

Enhanced nonlinear optical effects in drift-biased nonreciprocal graphene plasmonics

Nonlinear light-matter interactions are typically enhanced by increasing the local field and its interaction time with matter. Conventional methods to achieve these goals are based on resonances or slow-light effects. However, these methods suffer from various issues, including narrow operational bandwidths, large footprints, and material absorption. An interesting alternative approach to enhance the local field is offered by nonreciprocal systems: by blocking the path of a unidirectional wave in a terminated nonreciprocal waveguiding structure, broadband electromagnetic fields can be drastically enhanced and localized near the termination. This approach was previously studied only in three-dimensional gyrotropic material platforms where the need for external magnets and bulky materials make it less practical. Here, instead, we employ a magnet-free mechanism to break reciprocity in 2D plasmonic materials, e.g., graphene. Specifically, we employ high-speed drifting electrons on a voltage-biased graphene sheet to lift the forward/backward degeneracy of the surface plasmon-polariton dispersion, creating modes with different propagation properties parallel and antiparallel to the current. We show that controllable, asymmetric, and intense field hot-spots are generated at the edges of a suitably terminated graphene metasurface. We then theoretically demonstrate that such asymmetric field hot-spots offer an effective solution to enhance third-order nonlinear optical effects. As an example, we predict that, using realistic values of drift velocity, high third-harmonic conversion efficiencies of up to 0.3 percent are achievable around the plasmon resonance frequencies.

physics.optics↗

arXiv2022

Enhancement of Quantum Excitation Transport by Photonic Nonreciprocity

Enhanced interaction between two two-level emitters (e.g., atoms) by nonreciprocal photonic media can be of benefit to broad areas, from quantum information science to biological detection. Here we provide a detailed analysis on why nonreciprocal photon-mediated interaction enhances inter-atomic excitation transport efficiency. We investigate a system consisting of two two-level emitters embedded in a generic photonic environment. By comparing symmetric and asymmetric photon-exchange, we analytically show that breaking electromagnetic reciprocity makes it possible for the cooperative decay rate to exceed the spontaneous decay rate even in a translation-invariant homogeneous system. This means that the excitation of an emitter must decay mostly into the other emitter rather than leaking and dissipating into the reservoir photonic modes. We also provide an example where a chain of two-level emitters dominantly interact via the reciprocal modes of a plasmonic waveguide. We then show that breaking reciprocity in such a system via driving a DC current through the plasmonic material can drastically increase the probability of photon emission from one emitter to another, leading to an order-of-magnitude enhancement in quantum energy-transport efficiency.

quant-ph↗

arXiv2022

Chern Invariants of Topological Continua; a Self-Consistent Nonlocal Hydrodynamic Model

The bulk-edge correspondence is a fundamental principle of topological wave physics, which states that the difference in gap Chern numbers between the interfaced materials is equal to the net number of topological edge modes. Although this principle works well for periodic photonic topological insulators, difficulties arise in the case of continuous systems with no intrinsic periodicity, due to the absence of a finite Brillouin zone, which may lead to an ill-behaved response for diverging wavenumbers. This problem has been solved previously by introducing an ad hoc material model including a spatial cutoff wavenumber. However, this method introduces other difficulties, such as an unphysical response at large wavenumbers, and the need to interpolate the interfaced materials permittivity functions. In this work, we show that the inclusion of realistic spatial dispersion (e.g., hydrodynamic nonlocality) formally restores the bulk-edge correspondence by avoiding an unphysical response at large wavenumbers, forming a complete, self-consistent model. This model clarifies the subtle and rich topological wave physics of continuous media.

physics.optics↗

arXiv2021

Drifting Electrons: Nonreciprocal Plasmonics and Thermal Photonics

Light propagates symmetrically in opposite directions in most materials and structures. This fact -- a consequence of the Lorentz reciprocity principle -- has tremendous implications for science and technology across the electromagnetic spectrum. Here, we investigate an emerging approach to break reciprocity that does not rely on magneto-optical effects or spacetime modulations, but is instead based on biasing a plasmonic material with a direct electric current. Using a 3D Green function formalism and microscopic considerations, we elucidate the propagation properties of surface plasmon-polaritons (SPPs) supported by a generic nonreciprocal platform of this type, revealing some previously overlooked, anomalous, wave-propagation effects. We show that SPPs can propagate in the form of steerable, slow-light, unidirectional beams associated with inflexion points in the modal dispersion. We also clarify the impact of dissipation (due to collisions and Landau damping) on nonreciprocal effects and shed light on the connections between inflexion points, exceptional points at band-edges, and complex modal transitions in leaky-wave structures. We then apply these concepts to the important area of thermal photonics, and provide the first theoretical demonstration of drift-induced nonreciprocal near-field radiative heat transfer between two planar bodies. Our findings may open new opportunities toward the development of nonreciprocal magnet-free devices that combine the benefits of plasmonics and nonreciprocal photonics for wave-guiding and energy applications.

physics.optics↗

arXiv2020

Broadband field enhancement and giant nonlinear effects in terminated unidirectional plasmonic waveguides

Unidirectional wave propagation in nonreciprocal structures enables exciting opportunities to control and enhance wave-matter interactions in extreme ways. Within this context, here we investigate the possibility of using terminated unidirectional plasmonic waveguides to enhance typically weak nonlinear effects by orders of magnitude. We theoretically demonstrate remarkable levels of electric field enhancement and confinement (field hot-spots) when the unidirectional waveguiding structure is terminated with a suitable boundary that fully stops the one-way mode. Such a large field enhancement, originating from a nonresonant effect, is fundamentally different from the narrow-band field concentration effects in resonant plasmonic structures. Instead, it is analogous to the broadband response of plasmonic tapers, but without the need for any adiabatic impedance matching. We show that this effect can indeed lead to a substantial boosting of nonlinear light-matter interactions, exemplified by an improvement of several orders of magnitude in the third-harmonic-generation efficiency, which is of large significance for several applications. More broadly, our findings show the potential of extreme nonreciprocal configurations for enhanced wave-matter interactions.

physics.optics↗

arXiv2020

Exchange splitting and exchange-induced non-reciprocal photonic behavior of graphene in CrI3-graphene vdW heterostructures

The exchange splitting and resulting electromagnetic response of graphene in a monolayer chromium triiodide(CrI3) graphene van der Waals(vdW) heterostructure are considered using a combination of density functional theory and electromagnetic calculations. Although the effective exchange fields are in the hundreds of Tesla, for the equilibrium separation non reciprocal effects are found to be weak compared to those for a comparable external magnetic bias. For non equilibrium separations, nonreciprocal effects can be substantial.

cond-mat.mes-hall↗

arXiv2019

Physical Violations of the Bulk-Edge Correspondence in Topological Electromagnetics

In this Letter, we discuss two general classes of apparent violations of the bulk-edge correspondence principle for uniform topological photonic materials, associated with the asymptotic behavior of the surface modes for diverging wavenumbers. Considering a nonreciprocal plasma as a model system, we show that the inclusion of spatial dispersion (e.g., hydrodynamic nonlocality) formally restores the bulk-edge correspondence by avoiding an unphysical response at large wavenumbers. Most importantly, however, our findings show that, for the considered cases, the correspondence principle is physically violated for all practical purposes, as a result of the unavoidable attenuation of highly confined modes even if all materials are assumed perfect, with zero intrinsic bulk losses, due to confinement-induced Landau damping or nonlocality-induced radiation leakage. Our work helps clarifying the subtle and rich topological wave physics of continuous media.

physics.optics↗

arXiv2019

Do Truly Unidirectional Surface Plasmon-Polaritons Exist?

In this work, we revisit the topic of surface waves on nonreciprocal plasmonic structures, and clarify whether strictly unidirectional surface plasmon-polaritons are allowed to exist in this material platform. By investigating different three-dimensional configurations and frequency regimes, we theoretically show that, while conventional surface magneto-plasmons are not strictly unidirectional due to nonlocal effects, consistent with recent predictions made in the literature, another important class of one-way surface plasmon-polaritons, existing at an interface with an opaque isotropic material, robustly preserve their unidirectionality even in the presence of nonlocality, and for arbitrarily-small levels of dissipation. We also investigate the extreme behavior of terminated unidirectional wave-guiding structures, for both classes of surface waves, and discuss their counter-intuitive implications.

physics.optics↗

arXiv2019

Unidirectional and diffractionless surface plasmon-polaritons on three-dimensional nonreciprocal plasmonic platforms

Light-matter interactions in conventional nanophotonic structures typically lack directionality. Furthermore, surface waves supported by conventional material substrates do not usually have a preferential direction of propagation, and their wavefront tends to spread as it propagates along the surface, unless the surface or the excitation are properly engineered and structured. In this article, we theoretically demonstrate the possibility of realizing \emph{unidirectional and diffractionless surface-plasmon-polariton modes} on a nonreciprocal platform, namely, a gyrotropic magnetized plasma. Based on a rigorous Green function approach, we provide a comprehensive and systematic analysis of all the available physical mechanisms that may bestow the system with directionality, both in the sense of one-way excitation of surface waves, and in the sense of directive diffractionless propagation along the surface. The considered mechanisms include (i) the effect of strong and weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like topology of the modal dispersion surfaces, and (iii) the source polarization state, with the associated possibility of chiral surface-wave excitation governed by angular-momentum matching. We find that three-dimensional gyrotropic plasmonic platforms support a previously-unnoticed wave-propagation regime that exhibit several of these physical mechanisms simultaneously, allowing us to theoretically demonstrate, for the first time, unidirectional surface-plasmon-polariton modes that propagate as a single ultra-narrow diffractionless beam. We also assess the impact of dissipation and nonlocal effects. Our theoretical findings may enable a new generation of plasmonic structures and devices with highly directional response.

physics.optics↗

arXiv2019

Non-Markovian Transient Casimir-Polder force and population dynamics on excited and ground state atoms: weak and strong coupling regimes in generally non-reciprocal environments

The transient Casimir-Polder force on a two-level atom introduced into a three-dimensional, inhomogeneous, generally non-reciprocal environment is evaluated using non-Markovian Weisskopf-Wigner theory in the strong and weak coupling regimes. Ground-state and excited atoms are considered as two separate initial-value problems, and both the short-time and long-time atomic population and force are evaluated. The results are compared with various Markov approximation of the Weisskopf-Wigner theory, and with previous Markov results from the Heisenberg picture.

quant-ph↗

arXiv2019

Robust surface plasmon polaritons on gyrotropic interfaces

Unidirectional surface plasmon polaritons (SPPs) at the interface between a gyrotropic medium and a simple medium are studied in a newly-recognized frequency regime wherein the SPPs form narrow, beam-like patterns due to hyperbolic dispersion. The SPP beams are steerable by controlling parameters such as the cyclotron frequency (external bias) or the frequency of operation. The bulk band structure along different propagation directions is examined to ascertain a common bandgap, valid for all propagation directions, which the SPPs cross. The case of a finite-thickness gyrotropic slab is also considered, for which we present the Green function and examine the thickness and loss level required to maintain a unidirectional SPP.

physics.optics↗

arXiv2018

Optical torque on a two-level system near a strongly nonreciprocal medium

We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and non-resonant part, associated respectively with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green function. Our formulation is valid for any inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. We apply this general theory to a scenario in which the atom interacts with a material characterized by strong nonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is represented by the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevant physical insight into the role of these unidirectional surface waves in the emergence of non-trivial optical torque, we derive closed-form expressions for the induced torque under the quasi-static approximation. Finally, we investigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to align due to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydberg atoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may find broad application in the context of nano-mechanical and bio-mechanical systems.

physics.optics↗