arXiv subjects

M. S. Mirmoosa

Publications and source records attributed to M. S. Mirmoosa.

At least 19 recordsLinked to original sources

arXiv2026

Photonic time crystals assisted by quasi-bound states in the continuum

Photonic time crystals (PTCs) are characterized by the rapid modulation of the material properties in time, causing a momentum bandgap for light. However, the observation of these bandgaps at optical frequencies remains elusive as the necessary temporal modulation amplitudes to show notable momentum bandgaps are relatively high, inaccessible with available materials. While it has been known that structuring PTCs at the subwavelength scale can improve the bandgap size, we push this concept to the extreme by leveraging the nanophotonic toolbox. Specifically, we demonstrate that structures composed of scatterers supporting quasi-bound states in the continuum can substantially reduce the required modulation amplitudes by enhancing the interaction time between light and time-varying matter. This allows us to observe noticeable momentum bandgaps despite the weak temporal modulation. Our approach bridges the concepts of bound states in the continuum and time-varying metamaterials, paving the way toward realizable PTCs at optical frequencies.

physics.optics↗

arXiv2026

Multi-mode Photonic Time Crystals Based on Time-Modulated Metasurface Waveguides

Photonic time crystals are electromagnetic media with periodically time-varying parameters, enabling momentum band gaps, parametric amplification, and frequency conversion beyond what is possible in time-invariant systems. So far, they have been explored mainly in single-mode systems, which limits the range of accessible physical phenomena. Here, we introduce an impenetrable metasurface waveguide as a multimode time-varying platform supporting both guided surface modes and higher-order guided volume modes. We show that temporal modulation in this platform gives rise not only to conventional intramodal band gaps associated with same-branch coupling, but also to tilted intermodal band gaps originating from coupling between different guided-mode branches. Unlike intramodal band gaps, these intermodal band gaps are not restricted to half the modulation frequency and can enable directional wave amplification, where the amplified field carries energy along the waveguide even inside the band gap. We further show that the modulation phase difference provides an effective symmetry-control parameter: by exploiting temporal glide symmetry, one can selectively suppress or enhance gap opening for interactions between modes of the same or different symmetry. These results establish a versatile multimode platform for photonic time crystals, offering one of the simplest and most experimentally accessible routes to tilted band gaps compared with volumetric dispersive PTC implementations and, more broadly, opening new opportunities for time-varying electromagnetic systems.

physics.optics↗

arXiv2026

Photonic Temporal Illusion

Materials with unusual optical properties are central to advanced control of light. Yet, in nature, such materials may be exceedingly rare and often difficult to obtain. To overcome this limitation, here we introduce the concept of temporal illusion: A temporally dynamic framework in which carefully programmed temporal variations in effective parameters generate responses akin to those of, in principle, any arbitrary time-invariant structure. We theoretically demonstrate that proper modulation of the permittivity of a conventional dielectric in space and time replicates the optical behavior associated with exotic materials. Besides, we reveal that, beyond steady-state effects, temporal illusion also enables control over transient responses, for instance, by effectively lowering the time constant of high-quality-factor resonators, therefore, allowing faster energy accumulation. Moreover, by incorporating detuning between modulation and excitation, we show that the framework unlocks additional functionalities. The temporal illusion paradigm thus broadens the capabilities of space-time varying systems, offering a powerful route to synthesize material responses on demand and paving the way for new theoretical and experimental directions in optics and wave physics.

physics.optics↗

arXiv2025

Broadband amplification of light through adiabatic spatiotemporal modulation

Four-dimensional optics leverages the simultaneous control of materials in space and time to manipulate light. A key challenge in experimentally realizing many intriguing phenomena is the need for rapid modulation, which is hindered by the inherently adiabatic relaxation of optical materials. Here, we theoretically demonstrate that broadband amplification can be achieved without the need for sub-cycle temporal responses, instead leveraging adiabatic spatiotemporal modulation patterns. The proposed modulation scheme is compatible with recent demonstrations of the temporal modulation of epsilon-near-zero materials. We also show that the same phenomenon may be realized by modulating bianisotropic nonreciprocal media in time. This broadband gain mechanism opens new avenues for the generation of high-energy, ultrashort optical pulses, with potential impact in ultrafast optics and electron microscopy.

physics.optics↗

arXiv2024

Quantum Theory of Wave Scattering from Electromagnetic Time Interfaces

Modulating macroscopic parameters of materials in time offers innovative avenues for manipulating electromagnetic waves. Due to such enticing prospects, the general research subject of time-varying systems is expanding today in different branches of electromagnetism and optics. However, compared with the research efforts and progresses that have taken place in the realm of classical electrodynamics, the quantum aspects of this emerging subject have been less explored. Here, through the lens of quantum optics, we study the scattering of electromagnetic fields from an isotropic and nondispersive material with a suddenly changing refractive index, creating a time interface. We revisit the transformation of the bosonic mode operators and corresponding quantum states due to this interface, governed by the two-mode squeeze operator. Building on this foundation, more importantly, our analysis focuses on the photon statistics and quantum state engineering of the scattered light, elucidating various quantum optical phenomena and opportunities arising from the time interface. Notably, these include photon-pair production and destruction, photon bunching and antibunching, vacuum generation, quantum state discrimination, and quantum state freezing. To bridge theory and experiment, we propose a possible circuit quantum electrodynamics approach for validating our theoretical predictions. We hope that our work inspires experimental investigations and further quantum optical explorations of electromagnetic field interaction with photonic time crystals or with dispersive time-varying materials.

physics.optics↗

arXiv2024

Photonic time crystals: Theory and applications

This tutorial offers a comprehensive overview of photonic time crystals - artificial materials whose electromagnetic properties are periodically modulated in time at scales comparable to the oscillation period of light while remaining spatially uniform. Being the temporal analogs to traditional photonic crystals, photonic time crystals differ in that they exhibit momentum bandgaps instead of energy bandgaps. The energy is not conserved within momentum bandgaps, and eigenmodes with exponentially growing amplitudes exist in the momentum bandgap. Such properties make photonic time crystals a fascinating novel class of artificial materials from a basic science and applied perspective. This tutorial overviews the fundamental electromagnetic equations governing photonic time crystals and explores the groundbreaking physical phenomena they support. Based on these properties, we also oversee a diverse range of applications they unlock. Different material platforms suitable for creating photonic time crystals are discussed and compared. Furthermore, we elaborate on the connections between wave amplification in photonic time crystals and parametric amplification mechanisms in electrical circuits and nonlinear optics. The tutorial will be helpful for readers with physics or engineering backgrounds. It is designed to serve as an introductory guide for beginners and to establish a reference baseline reflecting the current understanding for researchers in the field.

physics.optics↗

arXiv2024

Expanding momentum bandgaps in photonic time crystals through resonances

The realization of photonic time crystals is a major opportunity but also comes with significant challenges. The most pressing one, potentially, is the requirement for a substantial modulation strength in the material properties to create a noticeable momentum bandgap. Reaching that noticeable bandgap in optics is highly demanding with current, and possibly also future, material platforms since their modulation strength is small by tendency. Here, we demonstrate that by introducing temporal variations in a resonant material, the momentum bandgap can be drastically expanded with modulation strengths in reach with known low-loss materials and realistic laser pump powers. The resonance can emerge from an intrinsic material resonance or a suitably spatially structured material supporting a structural resonance. Our concept is validated for resonant bulk media and optical metasurfaces and paves the way toward the first experimental realizations of photonic time crystals.

physics.optics↗

arXiv2024

Time-Varying Wireless Power Transfer Systems for Improving Efficiency

Conventional wireless power transfer systems are linear and time-invariant, which sets fundamental limitations on their performance, including a tradeoff between transfer efficiency and the level of transferred power. In this paper, we introduce and study a possibility of temporal modulation for inductive wireless power transfer systems and uncover that this tradeoff is avoided as a consequence of varying the inductive coupling strength in time. Our theoretical analysis reveals that under the optimal modulation depth and phase, the time modulation can yield a substantial improvement in the WPT efficiency, while the received power at the load is also improved compared to the static WPT reference system. We experimentally demonstrate the concept with a low-frequency system and observe a threefold improvement in efficiency over the reference static counterpart. This technical capability reconciles the inherent tradeoff between the WPT efficiency and transferred power, paving the way for simultaneous advancements in both efficiency and delivered power.

physics.app-ph↗

arXiv2023

Time Interfaces in Bianisotropic Media

Wave phenomena in bianisotropic media have been broadly scrutinized in classical electrodynamics, as these media offer additional degrees of freedom to engineer electromagnetic waves. However, all investigations concerning such systems have so far been limited to stationary (time-invariant) media. Temporally varying the magnetoelectric coupling manifesting bianisotropy engenders a unique prospect to manipulate wave-matter interactions in new ways. In this paper, we theoretically contemplate electromagnetic effects in weakly dispersive bianisotropic media of all classes when the corresponding magnetoelectric coupling parameter suddenly jumps in time, creating a time interface in spatially uniform bianisotropic media. We investigate scattering effects at such time interfaces, revealing novel polarization- and direction-dependent phenomena. We anticipate that our work paves the road for further exploration of time-varying bianisotropic metamaterials (metasurfaces) and bianisotropic photonic time crystals, thus opening up interesting possibilities to control wave polarization and amplitude in reciprocal and nonreciprocal manners.

physics.optics↗

arXiv2023

Electromagnetic Effects in Anti-Hermitian Media with Gain and Loss

Incorporating both gain and loss into electromagnetic systems provides possibilities to engineer effects in unprecedented ways. Concerning electromagnetic effects in isotropic media that have concurrently electric and magnetic responses, there is in fact a degree of freedom to distribute the gain and loss in different effective material parameters. In this paper, we analytically scrutinize wave interactions with those media, and, most importantly, we contemplate the extreme scenario where such media are anti-Hermitian. Considering various conditions for excitation, polarization, and geometry, we uncover important effects and functionalities such as lasing into both surface waves and propagating waves, conversion of evanescent source fields to transmitted propagating waves, full absorption, and enhancing backward to forward scattering ratio. We hope that these findings explicitly show the potential of anti-Hermiticity to be used in optical physics as well as microwave engineering for creating and using unconventional wave phenomena.

physics.app-ph↗

arXiv2023

Coherently time-varying metasurfaces

Known coherent metasurfaces control interference of waves of a given frequency with other coherent waves at the same frequency, either illuminating from a different direction or created as intermodulation products. In this paper, we introduce a class of metasurfaces that are modulated in time coherently with the illuminating radiation. Importantly, such modulation opens a possibility to control reflection, absorption, and transmission at multiple frequencies, including illuminations by two or more incoherent waves. In particular, we study dynamic resistive layers and show how to use them to design thin multi-frequency perfect absorbers that overcome the bandwidth limit for static linear absorbers. Furthermore, we demonstrate possibilities of remote tuning of the absorption level. We hope that this work opens up novel avenues in wave engineering using coherent modulation of metasurface parameters.

physics.app-ph↗

arXiv2023

Spin-dependent phenomena at chiral temporal interfaces

Temporally varying electromagnetic media have been extensively investigated recently to unveil new means for controlling light. However, spin-dependent phenomena in such media have not been explored thoroughly. Here, we reveal the existence of spin-dependent phenomena at a temporal interface between chiral and dielectric media. In particular, we show theoretically and numerically that due to the material discontinuity in time, linearly polarized light is split into forward-propagating right-handed and left-handed circularly polarized waves having different angular frequencies and it the same phase velocities. This salient effect allows complete temporal separation of the two spin states of light with high efficiency. In addition, a phenomenon of spin-dependent gain/loss is observed. Furthermore, we show that when the dielectric medium is switched again to the original chiral medium, the right- and left-handed circularly polarized light waves (with different angular frequencies) merge to form a linearly polarized wave. Our findings extend spin-dependent interactions of light from space to space-time.

physics.optics↗

arXiv2023

Emulating Bianisotropic Coupling Through Coherent Illumination

One of the main advantages of reciprocal bianisotropic metasurfaces is their capability to produce asymmetric scattering depending from which side they are illuminated and on the handedness of circularly polarized illuminations. For most applications, these metasurfaces are designed for illumination by a single source at a time. The resulting bianisotropic metasurface has a specific and usually complex geometrical structure that ensures the expected scattering produced under various illuminations. Here we show that geometrical asymmetry of metasurfaces can be emulated by using non-bianisotropic layers in presence of coherent illumination, which allows us to replicate and optically control the desired asymmetric scattering and chirality effects. In particular, the concept is developed on an example of emulating asymmetric scattering needed to create a 180$^\circ$ hybrid junction for plane waves. We show that this device can be realized either using a bianisotropic metasurface or a set of simple sheets with electric response under simultaneous illumination by two coherent waves.

physics.app-ph↗

arXiv2022

Tutorial on basics of time-varying electromagnetic systems and circuits

During the last decade, possibilities to realize new phenomena and create new applications by varying system properties in time have gained increasing attention in many research fields, spanning a wide range from acoustics to optics. While the interest in using time-modulation techniques for engineering electromagnetic response has got revitalized only in recent years, the field originates from the middle of the previous century, and a multitude of works have been published ever since. In this tutorial paper, we provide a historical picture and review the basic concepts in this field. In particular, we introduce the general theory of linear time-varying systems and discuss the means to properly account for frequency dispersion of nonstationary systems. Also, we elucidate models of time-varying electrical circuits and some useful effects that can be achieved by time modulation of circuit parameters. We hope that this paper will particularly help inexperienced researchers who would like to work on new ways of manipulating waves and signals by temporally varying the properties of their systems.

physics.class-ph↗

arXiv2021

Scattering from spheres made of time-varying and dispersive materials

Exploring the interaction of light with time-varying media is an intellectual challenge that, in addition to fundamental aspects, provides a pathway to multiple promising applications. Time modulation constitutes here a fundamental handle to control light on entirely different grounds. That holds particularly for complex systems simultaneously structured in space and time. However, a realistic description of time-varying materials requires considering their material dispersion. The combination thereof has barely been considered but is crucial since dispersion accompanies materials suitable for dynamic modulation. As a canonical scattering problem from which many general insights can be obtained, we develop and apply a self-consistent analytical theory of light scattering by a sphere made from a time-varying material exemplarily assumed to have a Lorentzian dispersion. We discuss the eigensolutions of Maxwell's equations in the bulk and present a dedicated Mie theory. The proposed theory is verified with full-wave simulations. We disclose effects such as energy transfer from the time-modulation subsystem to the electromagnetic field, amplifying carefully structured incident fields. Since many phenomena can be studied on analytical grounds with our formalism, it will be indispensable when exploring electromagnetic phenomena in time-varying and spatially structured finite objects of other geometries.

physics.optics↗

arXiv2021

Dipole polarizability of time-varying particles

Invariance under time translation (or stationarity) is probably one of the most important assumptions made when investigating electromagnetic phenomena. Breaking this assumption is expected to open up novel possibilities and result in exceeding conventional limitations. However, to explore the field of time-varying electromagnetic structures, we primarily need to contemplate the fundamental principles and concepts from a nonstationarity perspective. Here, we revisit one of those key concepts: The polarizability of a small particle, assuming that its properties vary in time. We describe the creation of induced dipole moment by external fields in a nonstationary, causal way, and introduce a complex-valued function, called temporal complex polarizability, for elucidating a nonstationary Hertzian dipole under time-harmonic illumination. This approach can be extended to any subwavelength particle exhibiting electric response. In addition, we also study the classical model of the polarizability of an oscillating electron using the equation of motion whose damping coefficient and natural frequency are changing in time. Next, we theoretically derive the effective permittivity corresponding to time-varying media (comprising free or bound electrons, or dipolar meta-atoms) and explicitly show the differences with the conventional macroscopic Drude-Lorentz model. This paper will hopefully pave the road towards better understanding of nonstationary scattering from small particles and homogenization of time-varying materials, metamaterials, and metasurfaces.

physics.app-ph↗

arXiv2020

Coherent Retroreflective Metasurfaces

Inhomogeneous metasurfaces have shown possibilities for unprecedented control of wave propagation and scattering. While it is conventional to shine a single incident plane wave from one side of these metastructures, illuminating by several waves simultaneously from both sides may enhance possibilities to control scattered waves, which results in additional functionalities and novel applications. Here, we unveil how using coherent plane-wave illumination of a properly designed inhomogeneous metasurface sheet it is possible to realize controllable retroreflection. We call these metasurfaces as "coherent retroreflectors" and explain the method for realizing them both in theory and practice. We show that coherent retroreflectors can be used for filtering undesired modes and creation of field-localization regions in waveguides. The latter application is in resemblance to bound states in the radiation continuum.

physics.app-ph↗

arXiv2019

Physical meaning of the dipole radiation resistance in lossless and lossy media

In this tutorial, we discuss the radiation from a Hertzian dipole into uniform isotropic lossy media of infinite extent. If the medium is lossless, the radiated power propagates to infinity, and the apparent dissipation is measured by the radiation resistance of the dipole. If the medium is lossy, the power exponentially decays, and the physical meaning of radiation resistance needs clarification. Here, we present explicit calculations of the power absorbed in the infinite lossy host space and discuss the limit of zero losses. We show that the input impedance of dipole antennas contains a radiation-resistance contribution which does not depend on the imaginary part of the refractive index. This fact means that the power delivered by dipole antennas to surrounding space always contains a contribution from far fields unless the real part of the refractive index is zero. Based on this understanding, we discuss the fundamental limitations of power coupling between two antennas and possibilities of removing the limit imposed by radiation damping.

physics.class-ph↗