arXiv subjects

Zeki Hayran

Publications and source records attributed to Zeki Hayran.

16 recordsLinked to original sources

arXiv2026

Reaching the optical propagation limit in temporal analog computing

A central goal of optical computing is to perform calculations on the timescale of light propagation. Yet many analog photonic solvers require feedback, storage or field build-up before the answer becomes available, introducing additional latency that limits real-time operation. Here we introduce the concept of two-time modulation for temporal analog computing, in which the material response is independently modulated along two temporal directions, allowing the computational operator to be constructed continuously as the waveform propagates. The solution can therefore form during optical transit, without an additional solution-formation timescale. This form of temporal control allows identical copies of a waveform separated only in time to reach entirely different target outputs within a single spatial channel, and enables nonlocal integral-equation solving in a single passage. The resulting framework brings compact, programmable, real-time analog computation within reach for ultrafast optical information processing.

physics.optics↗

arXiv2026

Pulse-driven photonic transitions and nonreciprocity in space-time modulated metasurfaces

Time-varying photonic systems open new possibilities for controlling light, enabling photonic time crystals, time reflection and refraction, frequency conversion, synthetic gauge fields, optical nonreciprocity, among others. These effects emerge from the dynamic modulation of optical properties, which can mediate photonic transitions between eigenstates of different frequencies and/or wavevectors. To achieve such transitions, conventional approaches rely on periodic modulation schemes that demand ultrafast modulation rates and continuous energy input, posing significant practical challenges at optical frequencies. Here, we demonstrate that periodic-modulation-driven photonic transitions within the radiation continuum can be effectively mimicked using a single-period ultrafast pulse modulation, eliminating the need for sustained continuous modulation. By leveraging dispersion engineering in metasurfaces to tailor the density of states in the radiation continuum, we achieve controlled frequency transitions and theoretically demonstrate strong nonreciprocity for free-space waves as a key application. Our findings may guide future experimental research on time-varying photonics using materials such as transparent conductive oxides and semiconductors, expanding the possibilities for ultrafast and reconfigurable optical technologies. More broadly, our work may establish a practical and energy-efficient framework for dynamic photonic systems, with potential applications ranging from spatio-temporal wavefront manipulation to photonic computing and ultrafast signal processing.

physics.optics↗

arXiv2025

Space-time refraction of space-time wave packets

Space-time modulation of refractive index can produce synthetically moving interfaces with arbitrary apparent velocities, including superluminal motion, offering new ways to control light in dynamic media. On the other hand, space-time wave packets are structured waves whose spatio-temporal spectra lie on tilted space-time planes, so their group velocity can be programmed, including superluminal values, even in a uniform medium. Here we develop a general theory of space-time refraction for such structured waves at a planar moving interface and show how a single boundary reshapes their velocity content. By identifying the invariants of a translating boundary, we obtain refraction laws for baseband, X-wave, and sideband packets that apply for arbitrary interface velocities and connect smoothly to static and purely temporal limits. These laws reveal regimes of "space-time anomalous optical push broom," where a moving interface compresses a wide range of incident velocities into a narrow transmitted band, and "velocity spectral optical fission," where an incident X-wave splits into two propagation-invariant branches with distinct velocities. The combined freedom to prepare waves with superluminal group velocity and to prescribe equally unconstrained interface speeds points toward reconfigurable time gating, optical buffering, velocity multiplexing, and controlled emission in moving media, and provides a route to photonic settings capable of emulating dynamical effects traditionally associated with gravitational or quantum processes.

physics.optics↗

arXiv2025

Chirality-driven all-optical image differentiation

Optical analog computing enables powerful functionalities, including spatial differentiation, image processing, and ultrafast linear operations. Yet, most existing approaches rely on resonant or periodic structures, whose performance is strongly wavelength-dependent, imposing bandwidth limitations and demanding stringent fabrication tolerances. Here, to address some of these challenges, we introduce a highly tunable platform for optical processing, composed of two cascaded uniform slabs exhibiting both circular and linear birefringence, whose response exhibits features relevant to optical processing without relying on resonances. Specifically, using a coupled-wave theory framework we show that sharp reflection minima, referred to as spectral holes, emerge from destructive interference between counter-propagating circularly polarized waves in uniform birefringent slabs, and can be engineered solely through parameter tuning without requiring any spatial periodicity. Unlike traditional Bragg scattering, this mechanism operates without a resonance condition and enables a comparatively broader spectral response through material parameter tuning in spatially uniform media. When operated in the negative refraction regime enabled by giant chirality, the proposed system acts as a polarization-selective Laplacian-like operator, whose functionality is evidenced by an edge-detection proof of concept. The required material parameters align closely with recent experimental demonstrations of giant, tunable chirality via meta-optics, presenting a promising pathway towards compact and reconfigurable platforms for all-optical pattern recognition and image restoration.

physics.optics↗

arXiv2025

Broadband Low-loss Unidirectional Reflection On-chip with Asymmetric Dielectric Metasurface

Metasurface has emerged as a powerful platform for controlling light at subwavelength thickness, enabling new functionalities for imaging, polarization manipulation, and angular momentum conversion within a flat surface. We explored an integrated asymmetric metasurface simultaneously achieving broadband, low loss forward power transmission, and significant back reflection sup-pression in multi-mode waveguides. The tapering along the direction of light propagation leads to low loss and space-efficient mode conversion. Enhanced by a double-flipped structure, a thin (2.5 micrometer) metasurface can simultaneously achieve high conversion efficiency (>80 percent), and back-reflection efficiency of 90 percent over a 200 nm wavelength range. Such single sided reflectors can be one of the enabling components for gain-integrated adaptive optics on a chip.

physics.optics↗

arXiv2024

Observation of broadband super-absorption of electromagnetic waves through space-time symmetry breaking

Using time as an additional design parameter in electromagnetism, photonics, and wave physics is attracting considerable research interest, motivated by the possibility to explore physical phenomena and engineering opportunities beyond the physical limits of time-invariant systems. However, despite substantial theoretical promise, the practical realization and observation of many new effects and capabilities leveraging such temporal degrees of freedom have remained elusive. Here, we report the first experimental demonstration of enhanced broadband absorption of electromagnetic waves in a continuously modulated time-varying system, exceeding one of the key theoretical limits of linear time-invariant absorbers. This is achieved by harnessing the frequency-wavevector transitions and enhanced interference effects enabled by breaking both continuous space- and time-translation symmetries in a periodically time-modulated absorbing structure operating at radio-frequencies. Furthermore, we demonstrate broadband coherent wave absorption using a secondary control wave, observing a nearly perfect, reconfigurable, anti-reflection effect over a broad continuous bandwidth. Our findings provide new insights to challenge existing paradigms on the limits of wave absorption and may pave the way to the development of devices that operate in a regime fundamentally beyond the reach of any linear time-invariant system.

physics.optics↗

arXiv2023

Comprehensive evidence of lasing from a 2D material enabled by a dual-resonance metasurface

Semiconducting transition metal dichalcogenides (TMDs) have gained significant attention as a gain medium for nanolasers, owing to their unique ability to be easily placed and stacked on virtually any substrate. However, the atomically thin nature of the active material in existing TMD nanolasers presents a challenge, as their limited output power makes it difficult to distinguish between true laser operation and other "laser-like" phenomena. Here, we present comprehensive evidence of lasing from a CVD-grown tungsten disulphide (WS$_2$) monolayer. The monolayer is placed on a dual-resonance dielectric metasurface with a rectangular lattice designed to enhance both absorption and emission; resulting in an ultralow threshold operation (threshold <1 W/cm$^2$). We provide a thorough study of the laser performance at room temperature, paying special attention to directionality, output power, and spatial coherence. Notably, our lasers demonstrated a coherence length of over 30 $\mu$m, which is several times greater than what has been reported for 2D material lasers so far. Our realisation of a single-mode laser from a wafer-scale CVD-grown monolayer presents exciting opportunities for integration and the development of novel applications.

physics.optics↗

arXiv2023

Beyond the Rozanov Bound on Electromagnetic Absorption via Periodic Temporal Modulations

Incorporating time-varying elements into electromagnetic systems has shown to be a powerful approach to challenge well-established performance limits, for example bounds on absorption and impedance matching. So far, the majority of these studies have concentrated on time-switched systems, where the material undergoes instantaneous modulation in time while the input field is entirely contained within it. This approach, however, necessitates accurate timing of the switching event and limits how thin the system can ultimately be due to the spatial width of the impinging pulse. To address these challenges, here we investigate the periodic temporal modulation of highly lossy materials, focusing on their relatively unexplored parametric absorption aspects. Our results reveal that, by appropriately selecting the modulation parameters, the absorption performance of a periodically modulated absorber can be greatly improved compared to its time-invariant counterpart, and can even exceed the theoretical bound for conventional electromagnetic absorbers, namely, the "Rozanov bound". Our findings thus demonstrate the potential of periodic temporal modulations to enable significant improvements in absorber performance while circumventing the limitations imposed by precise timing and material thickness in time-switched schemes, opening up new opportunities for the design and optimization of advanced electromagnetic absorber systems for various applications.

physics.optics↗

arXiv2022

$\hbar \omega$ versus $\hbar \boldsymbol{k}$: Dispersion and Energy Constraints on Time-Varying Photonic Materials and Time Crystals

Photonic time-varying systems have attracted significant attention owing to their rich physics and potential opportunities for new and enhanced functionalities. In this context, the duality of space and time in wave physics has been particularly fruitful to uncover interesting physical effects in the temporal domain, such as reflection/refraction at temporal interfaces and momentum-bandgaps in time crystals. However, the characteristics of the temporal/frequency dimension, particularly its relation to causality and energy conservation ($\hbar \omega$ is energy, whereas $\hbar \boldsymbol{k}$ is momentum), create challenges and constraints that are unique to time-varying systems and are not present in their spatially varying counterparts. Here, we overview two key physical aspects of time-varying photonics that have only received marginal attention so far, namely temporal dispersion and external power requirements, and explore their implications. We discuss how temporal dispersion, an inherent property of any causal material, makes the fields evolve continuously at sharp temporal interfaces and may limit the strength of fast temporal modulations and of various resulting effects. Furthermore, we show that changing the refractive index in time always involves large amounts of energy. We derive power requirements to observe a time-crystal response in one of the most popular material platforms in time-varying photonics, i.e., transparent conducting oxides, and we argue that these effects are almost always obscured by less exotic nonlinear phenomena. These observations and findings shed light on the physics and constraints of time-varying photonics, and may guide the design and implementation of future time-modulated photonic systems.

physics.optics↗

arXiv2022

Using Time-Varying Systems to Challenge Fundamental Limitations in Electromagnetics

Time-varying systems open intriguing opportunities to explore novel approaches in the design of efficient electromagnetic devices. While such explorations date back to more than half a century ago, recent years have experienced a renewed and increased interest into the design of dynamic electromagnetic systems. This resurgence has been partly fueled by the desire to surpass the performance of conventional devices, and to enable systems that can challenge various well-established performance bounds, such as the Bode-Fano limit, the Chu limit, and others. Here, we overview this emerging research area and provide a concise and systematic summary of the most relevant applications for which the relevant performance bounds can be overcome through time-varying elements. In addition to enabling devices with superior performance metrics, such research endeavors may open entirely new opportunities and offer insight towards future electromagnetic and optical technologies.

physics.optics↗

arXiv2021

Physical limitations on broadband invisibility based on fast-light media

This note is a comment on a recent article [Tsakmakidis, et al., Nat Commun 10 (2019)] that presents a thought-provoking proposal to overcome the bandwidth restrictions of invisibility cloaks based on using media that support superluminal (faster than light in free space) group and phase velocities. As illustrated in Fig. 1 of the original article, a wave packet propagating through such a fast-light cloak is alleged to be able to reach the side behind the cloaked object simultaneously with a corresponding wave packet propagating through the shorter, direct route in free space without the object, so that "no shadow or waveform distortion arises." As the authors claim, the "extra pathlength is balanced out by the correspondingly larger group velocity of the pulse in the cloak", which allows to "restore the incident field distribution all around the object in, both, amplitude and phase". This fast-light effect may be achieved in a broadband fashion using active (gain) materials. The authors claim that such a fast-light cloak can hide an object, even from time-of-flight detection techniques, and achieve invisibility "over any desired frequency band." We disagree with these claims and believe that a thorough clarification of the ideas put forward in the original article is important and necessary for the broad wave-physics community. Specifically, in this comment we clarify that invisibility cloaks based on fast-light media suffer from fundamental bandwidth restrictions that arise due to causality, the nature of superluminal wave propagation, and the stability issues of active systems. These limitations and issues were not addressed in [Tsakmakidis, et al., Nat Commun 10 (2019)]. Most importantly, we show that the material model considered in the original article is unphysical.

physics.optics↗

arXiv2020

Spectral Causality and the Scattering of Waves

Causality - the principle stating that the output of a system cannot temporally precede the input - is a universal property of nature. Here, we show that analogous input-output relations can also be realized in the spectral domain by leveraging the peculiar properties of time-modulated non-Hermitian photonic systems. Specifically, we uncover the existence of a broad class of complex time-modulated metamaterials which obey the time-domain equivalent of the well-established frequency-domain Kramers-Kronig relations (a direct consequence of causality). We find that, in the scattering response of such time-modulated systems, the output frequencies are inherently prohibited from spectrally preceding the input frequencies, hence we refer to these systems as 'spectrally causal'. We explore the consequences of this newly introduced concept for several relevant applications, including broadband perfect absorption, temporal cloaking of an 'event', and truly unidirectional propagation along a synthetic dimension. By emulating the concept of causality in the spectral domain and providing new tools to extend the field of temporally modulated metamaterials ("chrono-metamaterials") into the complex realm, our findings may open unexplored opportunities and enable relevant technological advances in various areas of photonics and, more broadly, of wave physics and engineering.

physics.optics↗

arXiv2020

Capturing Broadband Light in a Compact Bound State in the Continuum

Trapping and storing light for arbitrary time lengths in open cavities is a major goal of nanophotonics, with potential applications ranging from energy harvesting to optical information processing. Unfortunately, however, the resonance lifetime of conventional open resonators remains finite even in the limit of vanishing material absorption, as a result of radiation loss. In this context, bound states in the continuum (BiCs) have provided a unique way to achieve unbounded resonance lifetimes despite the presence of compatible radiation channels. However, physical constraints such as reciprocity, linearity, and delay-bandwidth limits prevent the possibility to externally excite such ideal bound states and make them interact with broadband sources. Here, we overcome these limitations and theoretically demonstrate that subwavelength open resonators undergoing a suitable temporal modulation can efficiently capture a broadband incident wave into a nonradiating eigenmode of the structure, leading to the first example of a BiC that is accessible to broadband light. Our findings unveil the dynamic capabilities of bound states in the continuum and extend their reach and potential impact for different applications.

physics.optics↗

arXiv2017

Invisibility on demand based on a generalized Hilbert transform

Designing invisible objects without the usage of extreme materials is a long-sought goal for photonic applications. Invisibility techniques demonstrated so far typically require high anisotropy, gain and losses, while also not being flexible. Here we propose an invisibility approach to suppress the scattering of waves from/to given directions and for particular frequency ranges, i.e. invisibility on demand. We derive a Born approximation-based generalized Hilbert transform for a specific invisibility arrangement relating the two quadratures of the complex permittivity of an object. The theoretical proposal is confirmed by numerical calculations, indicating that near-perfect invisibility can be attained for arbitrary objects with low-index contrast. We further demonstrate the cases where the idea can be extended to high-index objects or restricted to within practical limits by avoiding gain areas. The proposed concept opens a new route for the practical implementation of complex-shaped objects with arbitrarily suppressed scatterings determined on demand.

physics.optics↗

arXiv2017

All-dielectric self-cloaked structures

While practical realizations of optical invisibility have been achieved so far by various ingenious methods, they generally rely on complex materials which prevent the wide implementation of such schemes. Here, we propose an alternative indivisibility procedure to design objects (i.e. self-cloaked structures) that have optical properties identical to the surrounding environment and are, thereby, intrinsically invisible to an external observer as such (without the necessity of an external cloak). The proposed method is based on the uncoupling of the scattered waves from the incident radiation by judiciously manipulating the scattering potential of a given object. We show that such a procedure is able to yield optical invisibility for any arbitrarily shaped object within any specified frequency bandwidth by simply employing isotropic non-magnetic dielectric materials, without the usage of loss or gain material. The validity of the design principle has been verified by direct experimental observations of the spatial electric field profiles and scattering patterns at the microwave regime. Our alternative self-cloaking strategy may have profound implications especially in noninvasive probing and cloaked sensor applications, where the wave penetrability into the sensor region is essential together with its invisibility to minimize the field distortion.

physics.optics↗

arXiv2017

Rainbow Trapping in a Chirped Three-Dimensional Photonic Crystal

Light localization and intensity enhancement in a woodpile layer-by-layer photonic crystal, whose interlayer distance along the propagation direction is gradually varied, has been theoretically predicted and experimentally demonstrated. The phenomenon is shown to be related to the progressive slowing down and stopping of the incoming wave, as a result of the gradual variation of the local dispersion. The light localization is chromatically resolved, since every frequency component is stopped and reflected back at different spatial positions. It has been further discussed that the peculiar relation between the stopping distance and the wave vector distribution can substantially increase the enhancement factor to more than two orders of magnitude. Compared to previously reported one- and two-dimensional photonic crystal configurations, the proposed scheme has the advantage of reducing the propagation losses by providing a three-dimensional photonic bandgap confinement in all directions. The slowing down and localization of waves inside photonic media can be exploited in many applications that requires enhanced interaction of light and matter.

physics.optics↗