arXiv subjects

Mordechai Segev

Publications and source records attributed to Mordechai Segev.

At least 19 recordsLinked to original sources

arXiv2026

Extreme Ultrafast Dynamics of the Refractive Index in Transparent Conductive Oxides: Theory and Experiment

Recent experiments in transparent conductive oxides (TCOs) have revealed light-induced order-unity variations in the refractive index occurring at extreme time scales, as short as a few-femtoseconds. These experimental observations remain unexplained, especially the ultrafast 10-20 femtoseconds relaxation of the index change, that cannot be explained by known phonon-mediated relaxation processes. Here, we present a simplified model followed by comprehensive simulations describing the phenomena, relying on the microscopic dynamics of electrons in TCOs under powerful ultrafast laser pulses. With this physical model, we predict and experimentally observe the unexplored regime of intraband modulation of electrons in the conduction band, leading to ultrafast oscillations of the refractive index. The observation of the oscillations validates the theory as a predictive tool, utilizing it to design experiments targeting novel effects that hinge on extreme alterations of optical properties of materials, such as photonic time-crystals and a plethora of novel extreme ultrafast phenomena.

physics.optics↗

arXiv2026

Acceleration of Free Electrons by Photonic Time-Crystals

We study the quantum interaction between free electrons and photons in a time-varying media, and find that periodic modulation exponentially amplifies electron-photon coupling within momentum gaps, enabling arbitrarily large momentum transfer. By preparing the light in a two-mode squeezed vacuum state, the electron momentum grows faster than its spectral spreading, establishing time-modulated photonic media as a platform for accelerating free-electrons and shaping their quantum state.

quant-ph↗

arXiv2026

Demonstration of Broadband Non-Resonant Time-Crystal Amplification in Microwaves

We report an optically modulated experimental realization of a photonic time crystal (PTC) in the microwave regime, demonstrating for the first time that the PTC exponential growth can overcome losses and finite-size constraints of a practical spatio-temporal system and yield stable positive terminal gain over a continuous broadband frequency range. The developed experimental platform is a purely time-modulated capacitor (TMC) microwave circuit based on a microstrip transmission line, in which synchronized optical modulation of reverse-biased photodiodes generates strong (94.5 %) temporal modulation of the effective capacitance at 200 MHz. Broadband amplification consistent with a momentum band gap (MBG), a defining signature of photonic time-crystal physics, is observed, with a peak gain of 3.8 dB over a 65 MHz bandwidth. In addition, a narrow parametric resonance appears at the center of the band gap, reaching 4.8 dB. This sharp peak is associated with the spatial inhomogeneities of the lumped-element realization, while the corresponding homogeneous distributed system retains the Floquet-mode structure of a photonic time crystal. We show that finite microwave TMC implementations inherit the defining physics of PTCs, including phase-invariant non-resonant amplification and slow-light behavior inside the momentum band gap, while finite-size and loss mechanisms transform the ideal semicircular PTC gain profile into a continuous asymmetric non-Lorentzian gain band characterized by a Pearson type IV distribution.

physics.optics↗

arXiv2026

About Time: Observation of Time-Reflection at Optical Frequencies

Time-reflection occurs when a wave is propagating in a medium undergoing a large and abrupt change in its properties: the original wave splits into a time-refracted wave and a time-reflected wave, each displaying different features. The time-refracted wave continues along its original course but experiences a frequency shift, whereas the time-reflected wave is propagating backwards in space with a reversed phase, also with a shifted frequency. These phenomena are fundamental to any wave system, but the most interesting are electromagnetic (EM) waves, specifically at optical frequencies, where they can couple to light-matter interactions. However, time-reflection of EM waves was thus far observed only at RF frequencies, never at optical frequencies. This is because time-reflection requires an order-unity variation of the refractive index occurring faster than a single wave cycle, and conventional optical nonlinearities are either too weak or too slow by orders of magnitude. Here, we present the first observation of time-reflection at optical frequencies. We induce an order-unity refractive-index change with sub-cycle duration, observe the time-reflection, and study its fundamental properties. These results provide an experimental pathway to experimenting with time-interfaces, generating photonic time-crystals and exploring new regimes of light-matter interaction in time-varying media.

physics.optics↗

arXiv2026

Any DOF All at Once: Single Photon State Tomography in a Single Measurement Setup

Photonic quantum technologies utilize various degrees of freedom (DOFs) of light, such as polarization, frequency, and spatial modes, to encode quantum information. In the effort of further improving channel capacity of quantum communication, and for increasing the complexity of available quantum operations, high-dimensional and hyperentangled states are now gaining interest. However, efficiently measuring these high dimensional states is challenging due to the large number of measurements required for reconstructing the full density matrix via quantum state tomography (QST), and the fact that each measurement requires some modification in the experimental setup. Here, we propose a framework for reconstructing the density matrix of a single-photon hyperentangled across multiple DOFs using a single intensity-measurement obtainable from traditional cameras, and discuss extensions for multiphoton hyperentangled states. Our method hinges on the spatial DOF of the photon and uses it to encode the quantum information from the other DOFs. We numerically demonstrate this method for single-photon OAM-spin and OAM-frequency entangled states using an ideal coupler and a multimode fiber, to perform the information mixing and transfer the encoding to spatial information, where it is detected using a simple camera. This technique simplifies the experimental setup and reduces acquisition time compared to traditional QST-based methods. Moreover, it allows the recovery of DOFs that conventional cameras cannot detect, such as polarization, thus eliminating the need for projection measurements.

quant-ph↗

arXiv2026

Experimental Observation of Time-Domain Bound States in The Continuum

Bound states in the continuum (BICs) are spatially localized eigenmodes that remain perfectly confined even though their energies reside within a continuum of radiating modes. BICs were predicted in 1929, but their experimental realization awaited more than 8 decades. Following their experimental observation, BICs were explored in a variety of wave systems, and found to exhibit a plethora of fundamental features such nontrivial topology and extremely high Q-factor. Recently, with foundational advances in the new field of electromagnetic waves in time-varying media, BICs were predicted to exist in the time domain, with their wavenumber embedded in a continuum of unbound momentum modes. Here, we present the first experimental realization of the time-domain Bound States in the Continuum. We use a transmission-line network with a time-modulated wave-impedance and show that a sinusoidal wave launched into the network naturally evolves into a time-domain BIC with a well-defined peak and decaying-oscillating tails. We show that the time-domain BIC is anti-symmetric despite the symmetric nature of the modulation. These experiments pave the way for exploring new phenomena in the fields of BICs and time-varying wave-systems in nonconservative regimes where time-translation symmetry is broken.

physics.optics↗

arXiv2026

Sub-cycle time-refraction at optical frequencies

Large and abrupt variations in the electromagnetic properties of materials lead to dramatic effects: even a single step-like change in the refractive index induces striking phenomena, such as time-refraction and time-reflection. When the refractive index varies periodically in time, multiple time-refractions and -reflections interfere, giving rise to photonic time-crystals (PTCs). Importantly, PTCs display momentum bands separated by gaps in which the modes experience exponential amplification, drawing energy from the modulation in a non-resonant fashion. Ordinary nonlinear optics does not operate in this regime: the material response is either very weak or very slow. One of the immediate consequences is that time-reflection of light at optical frequencies has never been observed in experiments. Here, we experimentally realize an order-unity change in the refractive index occurring at sub-cycle rates, and explore the phenomena emerging from it. By varying the duration of the index change from extending over many cycles to being significantly below a single cycle, we observe that the frequency shift of the time-refraction is enhanced as the index variation occurs faster. Our experiment is the gateway for realizing sharp time-interfaces at optical frequencies, which are the key for experimenting with time-reflection, PTCs and new phenomena expected from light-matter interactions in time-varying media.

physics.optics↗

arXiv2025

Negative Index Makes a Perfect Time-Domain Lens, Generating Slow Playback of Ultrafast Events

We explore the effects of incorporating negative index materials into the physics of time-varying media and find that changing the refractive index from positive to negative creates a perfect time-reversed wave: a perfect time-domain lens. Unlike other mechanisms of phase conjugation, the perfect time-domain lens time-reverses both the propagating waves and the evanescent part of the spectrum. Moreover, we find that the time-reversed wave can be slowed down or accelerated, depending on the refractive index ratio. We show that this effect remains strong even when the refractive index varies arbitrarily slow, in sharp contradistinction to time-reflection which necessitates large index changes at sub-cycle rates. This is the first avenue found to yield significant negative-frequency waves using a temporal interface without the need for sub-cycle modulation or impedance matching. The effect can be used to record extreme ultrafast information and subsequently play it backwards at a slow rate, and vice-versa.

physics.optics↗

arXiv2025

Unsupervised Machine Learning for Experimental Detection of Quantum-Many-Body Phase Transitions

Quantum many-body (QMB) systems are generally computationally hard: the computing resources necessary to simulate them exactly can often exceed the existing computation resources by orders of magnitude. For this reason, Richard Feynman proposed the concept of a quantum simulator: quantum systems engineered to obey a prescribed evolution equation and repeating the experiment multiple times. Experimentally, however, as we explain below, the vast majority of observables describing the system are inaccessible. Thus, while Feynman's idea addresses the problem of simulating quantum dynamics, it leaves unsolved the equally fundamental problem of inferring the underlying physics from the limited observables accessible in experiments. Indeed, many complex phenomena associated with QMB systems remain elusive. Perhaps, the most important example is identifying phase transitions in QMB systems when no simple order-parameter exists, which poses major challenges to this day. Complicating the problem further is the fact that, in most cases, it is impossible to learn from numerical simulations, as the underlying systems are often too large to be computable, and small QMB can show strong finite size effects, masking the presence of the transition. Here, we present an unsupervised machine learning approach to study QMB experiments, specifically aimed at detecting phase transitions and crossovers directly from raw experimental measurements. We demonstrate our methodology on systems undergoing Many-Body Localization cross-over and Mott-to-Superfluid phase-transition, showing that it reveals collective phenomena from the very partial experimental data and without any model-specific prior knowledge of the system. This approach offers a general and scalable route for data-driven discovery of emergent phenomena in complex quantum many-body systems.

quant-ph↗

arXiv2025

Supercoherence: Harnessing Long-Range Interactions to Preserve Collective Coherence in Disordered Systems

Artificial quantum systems with synthetic dimensions enable exploring novel quantum phenomena difficult to create in conventional materials. These synthetic degrees of freedom increase the system's dimensionality without altering its physical structure, accessing higher-dimensional physics in lower-dimensional setups. However, synthetic quantum systems often suffer from intrinsic disorder, causing rapid decoherence that limits scalability, a major obstacle in quantum information science. Here, we show that introducing just a few long-range interactions can mitigate decoherence, creating persistent collective coherence in highly symmetric collective excited states. We term this universal phenomenon "supercoherence" and show its exceptional robustness against disorder up to a dynamical phase transition at critical interaction strength and disorder. Supercoherence stabilizes not only coherence but also all other quantum properties of the states, challenging traditional views on the inevitability of decoherence in disordered interacting quantum systems and suggesting new opportunities for quantum memory and information processing.

quant-ph↗

arXiv2025

2D Topological Edge States in Periodic Space-Time Interfaces

Topological edge states in systems of two (or more) dimensions offer scattering-free transport, exhibiting robustness to inhomogeneities and disorder. In a different domain, time-modulated systems, such as photonic time crystals (PTCs), offer non-resonant amplification drawing energy from the modulation. Combining these concepts, we explore topological systems that vary periodically in both time and space, manifesting the best of both worlds. We present topological phases and topological edge states in photonic space-time crystals - materials in which the refractive index varies periodically in both space and time, displaying bandgaps in both frequency and momentum. The topological nature of this system leads to topological invariants that govern the phase between refracted and reflected waves generated from both the spatial and the temporal interfaces. The 2D nature of this system leads to propagating edge states, and a unique edge state that grows exponentially in power whilst following the space-time edge.

physics.optics↗

arXiv2025

Tracking the Evolution of Near-Field Photonic Qubits into High-Dimensional Qudits via State Tomography

Quantum nanophotonics offers essential tools and technologies for controlling quantum states, while maintaining a miniature form factor and high scalability. For example, nanophotonic platforms can transfer information from the traditional degrees of freedom (DoFs), such as spin angular momentum (SAM) and orbital angular momentum (OAM), to the DoFs of the nanophotonic platform - and back, opening new directions for quantum information processing. Recent experiments have utilized the total angular momentum (TAM) of a photon as a unique means to produce entangled qubits in nanophotonic platforms. Yet, the process of transferring the information between the free-space DoFs and the TAM was never investigated, and its implications are still unknown. Here, we reveal the evolution of quantum information in heralded single photons as they couple into and out of the near-field of a nanophotonic system. Through quantum state tomography, we discover that the TAM qubit in the near-field becomes a free-space qudit entangled in the photonic SAM and OAM. The extracted density matrix and Wigner function in free-space indicate state preparation fidelity above 97%. The concepts described here bring new concepts and methodologies in developing high-dimensional quantum circuitry on a chip.

quant-ph↗

arXiv2025

Nonlinear Nanophotonics for High-Dimensional Quantum States

Quantum nanophotonics merges the precision of nanoscale light manipulation with the capabilities of quantum technologies, offering a pathway for enhanced light-matter interaction and compact realization of quantum devices. Here, we show how a recently-demonstrated nonlinear nanophotonic process can be employed to selectively create photonic high-dimensional quantum states (qudits). We utilize the nonlinearity on the surface of the nanophotonic device to dress, through the polarization of the pump field, the near-field modes carrying angular momentum and their superpositions. We then use this approach for the realization of a multilevel quantum key distribution protocol, which doubles the key rate compared to standard schemes. This idea is an important step towards experimental realizations of quantum state generation and manipulation through nonlinearity within nanophotonic platforms, and enables new capabilities for on-chip quantum devices.

physics.optics↗

arXiv2024

Long lived surface plasmons on the interface of a metal and a photonic time-crystal

We predict the existence of surface plasmons polaritons at the interface between a metal and a periodically modulated dielectric medium, and find an unusual multi-branched dispersion curve of surface and bulk modes. The branches are separated by momentum gaps indicating intense amplification of modes, and display high and low group velocity ranging from zero to infinity at short wavelengths. We simulate how these SPP modes are formed by launching a properly engineered laser beam onto the metallic interface and examine their space-time evolution. The amplification of the surface plasmons at the interface with a photonic time-crystal offers a path to overcome plasmonic losses, which have been a major challenge in plasmonics.

physics.optics↗

arXiv2024

Second Harmonic Generation and Nonlinear Frequency Conversion in Photonic Time-Crystals

We study the nonlinear process of second harmonic generation in photonic time-crystals, materials with refractive index that varies abruptly and periodically in time, and obtain the phase matching condition for this process. We find conditions for which the second harmonic generation is highly enhanced even in the absence of phase matching, governed by the exponential growth of the modes residing in the momentum gap of the photonic time crystal. Additionally, under these conditions, a cascade of higher order harmonics is generated at growing exponential rates. The process is robust, with no requirement for phase-matching, the presence of a resonance or a threshold, drawing energy from the modulation.

physics.optics↗

arXiv2024

Time-Domain Bound States in the Continuum

We present the concept of time-domain bound states in continuum. We show that a rapid judiciously-designed temporal modulation of the refractive index in a spatially homogenous medium gives rise to a bound state in time embedded in a continuum of wavenumbers. Mathematically, these bound states in the continuum (BIC) are analytic solutions of the Maxwell equations in time and one-dimensional space. Our results show the potential to extend known wave phenomena in space to the temporal domain, providing new avenues for light-matter interactions in time-varying media.

physics.optics↗

arXiv2024

Time-refraction and time-reflection above critical angle for total internal reflection

We study the time-reflection and time-refraction of waves caused by a spatial interface with a medium undergoing a sudden temporal change in permittivity. We show that monochromatic waves are transformed into a pulse by the permittivity change, and that time-reflection is enhanced at the vicinity of the critical angle for total internal reflection. In this regime, we find that the evanescent field is transformed into a propagating pulse by the sudden change in permittivity. These effects display enhancement of the time-reflection and high sensitivity near the critical angle, paving the way to experiments on time-reflection and photonic time-crystals at optical frequencies

physics.optics↗

arXiv2024

Third Harmonic Enhancement Harnessing Photoexcitation Unveils New Nonlinearities in Zinc Oxide

Nonlinear optical phenomena are at the heart of various technological domains such as high-speed data transfer, optical logic applications, and emerging fields such as non-reciprocal optics and photonic time crystal design. However, conventional nonlinear materials exhibit inherent limitations in the post-fabrication tailoring of their nonlinear optical properties. Achieving real-time control over optical nonlinearities remains a challenge. In this work, we demonstrate a method to switch third harmonic generation (THG), a commonly occurring nonlinear optical response. Third harmonic generation enhancements up to 50 times are demonstrated in zinc oxide films via the photoexcited state generation and tunable electric field enhancement. More interestingly, the enhanced third harmonic generation follows a quadratic scaling with incident power, as opposed to the conventional cubic scaling, which demonstrates a previously unreported mechanism of third harmonic generation. The THG can also be suppressed by modulating the optical losses in the film. This work shows that the photoexcitation of states can not only enhance nonlinearities, but can create new processes for third harmonic generation. Importantly, the proposed method enables real-time manipulation of the nonlinear response of a medium. The process is switchable and reversible, with the modulations occurring at picosecond timescale. Our study paves the way to boost or suppress the nonlinearities of solid-state media, enabling robust, switchable sources for nonlinear optical applications.

cond-mat.mtrl-sci↗