arXiv subjects

Vladimir M. Shalaev

Publications and source records attributed to Vladimir M. Shalaev.

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

Pathway to Optical-Cycle Dynamic Photonics: Extreme Electron Temperatures in Transparent Conducting Oxides

We theoretically demonstrate that transparent conducting oxides (TCOs) exhibit oscillatory and sign-reversing dynamic modulation in transmittance on the order of a few optical cycles under extreme electron temperatures, providing a possible explanation for TCO dynamics observed in earlier experiments. We present an inverse-designed multilayer cavity incorporating an ultrathin TCO layer, which supports an oscillatory optical response more pronounced than those previously observed experimentally in TCOs. This approach yields transmittance oscillations with a characteristic period of ~20fs, which corresponds to approximately four optical cycles of the 1.431μm probe beam. To achieve a similar oscillatory modulation in Δn, we incorporate a TCO electron-acceptor layer on top of the inverse-designed cavity, enabling thermionic carrier injection at the TCO junction. The resulting acceptor layer exhibits a striking Δn response as fast as 20fs, corresponding to only three optical cycles of the 1.8-2.0μm probe, and can be further tailored into the sub-optical-cycle regime. The findings could both clarify the previously unexplained transient dynamics in TCOs and, for the first time, demonstrate the critical role of electron temperatures in driving oscillatory dynamic responses.

physics.optics↗

arXiv2026

Nonlinearity Reversal in Epsilon-Near-Zero Indium Tin Oxide Driven by Few-Cycle Light Pulse

Recent breakthrough studies of nonlinearities at extreme pump intensities ($\sim$1 $\text{TW/cm}^2$) in transparent conducting oxides (TCOs) have rewritten our understanding of the dynamics in these materials. However, exploring TCO dynamics beyond these intensities is prohibited by the damage threshold of the material. In this work, we overcome this problem by using a few-cycle pump laser pulse (sub-8\,fs) to maximize the intensity while keeping the optical fluence below the damage threshold. We observe a reversal in the optical response trend starting at optical pump laser intensities of $\sim$5 $\text{TW/cm}^2$ similar to Segal et al. At the highest pump pulse intensities, we obtain a complete change in the sign of the modulation for both transmission and reflection, producing a full-cycle oscillation of the refractive index modulation within 300\,fs. The amplitude of the sign reversal scales quadratically with the intensity. We therefore propose a simple two-photon absorption (TPA) model to explain the observed behaviour. The TPA, which is normally forbidden by the Pauli blocking, is enabled here by intraband excitations from the lower to the upper non-equilibrium states of the conduction band (CB). Such excitations vacate the states at the bottom of the CB, lifting up the blocking and thus making interband TPA possible. The model is in good agreement with experimental results, capturing the essential trends in the observed data and revealing the dynamics of competing channels caused by the interplay between interband and intraband transitions. This intensity-controlled mechanism could be the key to unlocking new applications of TCOs for time-varying photonics such as photonic time crystals.

physics.optics↗

arXiv2026

Single-pump hybrid nonlinearities in transparent conductors

Low-index transparent conducting oxides have attracted significant attention because ultrafast optical excitation in these materials can induce exceptionally large temporal index gradients. Due to this remarkable nonlinear optical behaviour, this material platform enables sub-picosecond, all-optical control of photon energy and momentum, with growing relevance for integrated photonics, quantum optics, and optical computation. Owing to their hybrid electronic structure, transparent conductors exhibit both intraband and interband nonlinearities, previously accessed using dual-colour excitation with near-infrared and ultraviolet pumps. Here, we show that both excitation regimes can be activated using a single, intense near-infrared pump. Above a threshold intensity, the pump drives hot-electron intraband dynamics while simultaneously generating higher harmonics that trigger interband excitation. The interplay of these two effects sharpens the temporal features of the recorded transmissivity which in turn substantially broadens the effective material bandwidth. Finally, by comparing linear and circular pumping conditions, we further demonstrate that the observed interband nonlinearities originate from harmonic generation rather than from direct multiphoton absorption. Our results provide key insights into the strong-field optical response in these time-varying photonic materials, opening new frontiers for the ultra-fast manipulation of photons in both classic and quantum regimes.

physics.optics↗

arXiv2026

Anticipating Decoherence for Enhancing Coherence in Quantum Systems

Large-scale quantum systems require optical coherence between distant quantum devices, necessitating spectral indistinguishability. Scalable solid-state platforms offer promising routes to this goal. However, environmental disorders, including dephasing, spectral diffusion, and spin-bath interactions, influence the emitters' spectra and deteriorate the coherence. Using statistical theory, we identify correlations in spectral diffusion from slowly varying environmental coupling, revealing predictable dynamics extendable to other disorders. Importantly, this could enable the development of an anticipatory framework for forecasting and decoherence engineering in remote quantum emitters. To validate this framework, we demonstrate that a machine learning model trained on limited data can accurately forecast unseen spectral behavior. Realization of such a model on distinct quantum emitters could reduce the spectral shift by factors $\approx$ 2.1 to 15.8, depending on emitter stability, compared to no prediction. This work presents, for the first time, the application of anticipatory systems and replica theory to quantum technology, along with the first experimental demonstration of internal prediction that generalizes across multiple quantum emitters. These results pave the way for real-time decoherence engineering in scalable quantum systems. Such capability could lead to enhanced optical coherence and multi-emitter synchronization, with broad implications for quantum communication, computation, imaging, and sensing.

quant-ph↗

arXiv2025

PearSAN: A Machine Learning Method for Inverse Design using Pearson Correlated Surrogate Annealing

PearSAN is a machine learning-assisted optimization algorithm applicable to inverse design problems with large design spaces, where traditional optimizers struggle. The algorithm leverages the latent space of a generative model for rapid sampling and employs a Pearson correlated surrogate model to predict the figure of merit of the true design metric. As a showcase example, PearSAN is applied to thermophotovoltaic (TPV) metasurface design by matching the working bands between a thermal radiator and a photovoltaic cell. PearSAN can work with any pretrained generative model with a discretized latent space, making it easy to integrate with VQ-VAEs and binary autoencoders. Its novel Pearson correlational loss can be used as both a latent regularization method, similar to batch and layer normalization, and as a surrogate training loss. We compare both to previous energy matching losses, which are shown to enforce poor regularization and performance, even with upgraded affine parameters. PearSAN achieves a state-of-the-art maximum design efficiency of 97%, and is at least an order of magnitude faster than previous methods, with an improved maximum figure-of-merit gain.

cs.LG↗

arXiv2025

Sub-Terahertz Spin Relaxation Dynamics of Boron-Vacancy Centers in Hexagonal Boron Nitride

Quantum sensors based on spin-defect relaxation have become powerful tools for detecting faint magnetic signals, yet their operation has remained largely confined to low magnetic fields and gigahertz frequencies. Extending such sensors into high-field ($> 0.3$ T) and sub-terahertz regimes would enable quantum metrology across a wide range of electromagnetic phenomena and scientific applications, but has proven challenging. Here, we demonstrate that negatively charged boron vacancies ($\mathrm{V_B^-}$) in two-dimensional hexagonal boron nitride can function as relaxation-based quantum sensors operating up to 0.2 terahertz. Their uniform spin-orientation and persistent spin-contrast at high fields enable direct measurement of intrinsic spin relaxation across previously unexplored temperature and frequency regimes. We also reveal a crossover in relaxation behavior \textemdash initially decreasing at low fields before rising at higher fields \textemdash consistent with the emergence of single-phonon-induced resonant noise that becomes significant at sub-terahertz frequencies. These results establish $\mathrm{V_B^-}$ centers as a versatile platform for quantum sensing in the sub-terahertz, high-field regime.

quant-ph↗

arXiv2025

Spatio-Temporal Photonic Metalattice

When coherent light interacts with an ordered lattice whose periodicity is comparable to its wavelength, constructive interference produces a diffraction pattern as in crystallography, where x-rays are employed to reveal atomic structures. By asking 'when' the diffractive object exist, rather than 'where', we implicitly introduce time as a design parameter, thus enabling the creation of spatio-temporal metalattices. In these structures, temporal modulation of optical properties complements the spatial patterning, unlocking advanced functionalities such as dynamic reconfigurability, nonreciprocal behavior, coherent amplification, and tailored spectral response. However, for these effects to be relevant an extreme temporal modulation of the refractive index is necessary. In this work, we realize a two-dimensional spatio-temporal metalattice by integrating a physically patterned spatial modulation with an orthogonal temporal lattice induced by interfering ultrafast pulses, using highly nonlinear, low-index transparent conducting films. While the optical pumps experience a uniform medium, the lattice emerges through a strongly enhanced and internally generated third harmonic signal. The transient lattice shows comparable diffraction efficiency to the physical structure and is also dynamically reconfigurable via a broad range of parameters, including pump pulse delay, incidence angle, and wavelength, offering exceptional versatility for ultra-fast transient lithography and photon manipulation in both momentum and frequency. This approach shifts device design from fixed fabrication constraints to radiation engineering, opening new pathways towards ultrafast reconfigurable photonics.

physics.optics↗

arXiv2025

All-optical polarization control in time-varying low-index films via plasma symmetry breaking

Controlling the polarization state of light with sub-picosecond speed and subwavelength precision remains a key challenge for next-generation nanophotonic devices. Conventional methods such as birefringent crystals, liquid crystals, or electro-optic Pockels cells are limited in speed, compactness, and energy efficiency. While structured materials and two-dimensional heterostructures offer potential for on-chip ultrafast performance, achieving all-optical control remains an open problem. Here we introduce an all-optical scheme that employs femtosecond pumping of low-index, subwavelength isotropic films to achieve ultrafast control over birefringence, dichroism, and optical activity within a single material platform. When the material is probed near its crossover wavelength, linearly polarized pumping induces a transient phase retardation up to 0.1π per micrometer, accompanied by a dichroic absorption ratio of approximately twenty. Under circularly polarized excitation, the probe experiences nonreciprocal optical activity, leading to polarization rotation of about 1.1 degrees per micrometer. A universal hydrodynamic model quantitatively reproduces these effects and attributes them to pump-induced symmetry breaking in the photoexcited carrier plasma. This symmetry breaking enables coupling between orthogonal probe polarization components, mediated by a modified time-dependent damping term, which connects to the inverse Faraday effect induced by a circularly polarized pump. Our combined experimental and theoretical study establishes a reconfigurable, deep-subwavelength polarization-control mechanism operating on sub-picosecond timescales, suitable for compact ultrafast modulators, dynamic metasurfaces, and tunable nonreciprocal photonic devices, with implications for quantum optics, ultrafast logic, and time-resolved sensing.

physics.optics↗

arXiv2025

Topology-Optimized Dielectric Cavities for Enhanced Excitonic Light Emission from $\rm WSe_{2}$

Photonic inverse design and, especially, topology optimization, enable dielectric cavities with deeply sub-diffraction mode volumes and high quality factors, thus offering a powerful platform for enhanced light-matter coupling. Here, we design and fabricate arrays of CMOS-compatible silicon cavities on sapphire with extreme subwavelength transverse mode sizes of only 30-40 nm ($\rm V\simλ^3/2500$). These cavities are engineered for deterministic coupling to a monolayer (or few-layer) excitonic material, producing strong near-field localization directly beneath the 2D material. Photoluminescence (PL) measurements show reproducible tenfold enhancements relative to bare silicon, consistent with numerical simulations that account for material absorption and fabrication tolerances. Furthermore, time-resolved PL measurements reveal pronounced lifetime shortening and non-exponential dynamics, indicating cavity-mediated exciton-exciton interactions. The optimized cavity geometry enhances the far-field collection efficiency and supports scalable integration with van der Waals semiconductors. Our results show that the arrays of topology-optimized dielectric cavities are a versatile, scalable platform for controlling excitonic emission and interactions, which creates new opportunities in nonlinear optics, optoelectronics, and quantum photonics.

physics.optics↗

arXiv2025

Machine-Learning-Assisted Photonic Device Development: A Multiscale Approach from Theory to Characterization

Photonic device development (PDD) has achieved remarkable success in designing and implementing new devices for controlling light across various wavelengths, scales, and applications, including telecommunications, imaging, sensing, and quantum information processing. PDD is an iterative, five-step process that consists of: i) deriving device behavior from design parameters, ii) simulating device performance, iii) finding the optimal candidate designs from simulations, iv) fabricating the optimal device, and v) measuring device performance. Classically, all these steps involve Bayesian optimization, material science, control theory, and direct physics-driven numerical methods. However, many of these techniques are computationally intractable, monetarily costly, or difficult to implement at scale. In addition, PDD suffers from large optimization landscapes, uncertainties in structural or optical characterization, and difficulties in implementing robust fabrication processes. However, the advent of machine learning over the past decade has provided novel, data-driven strategies for tackling these challenges, including surrogate estimators for speeding up computations, generative modeling for noisy measurement modeling and data augmentation, reinforcement learning for fabrication, and active learning for experimental physical discovery. In this review, we present a comprehensive perspective on these methods to enable machine-learning-assisted PDD (ML-PDD) for efficient design optimization with powerful generative models, fast simulation and characterization modeling under noisy measurements, and reinforcement learning for fabrication. This review will provide researchers from diverse backgrounds with valuable insights into this emerging topic, fostering interdisciplinary efforts to accelerate the development of complex photonic devices and systems.

physics.optics↗

arXiv2025

Crystallization of the transdimensional electron liquid

Wigner crystallization of free electrons at room temperature is explored for a new class of metallic ultrathin (transdimensional) materials whose properties can be controlled by their thickness. Our calculations of the critical electron density, temperature and the melting curve show that by reducing the material thickness one can Wigner-crystallize free electrons at room temperature to get them pinned onto a two-dimensional triangular lattice of a supersolid inside of the crystalline material. Such a solid melts and freezes reversibly with increase and decrease of electron doping or temperature, whereby its resistivity behaves opposite to the free electron gas model predictions.

cond-mat.str-el↗

arXiv2025

Generation of photon pairs through spontaneous four-wave mixing in subwavelength nonlinear films

Pairs of entangled photons are crucial for photonic quantum technologies. The demand for integrability and multi-functionality suggests 'flat' platforms - ultrathin layers and metasurfaces - as sources of photon pairs. Despite the success in the demonstration of spontaneous parametric down-conversion (SPDC) from such sources, there are almost no works on spontaneous four-wave mixing (SFWM) - an alternative process to generate photon pairs. Meanwhile, SFWM can be implemented in any nanostructures, including ones made of isotropic materials, which are easier to fabricate than crystalline SPDC sources. Here, we investigate photon pair generation through SFWM in subwavelength films of amorphous silicon nitride (SiN) with varying nitrogen content. For all samples, we demonstrate two-photon quantum correlations, indicated by the normalized second-order correlation function g(2)(0): it exceeds 2 and decays as the pump power increases. By observing two-photon interference between SFWM from the SiN films and the fused silica substrate, we find the third-order susceptibilities of films with different nitrogen content.

physics.optics↗

arXiv2025

All-optical modulation with single-photons using electron avalanche

The distinctive characteristics of light, such as high-speed and low-loss propagation, low cross-talk and low power consumption, along with photons unique quantum properties, make it most suitable for various applications in communication, high-resolution imaging, optical computing, and emerging quantum information technologies. One limiting factor, though, is the weak optical nonlinearity of conventional media that poses challenges for the control of light with ultra-low intensities. In this work, we demonstrate all-optical modulation enabled by electron avalanche process in silicon, using a control beam with single-photon light intensities. The observed process corresponds to a record-high nonlinear refractive index of $n_{2}$~$1.3*10^{-2} m^2/W$, which is several orders of magnitude higher than the best known nonlinear optical materials. Our approach opens the possibility of gigahertz-speed, and potentially even faster, optical switching at the single-photon level, which could enable a family of novel on-chip photonic and quantum devices operating at room temperature.

physics.optics↗

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

High Harmonic Generation from a Noble Metal

High-harmonic generation (HHG) in solids has typically been explored in transparent dielectrics and semiconductors. Metals have long been dismissed due to their strong reflectivity at infrared wavelengths. Here, we demonstrate HHG from silver - a noble metal - using few-cycle near-infrared laser pulses at near-normal incidence. Our results show that sub-cycle electron dynamics within the material's penetration depth can drive high-order harmonics, challenging the prevailing notion that metals are unsuited for infrared-driven strong-field processes. Despite silver's high reflectivity and large free-electron density, we observe nonperturbative harmonics extending into the extreme ultraviolet (up to 20 eV). Moreover, silver's multi-shot damage threshold proves surprisingly high (30 TW/cm^2) - comparable to large-bandgap dielectrics like magnesium oxide - thereby enabling intense strong-field processes in a metallic environment. Measuring the orientation dependence of the emitted harmonics reveals that the process arises from coherent electron dynamics in the crystal lattice, rather than from a plasma-driven mechanism. Time-dependent density-matrix simulations based on maximally localized Wannier functions show that low-order harmonics predominantly originate from conduction electrons near the Fermi surface (s- and p-type orbitals), whereas higher harmonics rely on bound d-electron excitations. These findings establish metals - long thought unfavorable for HHG - as a promising platform for ultrafast strong-field physics, extending high-harmonic spectroscopy to regimes in which lattice order and plasma formation directly intersect. This work expands the frontier of solid-state HHG to all-metallic attosecond pulse generation and underscores the potential of metals as robust XUV sources for advanced attosecond metrology.

physics.optics↗

arXiv2025

Spatio-spectral optical fission in time-varying subwavelength layers

Transparent conducting oxides are highly doped semiconductors that exhibit favourable characteristics when compared to metals, including reduced material losses, tuneable electronic and optical properties, and enhanced damage thresholds. Recently, the photonic community has renewed its attention towards these materials, recognizing their remarkable nonlinear optical properties in the near-infrared spectrum, a feature previously overlooked despite their long-standing application in photovoltaics and touchscreens. The exceptionally large and ultra-fast change of the refractive index, which can be optically induced in these compounds, extends beyond the boundaries of conventional perturbative analysis and makes this class of materials the closest approximation to a time-varying system, and a unique playground for studying a variety of novel phenomena within the domain of photon acceleration. Here we report the spatio-spectral fission of an ultra-fast pulse trespassing a thin film of aluminium zinc oxide with a non-stationary refractive index. By applying phase conservation to this time-varying layer, our model can account for both space and time refraction and explain in quantitative terms, the spatial separation of both the spectrum and energy. Our findings represent an example of extreme nonlinear phenomena on subwavelength propagation distances and shed light on the nature of several nonlinear effects recently reported not accounting for the full optical field distribution. Our work also provides new important insights into transparent conducting oxides transient optical properties which are critical for the ongoing research in photonic time crystals, on-chip generation of nonclassical states of light, integrated optical neural networks as well as ultra-fast beam steering and frequency division multiplexing

physics.optics↗

arXiv2024

Floquet Engineering of Polaritonic Amplification in Dispersive Photonic Time Crystals

In this study, we investigate the dynamics of dispersive photonic time crystals (PTCs) and their potential applications for controlling light-matter interaction. By employing the Lorentz-Drude model, we analyze theoretically and via numerical simulation the effects of periodic modulation of dispersion parameters, revealing the emergence of hybrid bandgaps from interaction of polaritonic branches with unique characteristics. Our study demonstrates that dispersive PTCs offer novel excitation channels and amplification possibilities, that require lower modulation frequencies compared to non-dispersive systems thus alleviating experimental challenges for the realization of PTCs in the optical regime. These findings pave the way for advancements in polaritonic lasing and resonant Raman scattering.

physics.optics↗