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Yonatan Sivan

Publications and source records attributed to Yonatan Sivan.

At least 19 recordsLinked to original sources
arXiv2026

Origin, strength, and speed of the nonlinear optical response of transparent conducting oxides to single-cycle optical pulses

We present a first-principles study of the nonlinear optical response of transparent conducting oxides at the nanoscale due to excitation by intense, extremely short pulses based on a density matrix framework. We identify a strong ($O(1)$) thermal nonlinearity, which is complemented with stimulated emission and excited-state absorption; it yields a cumulative permittivity change decorated by quantum coherent oscillations. Further, rigorous calculations under far-from-equilibrium conditions show that electron-electron thermalization occurs within a few femtoseconds, supporting interpretations of high-harmonic generation measurements and in agreement with a generalization of Fermi liquid theory.

physics.optics↗
arXiv2026

Quantum-optical theory of the few femtosecond nonlinear optical response of Drude metals with a non-parabolic conduction band

We develop an energy-space density matrix framework to investigate the interaction of extremely short optical pulses (ESPs) with transparent conducting oxides (TCOs). This approach captures not only electron populations, material polarization, and the permittivity, but also the quantum coherences between states. Compared to traditional momentum-space models, the energy-space formulation offers substantial computational simplification while retaining accuracy. Building on but going beyond the scope of Ref.~\cite{single_cycle_nlty_Letter}, we focus on dynamical features previously unexplored. Our formulation reveals clear signatures of quantum coherence in the net absorption dynamics and highlights the emergence of strong excited-state absorption under intense excitation. It also clarifies that spontaneous emission can be neglected in this regime. Furthermore, we investigate the influence of pump pulse intensity on the local field's duration, spectral broadening and shift, and phase induced by carrier dynamics, highlighting the absorptive nature of the nonlinear response. Our results provide a unified framework for understanding nonlinear light-matter interaction in dispersive, low-density electron systems driven far from equilibrium by intense broadband excitation.

physics.optics↗
arXiv2025

Photoluminescence from metal nanostructures -- dependence on size

For decades, there have been multiple seemingly contradicting experimental reports on the dependence of the photoluminescence from metal nanostructures on their size. We reconcile these reports using a simple analytic formula which is found to match well photoluminescence measurements for a range of structures and illumination conditions. Our expression requires only knowledge of the local electric field and temperature, and it points to the physical quantities that determine the emission strength and its dependence on size.

cond-mat.mtrl-sci↗
arXiv2024

Ballistic vs. diffusive transport in metals

Using the Boltzmann transport model, we show that, somewhat unintuitively, ballistic transport of electrons in metals is weaker than diffusive transport. This happens because the femtosecond-scale collision rates of the non-thermal electrons makes their mean-free path negligible. Our predictions are correlated with various photoluminescence and nonlinear optics experimental examples both for Continuous Wave (CW) and pulsed illumination, and open the way to easy modelling of the non-thermal electron distributions in metal nanostructures of arbitrary complexity.

cond-mat.mes-hall↗
arXiv2022

An electronic-based model of the optical nonlinearity of low-electron-density-Drude materials

Low electron density Drude (LEDD) materials such as indium tin oxide (ITO) are receiving considerable attention because of their combination of CMOS compatibility, unique epsilon-near-zero (ENZ) behavior, and giant ultrafast nonlinear thermo-optic response. However, the understanding of the electronic and optical response of LEDD materials is so far based on simplistic extensions of known models of noble metals, frequently without the inclusion of the interplay among the lower electron density, relatively high Debye energy, and the non-parabolic band structure. To bridge this knowledge gap, this work provides a complete understanding of the nonlinear electronic-thermal-optical response of LEDD materials. In particular, we rely on state-of-the-art electron dynamics modeling, as well as the newly derived time-dependent permittivity model for LEDD materials under optical pumping within the adiabatic approximation. We find that unlike noble metals, the electron temperatures can reach the Fermi temperature, in which case the effective chemical potential dramatically decreases and even becomes negative, thus, transiently converting the Drude metal into a semiconductor. We further show that the nonlinear optical response of LEDD materials originating from the changes to the real part of the permittivity is due to the generation of non-thermal electrons. This resolves the argument about the rise time of the permittivity and shows that it is instantaneous. In this vein, we show that referring to the LEDD permittivity as having a ``saturable'' nonlinearity is unsuitable since its permittivity dynamics does not originate from population inversion. Finally, we analyze the probe pulse dynamics and unlike previous work, we obtain a quantitative agreement with the results of recent experiments.

physics.optics↗
arXiv2022

The electronic and thermal response of low electron density Drude materials to ultrafast optical illumination

Many low electron density Drude (LEDD) materials such as transparent conductive oxide or nitrides have recently attracted interest as alternative plasmonic materials and future nonlinear optical materials. However, the rapidly growing number of experimental studies has so far not been supported by a systematic theory of the electronic, thermal and optical response of these materials. Here, we use the techniques previously derived in the context of noble metals to go beyond a simple electromagnetic modelling of low electron density Drude materials and provide an electron dynamics model for their electronic and thermal response. We find that the low electron density makes momentum conservation in electron-phonon interactions more important, more complex and more sensitive to the temperatures compared with noble metals; moreover, we find that electron-electron interactions are becoming more effective due to the weaker screening. Most importantly, we show that the low electron density makes the electron heat capacity much smaller than in noble metals, such that the electrons in LEDD materials tend to heat up much more and cool down faster compared to noble metals. While here we focus on indium tin oxide (ITO), our analytic results can be easily applied to any LEDD materials.

physics.optics↗
arXiv2022

Theory of Non-equilibrium "Hot" Carriers in Direct Band-gap Semiconductors Under Continuous Illumination

The interplay between the illuminated excitation of carriers and subsequent thermalization and recombination leads to the formation of non-equilibrium distributions for the "hot" carriers and to heating of both electrons, holes and phonons. In spite of the fundamental and practical importance of these processes, there is no theoretical framework which encompasses all of them and provides a clear prediction for the non-equilibrium carrier distributions. Here, a self-consistent theory accounting for the interplay between excitation, thermalization, and recombination in continuously-illuminated semiconductors is presented, enabling the calculation of non-equilibrium carrier distributions. We show that counter-intuitively, distributions deviate more from equilibrium under weak illumination than at high intensities. We mimic two experimental procedures to extract the carrier temperatures and show that they yield different dependence on illumination. Finally, we provide an accurate way to evaluate photoluminescence efficiency, which, unlike conventional models, predicts correctly the experimental results. These results provide a starting point towards examining how non-equilibrium features will affect properties hot-carrier based application.

physics.optics↗
arXiv2022

The photothermal nonlinearity in plasmon-assisted photocatalysis

Understanding the intricate relation between illumination and temperature in metallic nano-particles is crucial for elucidating the role of illumination in various physical processes which rely on plasmonic enhancement but are also sensitive to temperature. Recent studies have shown that the temperature rise in optically-thick ensembles of metal nanoparticles under intense illumination is dominated by the thermal conductivity of the host, rather than by the optical properties of the metal or the host. Here, we show that the temperature dependence of the thermal conductivity of the host dominates the nonlinear photothermal response of these systems. In particular, this dependence typically causes the temperature rise to become strongly sublinear, reaching even several tens of percent. We then show that this can explain experimental observations in several recent plasmon-assisted photocatalysis experiments. This shows that any claim for dominance of non-thermal electrons in plasmon-assisted photocatalysis must account first for this photothermal nonlinear mechanism.

physics.chem-ph↗
arXiv2022

Distinguishing thermal from non-thermal ("hot") carries in illuminated molecular junctions

The search for the signature of non-thermal (so-called ``hot'') electrons in illuminated plasmonic nanostructures requires a detailed understanding of the non-equilibrium electron distribution under illumination, as well as a careful design of the experimental system employed to distinguish non-thermal electrons from thermal ones. Here, we provide a theory for using plasmonic molecular junctions to achieve this goal. We show how non-thermal electrons can be measured directly and separately from the unavoidable thermal response, and discuss the relevance of our theory to recent experiments.

cond-mat.mes-hall↗
arXiv2021

Matters Arising in -- "Plasmon-driven carbon-fluorine (C(sp^$3$)-F) bond activation with mechanistic insights into hot-carrier-mediated pathways"

In a recent paper~[Nature Catalysis 3, 573 (2020)], Robatjazi {\em et al.} demonstrate hydrodefluorination on Al nanocrystals decorated by Pd islands under illumination and under external heating. They conclude that photocatalysis accomplishes the desired transformation \ce{CH3F + D2 -> CH3D + DF} efficiently and selectively due to "hot" electrons, as evidenced by an illumination-induced reduction of the activation energy. Although some of the problems identified in prior work by the same group have been addressed, scrutiny of the data in~[Nature Catalysis 3, 573 (2020)] raises doubts about both the methodology and the central conclusions. First, we show that the thermal control experiments in~[Nature Catalysis 3, 573 (2020)] do not separate thermal from "hot electron" contributions, and therefore any conclusions drawn from these experiments are invalid. We then show that an improved thermal control implies that the activation energy of the reaction does not change, and that an independent purely thermal calculation (based solely on the sample parameters provided in the original manuscript) explains the measured data perfectly. For the sake of completeness, we also address technical problems in the calibration of the thermal camera, an unjustifiable disqualification of some of the measured data, as well as concerning aspects of the rest of the main results, including the mass spectrometry approach used to investigate the selectivity of the reaction, and claims about the stoichiometry and reaction order. All this shows that the burden of proof for involvement of hot electrons has not been met.

physics.chem-ph↗
arXiv2021

The role of heat generation and fluid flow in plasmon-enhanced reduction-oxidation reactions

Recently, we have shown that thermal effects play a crucial role in speeding up the rate of bond-dissociation reactions. This was done by applying a simple temperature-shifted Arrhenius Law to the experimental data, corroborated with detailed account of the heat diffusion occurring within the relevant samples and identification of errors in the temperature measurements. Here, we provide three important extensions of our previous studies. First, we analyze thermal effects in reduction-oxidation (redox) reactions, where charge transfer is an integral part of the reaction. Second, we analyze not only the spatial distribution of the temperature, but also its temporal dynamics. Third, we also model the fluid convection and stirring. An analysis of two exemplary experimental studies allows us to show that thermal effects can explain the experimental data in one of experiments (Baumberg and coworkers), but not in the other (Jain and coworkers), showing that redox reactions are not necessarily driven by non-thermal charge carriers.

physics.chem-ph↗
arXiv2020

Recent developments in plasmon-assisted photocatalysis -- a personal perspective

It has been known for many years that metallic nanoparticles can catalyse various chemical reactions, both in the dark and under illumination, through different mechanisms. In the last decade or so, many claims of plasmon-assisted "hot" electron driven catalysis of bond-dissociation reactions have been put forward. These claims were \XYZ{challenged} in a recent series of papers, where both the underlying theory of "hot" electron generation and the use of specific experimental setups to discover them in chemical reactions were examined in detail. The conclusion that arose from these works is that as long as temperature gradients exist inside the system (as for typical experimental setups) a quantification of non-thermal effects is close to impossible. Instead, a standard thermal theory was shown to be capable of explaining the experimental findings quite accurately. Here, we review the central lines of thought that led to these conclusions, from a personal perspective. We lay out the key aspects of the theory, and point to the specific caveats one must be aware of in performing photo-catalysis experiments. Finally, we provide some future directions of study.

physics.optics↗
arXiv2020

Optimization of second-harmonic generation from touching plasmonic wires

We employ transformation optics to optimize the generic nonlinear wave interaction of second-harmonic generation from a pair of touching metallic wires. We demonstrate a 10 orders of magnitude increase in the second-harmonic scattering cross-section by increasing the background permittivity and a 5 orders of magnitude increase in efficiency with respect to a single wire. These results have clear implications for the design of nanostructured metallic frequency-conversion devices. Finally, we exploit our analytic solution of a non-trivial nanophotonic geometry as a platform for performing a critical comparison of the strengths, weaknesses and validity of other prevailing theoretical approaches previously employed for nonlinear wave interactions at the nanoscale.

physics.optics↗
arXiv2020

Parametric study of temperature distribution in plasmon-assisted photocatalysis

Recently, there has been a growing interest in the usage of mm-scale composites of plasmonic nanoparticles for enhancing the rates of chemical reactions; the effect was shown recently to be predominantly associated with the elevated temperature caused by illumination. Here, we study the parametric dependence of the temperature distribution in these samples, and provide analytic expressions for simple cases. We show that since these systems are usually designed to absorb all the incoming light, the temperature distribution in them is weakly-dependent on the illumination spectrum, pulse duration, particle shape, size and density. Thus, changes in these parameters yield at most modest quantitative changes. We also show that the temperature distribution is linearly dependent on the beam radius and the thermal conductivity of the host. Finally, we study the sensitivity of the reaction rate to these parameters as a function of the activation energy and interpret various previous experimental reports. These results would simplify the optimization of photocatalysis experiments, as well as for other energy-related applications based on light harvesting for heat generation.

physics.app-ph↗
arXiv2020

Resolving the Gibbs phenomenon via a discontinuous basis in a mode solver for open optical systems

Partial differential equations are frequently solved using a global basis, such as the Fourier series, due to excellent convergence. However, convergence becomes impaired when discontinuities are present due to the Gibbs phenomenon, negatively impacting simulation speed and possibly generating spurious solutions. We resolve this by supplementing the smooth global basis with an inherently discontinuous basis, incorporating knowledge of the location of the discontinuities. The solution's discontinuities are reproduced with exponential convergence, expediting simulations. The highly constrained discontinuous basis also eliminates the freedom to generate spurious solutions. We employ the combined smooth and discontinuous bases to construct a solver for the modes of a resonator in an open electromagnetic system. These modes can then expand any scattering problem for any source configuration or incidence condition without further numerics, enabling ready access and physical insight into the spatial variation of Green's tensor. Solving for the modes is the most numerically intensive and difficult step of modal expansion methods, so our mode solver overcomes the last major impediment to the use of modal expansion for open systems.

physics.comp-ph↗
arXiv2020

Effective electron temperature measurement using time-resolved anti-Stokes photoluminescence

Anti-Stokes photoluminescence of metal nanoparticles, in which emitted photons have a higher energy than the incident photons, is an indicator of the temperature prevalent within a nanoparticle. Previous work has shown how to extract the temperature from a gold nanoparticle under continuous-wave monochromatic illumination. We extend the technique to pulsed illumination and introduce pump-probe anti-Stokes spectroscopy. This new technique enables us not only to measure an effective electron temperature in a gold nanoparticle ($\sim 10^3$ K under our conditions), but also to measure ultrafast dynamics of a pulse-excited electron population, through its effect on the photoluminescence, with sub-picosecond time resolution. We measure the heating and cooling, all within picoseconds, of the electrons and find that, with our sub-picosecond pulses, the highest apparent temperature is reached $0.6$ ps before the maximum change in magnitude of the extinction signal

physics.optics↗
arXiv2020

Ultrafast dynamics of optically-induced heat gratings in metals -- more complicated than expected

Diffusion of heat in metals is a fundamental process which is crucial for a variety of applications of metal nanostructures. Surprisingly, however, {\em ultrafast} heat diffusion received only limited attention so far. Here, we show that heat diffusion can be made faster than $e-ph$ energy transfer rate, in which case, it dominates the spatio-temporal dynamics of the temperature. This enables the metals to overcome the conventional limitations of the nonlinear optical response of materials - it can be simultaneously fast and strong. As a specific example, we identify the underlying (femtosecond and few picosecond) time scales responsible for the generation and erasure of optically-induced transient Bragg gratings in thin metal films. Further, we show that heat diffusion gives rise to a significant nonlocal thermo-optic nonlinearity - it affects also the nonlinear optical response such that the overall change of the permittivity (hence, reflectivity of the transient grating) has a significant dependence also on the illumination period rather than only on the illumination intensity.

physics.optics↗