arXiv subjects

Ieng-Wai Un

Publications and source records attributed to Ieng-Wai Un.

8 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↗

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

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↗

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↗

arXiv2019

Size-Dependence of the photothermal response of a single metal nanosphere

We study the thermal response of a single spherical metal nanoparticle to continuous wave illumination as a function of its size. We show that the particle temperature increases non-monotonically as the particle size increases, indicating that the photo-thermal response can be optimized by tuning the particle size and illumination wavelength. We also compare the size-effect on the photo-thermal effects of gold and silver nanoparticles and find somewhat surprisingly that Ag NPs are more efficient heat generators only for sufficiently small sizes. These results have importance primarily for application such as plasmon-assisted photo-catalysis, photothermal cancer therapy, etc., and provide a first step toward the study of the size-dependence of the thermo-optic nonlinearity of metal nanospheres.

physics.optics↗