arXiv subjects

Thomas F. Allard

Publications and source records attributed to Thomas F. Allard.

10 recordsLinked to original sources

arXiv2026

Microscopic quantum description of surface plasmon polaritons: Revealing intrinsic ultrastrong light-matter coupling

We develop a microscopic quantum theory of surface plasmon polaritons valid for arbitrary metal-dielectric geometries. Our framework is based on the Power-Zienau-Woolley representation of quantum electrodynamics, which provides an optimal separation between electronic and photonic degrees of freedom and is therefore particularly well suited for constructing quantum descriptions of polaritonic excitations in strongly dispersive media. Within this formulation, the fundamental electronic oscillator is identified as the bulk plasmon mode, which is nonperturbatively coupled to the radiative continuum of free photon modes. This coupling induces a geometry-dependent renormalization of the bulk plasma frequency, giving rise to confined plasmonic resonances. As specific applications, we recover the localized surface plasmon modes of metallic nanoparticles, including radiative frequency shifts and decay, as well as the exact dispersion relation of propagating surface plasmon polaritons at planar interfaces. Our quantum treatment further reveals that light-matter interactions at metal-dielectric interfaces are inherently in the ultrastrong coupling regime. As a result, in the quasistatic limit, the system exhibits unconventional ground-state quantum fluctuations that can be controlled through the refractive index. These results open new intriguing perspectives in the field of quantum plasmonics.

physics.optics↗

arXiv2026

Fluctuational Quantum Electrodynamics of Dispersive Time-Varying Media

We present the theoretical framework of fluctuational quantum electrodynamics in frequency-dispersive and dissipative time-varying media. Our theory accounts for dispersion and losses in the temporal modulation, which is treated in an exact manner, without relying on perturbative methods. Thus, our work constitutes the first consistent quantization of the electromagnetic field in time-modulated material bodies. We derive a Fermi Golden Rule for time-varying media and use it to define the local density of states for these time-dependent systems, which includes both loss and gain contributions. Additionally, we prove the equivalence between the quantum Fermi Golden Rule and the power emitted by a classical harmonic point dipole. Moreover, we show that neglecting the dispersive and dissipative nature of the time modulation leads to erroneous predictions for both slow and fast modulations. Furthermore, we analyze the thermal radiation emitted by a time-varying material body, revealing new features in the enhancement of thermal emission in time-varying media. Finally, we study the dynamical Casimir effect, showing how the time modulation amplifies vacuum fluctuations and generates entangled pairs of polaritons exhibiting non-local spatial correlations.

physics.optics↗

arXiv2026

Reentrant localization transition in a dimerized quasiperiodic dipolar chain

Reentrant localization transitions, that is, the transitions of a portion of the eigenspectrum from localized to critical and then again to localized as the quasiperiodic modulation strength is increased, have been recently unveiled in various quasiperiodic models. However, both the physical mechanisms underlying these transitions and how they may extend to systems with long-range coupling and dissipation remain elusive. Here we investigate the fate of such a phenomenon in a dimerized quasiperiodic chain of lossy dipolar emitters with all-to-all coupling. We demonstrate that in this model, reentrant transitions survive to all-to-all couplings and occur from an interplay between the chain dimerization and an asymmetric quasiperiodic modulation of the emitter spacings. Transport simulations through a driven-dissipative open quantum system approach complete our study and reveal the detrimental effects of emitter losses on the reentrant localization transition.

cond-mat.mes-hall↗

arXiv2026

Broadband Dipole Absorption in Dispersive Photonic Time Crystals

Photonic media modulated periodically in time, termed photonic time crystals (PTCs), have attracted considerable attention for their ability to open momentum bandgaps hosting amplifying modes. These momentum gaps, however, generally appear only at the system's parametric resonance condition which constrain many features derived from amplification to a narrow frequency band. Moreover, they are accompanied by exceptional points (EPs) and may drive the system into an instability, which render their analysis more intricate. Here, we show that a careful consideration of dispersion and absorption can overcome these issues. By investigating the dissipated power of a point-dipole embedded in a dispersive and absorptive PTC, we unveil that temporal modulation enables the conversion of dipole emission into dipole absorption within a broadband frequency window free of EPs. We demonstrate that this effect is general, emerging in both the stable and unstable regimes, and occurs from weak modulation strength to low modulations frequencies that could be achieved for various material platforms.

physics.optics↗

arXiv2026

Interface States in Space-Time Photonic Crystals: Topological Origin, Propagation and Amplification

Studying the topology of spatiotemporal media poses a fundamental challenge: their remarkable properties stem from breaking spatial and temporal symmetries, yet this same breaking obscures their topological characterization. Here, we show that space-time symmetries persist in crystals with travelling-wave modulations whose velocities can be either lower (subluminal) or higher (superluminal) than the speed of light, enabling the study of their topological properties and the prediction of spatiotemporal interface states. For each modulation regime, we use a Lorentz transformation to a frame in which the modulation depends on only one of the transformed variables. Then, we identify a conserved joint parity-time-reversal symmetry in the new variables that enforces the quantization of a spatiotemporal Zak phase, elevating it to a $\mathbb{Z}_2$ topological invariant. Finally, we calculate the associated interface states and uncover unique features arising from time-varying effects, including selective directional amplification, propagation along subluminal and superluminal boundaries, frequency- and momentum-converted replicas, and broadband amplification even in the absence of momentum gaps. Our framework holds for spatiotemporal modulations of any velocity, unifying a wide class of systems that includes photonic time crystals, and clarifying their topological origin.

physics.optics↗

arXiv2025

Near-Field Gain and Far-Field Control via a Plasmonic Time Crystal Slab

Light matter interactions can be substantially altered in the presence of time varying media. We study the interaction between a harmonic electric dipole and a plasmonic time crystal slab. Temporal modulation of the plasma frequency enables near field gain, allowing the dipole to absorb rather than emit energy, suppressing nonradiative losses. At the parametric resonance condition, the slab radiates strongly to the far field, producing hundred per cent oscillations in the radiated power at distances up to a thousand times the epsilon near zero wavelength. These findings reveal a new mechanism for controlling light matter interaction in time varying plasmonic systems

physics.optics↗

arXiv2024

Mirror-induced effects in cavity polaritonics: Influence on edge states

Optical cavities are widely used to induce strong light-matter coupling and thereby enable the presence of polaritons. While polaritons are at the source of most of the observed physics, the mirrors forming the cavity may also themselves be responsible for a number of phenomena, independently of the strong light-matter coupling regime. Here we use a toy model of a chain of dipolar emitters coupled to a cuboidal cavity. We unveil several effects originating solely from the boundary conditions imposed by the cavity mirrors, that are dominant when the distances of the emitters to the cavity walls are of the order of the interdipole separation. In particular, we show that mirrors in the direction transverse to the chain may act as effective defects, leading to the emergence of Tamm edge states. Considering a topological chain, we demonstrate that such transverse mirrors may also protect edge states against the effects of the strong light-matter coupling. Finally, we find that mirrors parallel to the chain, by the image charges they involve, induce topological phase transitions even in the case of highly off-resonant photons.

cond-mat.mes-hall↗

arXiv2023

Multiple polaritonic edge states in a Su-Schrieffer-Heeger chain strongly coupled to a multimode cavity

A dimerized chain of dipolar emitters strongly coupled to a multimode optical waveguide cavity is studied. By integrating out the photonic degrees of freedom of the cavity, the system is recast in a two-band model with an effective coupling, so that it mimics a variation of the paradigmatic Su-Schrieffer-Heeger model, which features a nontrivial topological phase and hosts topological edge states. In the strong-coupling regime, the cavity photons hybridize the bright dipolar bulk band into a polaritonic one, renormalizing the eigenspectrum and strongly breaking chiral symmetry. This leads to a formal loss of the in-gap edge states present in the topological phase while they merge into the polaritonic bulk band. Interestingly, however, we find that bulk polaritons entering in resonance with the edge states inherit part of their localization properties, so that multiple polaritonic edge states are observed. Although these states are not fully localized on the edges, they present unusual properties. In particular, due to their delocalized bulk part, owing from their polaritonic nature, such edge states exhibit efficient edge-to-edge transport characteristics. Instead of being degenerate, they occupy a large portion of the spectrum, allowing one to probe them in a wide driving frequency range. Moreover, being reminiscent of symmetry-protected topological edge states, they feature a strong tolerance to positional disorder.

cond-mat.mes-hall↗

arXiv2023

Disorder-enhanced transport in a chain of lossy dipoles strongly coupled to cavity photons

We study the interplay between disorder and light-matter coupling by considering a disordered one-dimensional chain of lossy dipoles coupled to a multimode optical cavity, through a microscopically derived Hamiltonian. Such a system, hosting polaritonic excitations, may be realized experimentally in a wide range of platforms under strong light-matter coupling. By analyzing both the eigenspectrum and the driven-dissipative transport properties of our system, we find that in the strong-coupling regime, increasing disorder leads almost uncoupled dark states to acquire a photonic part, allowing them to inherit polaritonic long-range transport characteristics. Crucially, we show that this disorder-enhanced transport mechanism is increasingly noticeable when the considered dipoles are lossier.

cond-mat.mes-hall↗

arXiv2021

Quantum theory of plasmon polaritons in chains of metallic nanoparticles: From near- to far-field coupling regime

We develop a quantum theory of plasmon polaritons in chains of metallic nanoparticles, describing both near- and far-field interparticle distances, by including plasmon-photon Umklapp processes. Taking into account the retardation effects of the long-range dipole-dipole interaction between the nanoparticles, which are induced by the coupling of the plasmonic degrees of freedom to the photonic continuum, we reveal the polaritonic nature of the normal modes of the system. We compute the dispersion relation and radiative linewidth, as well as the group velocities of the eigenmodes, and compare our numerical results to classical electrodynamic calculations within the point-dipole approximation. Interestingly, the group velocities of the polaritonic excitations present an almost periodic sign change and are found to be highly tunable by modifying the spacing between the nanoparticles. We show that, away from the intersection of the plasmonic eigenfrequencies with the free photon dispersion, an analytical perturbative treatment of the light-matter interaction is in excellent agreement with our fully retarded numerical calculations. We further study quantitatively the hybridization of light and matter excitations, through an analysis of Hopfield's coefficients. Finally, we consider the limit of infinitely spaced nanoparticles and discuss some recent results on single nanoparticles that can be found in the literature.

physics.optics↗