arXiv subjects

A. I. Fernández-Domínguez

Publications and source records attributed to A. I. Fernández-Domínguez.

At least 19 recordsLinked to original sources

arXiv2025

Constructing qubit edge states by inverse-designing the electromagnetic environment

Building on advances in topological photonics and computational optimization, we inversedesign a periodic dielectric structure surrounding a chain of interacting qubits, emulating an extended, dimerized Su-Schrieffer-Heeger (SSH) excitonic model. Our approach enables precise control over photon-mediated interactions, allowing us to explore the emergence of topological edge states in the qubit chain. By systematically tuning structural parameters to address both coherent evolution and dissipative effects, we demonstrate that edge states remain robust and isolated from the bulk, even in the presence of long-range coupling and disorder, and preserving key topological properties despite deviations from complete chiral symmetry preservation. This work highlights the potential of inverse design in stabilizing topological excitonic states, opening new possibilities for advanced quantum technologies.

quant-ph↗

arXiv2024

Multi-qubit quantum state preparation enabled by topology optimization

Using topology optimization, we inverse-design nanophotonic cavities enabling the preparation of pure states of pairs and triples of quantum emitters. Our devices involve moderate values of the dielectric constant, operate under continuous laser driving, and yield fidelities to the target (Bell and W) states approaching unity for distant qubits (several natural wavelengths apart). In the fidelity optimization procedure, our algorithm generates entanglement by maximizing the dissipative coupling between the emitters, which allows the formation of multipartite pure steady states in the driven-dissipative dynamics of the system. Our findings open the way towards the efficient and fast preparation of multiqubit quantum states with engineered features, with potential applications for nonclassical light generation, quantum simulation, and quantum sensing.

quant-ph↗

arXiv2023

Electron-assisted manipulation of polaritonic light-matter states

Thanks to their exceptional spatial, spectral and temporal resolution, highly-coherent free-electron beams have emerged as powerful probes for material excitations, enabling their characterization even in the quantum regime. Here, we investigate strong light-matter coupling through monochromatic and modulated electron wavepackets. In particular, we consider an archetypal target, comprising a nanophotonic cavity next to a single two-level emitter. We propose a model Hamiltonian describing the coherent interaction between the passing electron beam and the hybrid photonic-excitonic target, which is constructed using macroscopic quantum electrodynamics and fully parameterized in terms of the electromagnetic Dyadic Green's function. Using this framework, we first describe electron-energy-loss and cathodoluminescence spectroscopies, and photon-induced near-field electron emission microscopy. Finally, we show the power of modulated electrons beams as quantum tools for the manipulation of polaritonic targets presenting a complex energy landscape of excitations.

quant-ph↗

arXiv2023

Resonance fluorescence of two asymmetrically pumped and coupled two-level systems

We study a driven-dissipative duo of two-level systems in an open quantum systems approach, modelling a pair of atoms or (more generally) meta-atoms. Allowing for complex-valued couplings in the setup, which are of both a coherent and incoherent character, gives rise to a diverse coupling landscape. We consider several points on this landscape, for example where the coupling between the two coupled two-level systems is dominated by coherent, incoherent, unsymmetrical and even unidirectional interactions. Traversing the coupling terrain leads to remarkable features in the populations of the pair, correlations and optical spectra. Most notably, the famous Mollow triplet spectrum for a single atom may be superseded for a pair by a Mollow quintuplet (or even by a spectral singlet) and the setup allows for population trapping to arise, all depending upon the precise nature of the coupling between the two-level systems.

quant-ph↗

arXiv2022

Inverse-designed dielectric cloaks for entanglement generation

We investigate the generation of entanglement between two quantum emitters through the inverse-design engineering of their photonic environment. By means of a topology-optimization approach acting at the level of the electromagnetic Dyadic Green's function, we generate dielectric cloaks operating at different inter-emitter distances and incoherent pumping strengths. We show that the structures obtained maximize the dissipative coupling between the emitters under extremely different Purcell factor conditions, and yield steady-state concurrence values much larger than those attainable in free space. Finally, we benchmark our design strategy by proving that the entanglement enabled by our devices approaches the limit of maximum-entangled-mixed-states.

quant-ph↗

arXiv2021

Plexcitonic quantum light emission from nanoparticle-on-mirror cavities

We investigate the quantum-optical properties of the light emitted by a nanoparticle-on-mirror cavity filled with a single quantum emitter. Inspired by recent experiments, we model a dark-field set-up and explore the photon statistics of the scattered light under grazing laser illumination. Exploiting analytical solutions to Maxwell's equations, we quantize the nanophotonic cavity fields and describe the formation of plasmon exciton polaritons (or plexcitons) in the system. This way, we reveal that the rich plasmonic spectrum of the nanocavity offers unexplored mechanisms for nonclassical light generation that are more efficient than the resonant interaction between the emitter natural transition and the brightest optical mode. Specifically, we find three different sample configurations in which strongly antibunched light is produced. Finally, we illustrate the power of our approach by showing that the introduction of a second emitter in the platform can enhance photon correlations further.

cond-mat.mes-hall↗

arXiv2020

Theory of Energy Transfer in Organic Nanocrystals

Recent experiments have shown that highly efficient energy transfer can take place in organic nanocrystals at extremely low acceptor densities. This striking phenomenon has been ascribed to the formation of exciton polaritons thanks to the photon confinement provided by the crystal itself. We propose an alternative theoretical model that accurately reproduces fluorescence lifetime and spectrum measurements in these systems without such an assumption. Our approach treats molecule-photon interactions in the weak-coupling regime, and describes the donor and acceptor population dynamics by means of rate equations with parameters extracted from electromagnetic simulations. The physical insight and predictive value of our model also enables us to propose nanocrystal configurations in which acceptor emission dominates the fluorescence spectrum at densities orders of magnitude lower than the experimental ones.

physics.chem-ph↗

arXiv2020

Asymmetric coupling between two quantum emitters

We study a prototypical model of two coupled two-level systems, where the competition between coherent and dissipative coupling gives rise to a rich phenomenology. In particular, we analyze the case of asymmetric coupling, as well as the limiting case of chiral (or one-way) coupling. We investigate various quantum optical properties of the system, including its steady state populations, power spectrum, and second-order correlation functions, and outline the characteristic features which emerge in each quantity as one sweeps through the nontrivial landscape of effective complex couplings. Most importantly, we reveal instances of population trapping, unexpected spectral features and strong photon correlations.

cond-mat.mes-hall↗

arXiv2020

On the applicability of quantum-optical concepts in strong-coupling nanophotonics

Rooted in quantum optics and benefiting from its well-established foundations, strong coupling in nanophotonics has experienced increasing popularity in recent years. With nanophotonics being an experiment-driven field, the absence of appropriate theoretical methods to describe ground-breaking advances has often emerged as an important issue. To address this problem, the temptation to directly transfer and extend concepts already available from quantum optics is strong, even if a rigorous justification is not always available. In this Review we discuss situations where, in our view, this strategy has indeed overstepped its bounds. We focus on exciton--plasmon interactions, and particularly on the idea of calculating the number of excitons involved in the coupling. We analyse how, starting from an unfounded interpretation of the term N/V that appears in theoretical descriptions at different levels of complexity, one might be tempted to make independent assumptions for what the number N and the volume V are, and attempt to calculate them separately. Such an approach can lead to different, often contradictory results, depending on the initial assumptions (e.g. through different treatments of $V$ as the -- ambiguous in plasmonics -- mode volume). We argue that the source of such contradictions is the question itself -- How many excitons are coupled?, which disregards the true nature of the coupled components of the system, has no meaning and often not even any practical importance. If one is interested in validating the quantum nature of the system -- which appears to be the motivation driving the pursuit of strong coupling with small N -- one could instead focus on quantities such as the photon emission rate or the second-order correlation function.

cond-mat.mes-hall↗

arXiv2020

Plasmonic Purcell Effect in Organic Molecules

By means of quantum tensor network calculations, we investigate the large Purcell effect experienced by an organic molecule placed in the vicinity of a plasmonic nanostructure. In particular, we consider a donor-π bridge-acceptor dye at the gap of two Ag nanospheres. Our theoretical approach allows for a realistic description of the continua of both molecular vibrations and optical nanocavity modes. We analyze both the exciton dynamics and the corresponding emission spectrum, showing that these magnitudes are not accurately represented by the simplified models used up to date. By disentangling the molecule coupling to radiative and non-radiative plasmonic modes, we also shed light into the quenching phenomenology taking place in the system.

cond-mat.mes-hall↗

arXiv2019

Plasmon-assisted Förster resonance energy transfer at the single-molecule level in the moderate quenching regime

Metallic nanoparticles were shown to affect Förster energy transfer between fluorophore pairs. However, to date, the net plasmonic effect on FRET is still under dispute, with experiments showing efficiency enhancement and reduction. This controversy is due to the challenges involved in the precise positioning of FRET pairs in the near field of a metallic nanostructure, as well as in the accurate characterization of the plasmonic impact on the FRET mechanism. Here, we use the DNA origami technique to place a FRET pair 10 nm away from the surface of gold nanoparticles with sizes ranging from 5 to 20 nm. In this configuration, the fluorophores experience only moderate plasmonic quenching. We use the acceptor bleaching approach to extract the FRET rate constant and efficiency on immobilized single FRET pairs based solely on the donor lifetime. This technique does not require a posteriori correction factors neither a priori knowledge of the acceptor quantum yield, and importantly, it is performed in a single spectral channel. Our results allow us to conclude that, despite the plasmon-assisted Purcell enhancement experienced by donor and acceptor partners, the gold nanoparticles in our samples have a negligible effect on the FRET rate, which in turns yields a reduction of the transfer efficiency.

physics.chem-ph↗

arXiv2019

Dipolar and Quadrupolar Excitons Coupled to a Nanoparticle-on-a-Mirror Cavity

We investigate plasmon-emitter interactions in a nanoparticle-on-a-mirror cavity. We consider two different sorts of emitters, those that sustain dipolar transitions, and those hosting only quadrupolar, dipole-inactive, excitons. By means of a fully analytical two-dimensional transformation optics approach, we calculate the light-matter coupling strengths for the full plasmonic spectrum supported by the nanocavity. We reveal the impact of finite-size effects in the exciton charge distribution and describe the population dynamics in a spontaneous emission configuration. Pushing our model beyond the quasi-static approximation, we extract the plasmonic dipole moments, which enables us to calculate the far-field scattering spectrum of the hybrid plasmon-emitter system. Our findings, tested against fully numerical simulations, reveal the similarities and differences between the strong coupling phenomenology for bright and dark excitons in nanocavities.

cond-mat.mes-hall↗

arXiv2019

Quasi-Chiral Interactions between Quantum Emitters at the Nanoscale

We present a combined classical and quantum electrodynamics description of the coupling between two circularly-polarized quantum emitters held above a metal surface supporting surface plasmons. Depending on their position and their natural frequency, the emitter-emitter interactions evolve from being reciprocal to nonreciprocal, which makes the system a highly tunable platform for chiral coupling at the nanoscale. By relaxing the stringent material and geometrical constraints for chirality, we explore the interplay between coherent and dissipative coupling mechanisms in the system. Thus, we reveal a quasi-chiral regime in which its quantum optical properties are governed by its subradiant state, giving rise to extremely sharp spectral features and strong photon correlations.

cond-mat.mes-hall↗

arXiv2018

Magnetic-field controlled anomalous refraction in doped semiconductors

We predict here that a slab made of a doped semiconductor can exhibit anomalous refraction under the application of a static magnetic field. This anomalous refraction takes place in the far-infrared range and it occurs for any angle of incidence. We show that this effect is due to the fact that a doped semiconductor under a magnetic field can behave, to some extent, as a hyperbolic metamaterial. We also show that the occurrence of this anomalous refraction enables a semiconductor slab under a magnetic field to partially focus the electromagnetic radiation. The remarkable thing in our case is that we deal with naturally occurring materials and the anomalous refraction can be tuned at will with an external field.

physics.optics↗

arXiv2018

Organic Polaritons Enable Local Vibrations to Drive Long-Range Energy Transfer

Long-range energy transfer in organic molecules has been experimentally obtained by strongly coupling their electronic excitations to a confined electromagnetic cavity mode. Here, we shed light into the polariton-mediated mechanism behind this process for different configurations: donor and acceptor molecules either intermixed or physically separated. We numerically address the phenomenon by means of Bloch-Redfield theory, which allows us to reproduce the effect of complex vibrational reservoirs characteristic of organic molecules. Our findings reveal the key role played by the middle polariton as the non-local intermediary in the transmission of excitations from donor to acceptor molecules. We also provide analytical insight on the key physical magnitudes that helps to optimize the efficiency of the long-range energy transfer.

quant-ph↗

arXiv2018

Light-Forbidden Transitions in Plasmon-Emitter Coupling

We investigate the impact that light-forbidden exciton transitions have in the near-field population dynamics and far-field scattering spectrum of hybrid plasmon-emitter systems. Specifically, we consider a V-type quantum emitter, sustaining one dipolar and one quadrupolar (dipole-inactive) excited states, placed at the nanometric gap of a particle-on-a-mirror metallic cavity. Our fully analytical description of plasmon-exciton coupling for both exciton transitions enables us to reveal the conditions in which the presence of the latter greatly alters the Purcell enhancement and Rabi splitting phenomenology in the system.

cond-mat.mes-hall↗

arXiv2018

Photon Statistics in Collective Strong Coupling: Nano- and Microcavities

There exists a growing interest in the properties of the light generated by hybrid systems involving a mesoscopic number of emitters as a means of providing macroscopic quantum light sources. In this work, the quantum correlations of the light emitted by a collection of emitters coupled to a generic optical cavity are studied theoretically using an effective Hamiltonian approach. Starting from the single-emitter level, we analyse the persistence of photon antibunching as the ensemble size increases. Not only is the photon blockade effect identifiable, but photon antibunching originated from destructive interference processes (the so-called unconventional antibunching) is also present. We study the dependence of these two types of negative correlations on the spectral detuning between cavity and emitters, as well as its evolution as the time delay between photon detections increases. Throughout this work, the performance of plasmonic nanocavities and dielectric microcavities is compared: despite the distinct energy scales and the differences introduced by their respectively open and closed character, the bunching and antibunching phenomenology presents remarkable similarities in both types of cavities.

cond-mat.mes-hall↗

arXiv2017

Enhancing Photon Correlations through Plasmonic Strong Coupling

There is an increasing scientific and technological interest on the design and implementation of nanoscale sources of quantum light. Here, we investigate the quantum statistics of the light scattered from a plasmonic nanocavity coupled to a mesoscopic ensemble of emitters under low coherent pumping. We present an analytical description of the intensity correlations taking place in these systems, and unveil the fingerprint of plasmon-exciton-polaritons in them. Our findings reveal that plasmonic cavities are able to retain and enhance excitonic nonlinearities even when the number of emitters is large. This makes plasmonic strong coupling a promising route for generating nonclassical light beyond the single emitter level.

cond-mat.mes-hall↗