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Optics Letters

Permanent dipole moments improve quantum coherence near plasmonic structures

Niya Petkova, Piotr Gładysz, Karolina Słowik

Published Nov 12, 2025• 0 citations
Publisher

Abstract

Plasmonic nanostructures enable strong, localized enhancement of electromagnetic fields, making them powerful platforms for mediating light interactions with quantum emitters (QEs) at the nanoscale. However, the spatial inhomogeneity of these fields imposes sub-nanometric precision requirements on emitter placement, limiting the robustness and scalability of quantum photonic systems. Here, we investigate how permanent dipole moments (PDMs) in polar QEs affect their optical response in such environments. In our model, PDMs introduce a nonlinear dependence of coupling strength and spontaneous emission rate on the local field amplitude. Our simulations of QEs near a plasmonic nanosphere show that PDMs can markedly reduce spatial variations in interaction strength and decay rate, yielding stabilized Rabi oscillations and extended coherence times even in strong-field gradients. Exploiting systems with PDMs thus offers a route to more robust QEs in confined optical fields, with potential benefits for quantum control, information processing, and nanophotonic device design.

DOI
10.1364/ol.576863

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Plasmonic nanostructures enable strong, localized enhancement of electromagnetic fields, making them powerful platforms for mediating light interactions with quantum emitters (QEs) at the nanoscale. However, the spatial inhomogeneity of these fields imposes sub-nanometric precision requirements on emitter placement, limiting the robustness and scalability of quantum photonic systems. Here, we investigate how permanent dipole moments (PDMs) in polar QEs affect their optical response in such environments. In our model, PDMs introduce a nonlinear dependence of coupling strength and spontaneous emission rate on the local field amplitude. Our simulations of QEs near a plasmonic nanosphere show that PDMs can markedly reduce spatial variations in interaction strength and decay rate, yielding stabilized Rabi oscillations and extended coherence times even in strong-field gradients. Exploiting systems with PDMs thus offers a route to more robust QEs in confined optical fields, with potential benefits for quantum control, information processing, and nanophotonic device design.
Spontaneous emission from ensembles of quantum emitters (QEs) such as atoms, molecules or semiconductor quantum dots can be greatly enhanced by cooperative effects arising from electromagnetic correlations. We describe a cooperative mechanism for emission of light by an ensemble of QEs based upon cooperative energy transfer from QEs to localized surface plasmons in metal-dielectric structures followed by plasmon radiation at a rate that scales with the ensemble size. For large QE ensembles saturating the plasmon mode volume, we derive universal, i.e. independent of local field distribution, expression for cooperative Purcell factor that can by far exceed the field enhancement limits for individual QE in a hot spot. We also derive radiated power spectrum that retains the plasmon resonance lineshape and, in contrast to common cooperative mechanisms, is insensitive to natural variations of QEs' emission frequencies.
We investigate the dynamics of polar systems coupled to classical external beams in the ultrastrong coupling regime. The permanent dipole moments (PDMs) sustained by polar systems can couple to the electromagnetic field, giving rise to a variety of processes such as difference frequency and harmonic generation, reflected in the complicated dynamics of the atomic system. Here, we demonstrate that the atomic evolution can be described simply in a dynamic reference frame. We derive a Jaynes-Cummings-like framework with effective parameters describing the frequency shift, coupling strength with the driving field, and a rescaled relaxation rate. The familiar linear scaling of the interaction strength with the field amplitude is replaced with a nonlinear dependence, suggesting potential applications for improving the coherence of quantum system ensembles with permanent dipoles.
Summary form only given. In the pioneering work of Purcell, the possibility to control spontaneous emission lifetimes of quantum emitters by tailoring their electromagnetic surroundings was widely investigated and verified in various types of cavities or band-gap environments. This includes semiconductor microstructures, photonic crystals, and plasmonic nanoparticles. Due to the extraordinary ability of plasmonic nanostructures to confine light into subwavelength spatial domains, steps beyond the paradigmatic electric dipole approximation in the theoretical description of this process may be necessary. In this contribution, we focus on the case of multiple in general nonidentical two-level emitters, sharing the same electromagnetic surroundings, whose local density of states could be tailored by a neighbouring nanostructure. In this case, the surroundings may serve as a long-distance emitter-emitter interaction carrier, or it may trigger collective emission processes, in analogy to Dicke superradiance. We describe these phenomena taking into account the electric dipole, magnetic dipole and electric quadrupole components of light-matter interaction Hamiltonian.
We develop a framework that computes two-photon spontaneous emission (TPSE) spectra of a quantum emitter near an arbitrarily shaped nanostructure. The model considers the interaction up to the electric quadrupolar order, which is relevant for nanophotonic structures sustaining strongly confined fields that are used to enhance and to tailor spontaneous emission processes. Moreover, we consider interference effects between multipolar two-photon emission channels, for the first time to our knowledge. First, we show for a s → s transition of a hydrogen atom placed under a silver plasmonic nanodisk a substantial enhancement in the photon-pair emission rates by 5 and 11 orders of magnitude for the two-electric dipole (2ED) and two-electric quadrupole (2EQ) transitions, respectively. Then for the same emitter under a plasmonic graphene nanotriangle, we demonstrate a breakdown of the electric dipole approximation in the TPSE process where the interference between the 2ED and 2EQ transitions is important, as it increases the total rate by 63 %. Third, we explore platforms where entangled photons of different energy are emitted in the far-field in different directions. In the end, our framework is a complete tool to design emitters and nanostructures for the TPSE process, leading to a rich assortment of functional nanoantennas.
Hybridization of quantum emitters and plasmonic nanostructures has attracted much attention over the last years, due to their potential use as plasmon-based nanolasersor to achieve long-range quantum bit entanglement. Recent theoretical studies suggest that the plasmonic field can induce efficient cross-talking between emitters and lead to the formation of collective superradiant states. In this thesis, we developed a theoretical modelable to analyse collective effects in large ensemble of dipoles coupled by an electromagnetic nanoresonator. We experimentally investigated the plasmon-mediated superradiance of organic emitters grafted at a well-controlled distance from a metal nanosphere at room temperature. We report on the measured decay rates of these hybrid structures at the ensemble and single object levels. We find that the decay rate increases i) with the number ofemitters and ii) as the spacing between the emitters and the metal core decreases, a direct and clear evidence of plasmonic superradiance. This trend was observed for two types of hybrid structures, differing both by the size of the metal core and the type of organic dye used as emitter. The observation of plasmonic superradiance at room temperature opens questions about the robustness of these collective states against decoherence mechanisms.This robustness is of major interest for potential applications of quantum systems at room temperature.
Electrically tunable graphene plasmons are anticipated to enable strong light-matter interactions with resonant quantum emitters. However, plasmon resonances in graphene are typically limited to infrared frequencies, below those of optical excitations in robust quantum light sources and many biologically interesting molecules. Here we propose to utilize near fields generated by the plasmon-assisted nonlinear optical response of nanostructured graphene to resonantly couple with proximal quantum emitters operating in the near infrared. We show that the nonlinear near field produced by a graphene nanodisk can strongly excite and coherently control quantum states in two- and three-level atomic systems when the third harmonic of its plasmon resonance is tuned to a particular electronic transition. In the present scheme, emitter and plasmon resonances are nondegenerate, circumventing strong enhancement of spontaneous emission. We envision potential applications for the proposed nonlinear plasmonic coupling scheme in sensing and temporal quantum control.