5 research outputs found
Quantum-informed plasmonics for strong coupling: the role of electron spill-out
The effect of nonlocality on the optical response of metals lies at the
forefront of research in nanoscale physics and, in particular, quantum
plasmonics. In alkali metals, nonlocality manifests predominantly as electron
density spill-out at the metal boundary, and as surface-enabled Landau damping.
For an accurate description of plasmonic modes, these effects need be taken
into account in the theoretical modelling of the material. The resulting modal
frequency shifts and broadening become particularly relevant when dealing with
the strong interaction between plasmons and excitons, where hybrid modes emerge
and the way they are affected can reflect modifications of the coupling
strength. Both nonlocal phenomena can be incorporated in the classical local
theory by applying a surface-response formalism embodied by the Feibelman
parameters. Here, we implement surface-response corrections in Mie theory to
study the optical response of spherical plasmonic--excitonic composites in
core--shell configurations. We investigate sodium, a jellium metal dominated by
spill-out, for which it has been anticipated that nonlocal corrections should
lead to an observable change in the coupling strength, appearing as a
modification of the width of the mode splitting. We show that, contrary to
expectations, the influence of nonlocality on the anticrossing is minimal, thus
validating the accuracy of the local response approximation in strong-coupling
photonics
Electron beams traversing spherical nanoparticles: analytic and numerical treatment
We present an analytic, Mie-theory based solution for the energy-loss and the
photon-emission probabilities in the interaction of spherical nanoparticles
with electrons passing nearby and through them, in both cathodoluminescence
(CL) and electron energy-loss spectroscopies (EELS). In particular, we focus on
the case of penetrating electron trajectories, for which the complete fully
electrodynamic and relativistic formalism has not been reported as yet. We
exhibit the efficiency of this method in describing collective excitations in
matter through calculations for a dispersive and lossy system, namely a sphere
described by a Drude permittivity, and discuss possible complications when
computing contributions from higher-order modes. Subsequently, we use the
analytic solution to corroborate the implementation of electron-beam sources in
a state-of-the-art numerical method methods. We show that the two approaches
produce spectra in good mutual agreement, and demonstrate the versatility of
DGTD via simulations of spherical nanoparticles characterized by surface
roughness. The possibility of simultaneously employing both kinds of
calculations (analytic and numerical) facilitates a better understanding of the
rich optical response of nanophotonic architectures excited by fast electron
beams.Comment: 10 pages, 6 figure
Quantum surface effects in the electromagnetic coupling between a quantum emitter and a plasmonic nanoantenna: time-dependent density functional theory vs. semiclassical Feibelman approach
We use time-dependent density functional theory (TDDFT) within the jellium model to study the impact of quantum-mechanical effects on the self-interaction Green’s function that governs the electromagnetic interaction between quantum emitters and plasmonic metallic nanoantennas. A semiclassical model based on the Feibelman parameters, which incorporates quantum surface-response corrections into an otherwise classical description, confirms surface-enabled Landau damping and the spill out of the induced charges as the dominant quantum mechanisms strongly affecting the nanoantenna–emitter interaction. These quantum effects produce a redshift and broadening of plasmonic resonances not present in classical theories that consider a local dielectric response of the metals. We show that the Feibelman approach correctly reproduces the nonlocal surface response obtained by full quantum TDDFT calculations for most nanoantenna–emitter configurations. However, when the emitter is located in very close proximity to the nanoantenna surface, we show that the standard Feibelman approach fails, requiring an implementation that explicitly accounts for the nonlocality of the surface response in the direction parallel to the surface. Our study thus provides a fundamental description of the electromagnetic coupling between plasmonic nanoantennas and quantum emitters at the nanoscale.Villum Fonden (16498); Department of Education of the Basque Government (IT1164-19); MCIN/AEI/10.13039/501100011033/ (PID2019-107432GB-I00). The Department of Education of the Basque Government for a predoctoral fellowship (Grant No.
PRE2017_1_0267). N. A. M. is a VILLUM Investigator supported by VILLUM FONDEN (Grant No. 16498). P. E. S. is the recipient of the Zonta Denmark’s Scholarship for female PhD students in Science and Technology 2021.Peer reviewe
Disentangling Cathodoluminescence Spectra in Nanophotonics:Particle Eigenmodes vs Transition Radiation
Cathodoluminescence spectroscopy performed in an electron microscope has
proven a versatile tool for analysing the near- and far-field optical response
of plasmonic and dielectric nanostructures. Nevertheless, the transition
radiation produced by electron impact is often disregarded in the
interpretation of the spectra recorded from resonant nanoparticles. Here we
show, experimentally and theoretically, that transition radiation can by itself
generate distinct resonances which, depending on the time of flight of the
electron beam inside the particle, can result from constructive or destructive
interference in time. Superimposed on the eigenmodes of the investigated
structures, these resonances can distort the recorded spectrum and lead to
potentially erroneous assignment of modal characters to the spectral features.
We develop an intuitive analogy that helps distinguish between the two
contributions. As an example, we focus on the case of silicon nanospheres, and
show that our analysis facilitates the unambiguous interpretation of
experimental measurements on Mie-resonant nanoparticles