Visible quantum plasmonics from metallic nanodimers

We report theoretical evidence that bulk nonlinear materials weakly interacting with highly localized plasmonic modes in ultra-sub-wavelength metallic nanostructures can lead to nonlinear effects at the single plasmon level in the visible range. In particular, the two-plasmon interaction energy in such systems is numerically estimated to be comparable with the typical plasmon linewidths. Localized surface plasmons are thus predicted to exhibit a purely nonclassical behavior, which can be clearly identified by a sub-Poissonian second-order correlation in the signal scattered from the quantized plasmonic field under coherent electromagnetic excitation. We explicitly show that systems sensitive to single-plasmon scattering can be experimentally realized by combining electromagnetic confinement in the interstitial region of gold nanodimers with local infiltration or deposition of ordinary nonlinear materials. We also propose configurations that could allow to realistically detect such an effect with state-of-the-art technology, overcoming the limitations imposed by the short plasmonic lifetime

Josephson surface plasmons in spatially confined cuprate superconductors

In this work, we generalize the theory of localized surface plasmons to the case of high-Tc cuprate superconductors, spatially confined in the form of small spherical particles. At variance from ordinary metals, cuprate superconductors are characterized by a low-energy bulk excitation known as the Josephson plasma wave (JPW), arising from interlayer tunneling of the condensate along the c-axis. The effect of the JPW is revealed in a characteristic spectrum of surface excitations, which we call Josephson surface plasmons. Our results, which apply to any material with a strongly anisotropic electromagnetic response, are worked out in detail for the case of multilayered superconductors supporting both low-frequency (acoustic) and transverse-optical JPW. Spatial confinement of the Josephson plasma waves may represent a new degree of freedom to engineer their frequencies and to explore the link between interlayer tunnelling and high-Tc superconductivity

Quantum Theory of Surface Plasmon Polaritons: Planar and Spherical Geometries

A quantum theory of retarded surface plasmons on a metal-vacuum interface is formulated, by analogy with the well-known and widely exploited theory of exciton-polaritons. The Hamiltonian for mutually interacting instantaneous surface plasmons and transverse electromagnetic modes is diagonalized with recourse to a Hopfield-Bogoljubov transformation, in order to obtain a new family of modes, to be identified with retarded plasmons. The interaction with nearby dipolar emitters is treated with a full quantum formalism based on a general definition of modal effective volumes. The illustrative cases of a planar surface and of a spherical nanoparticle are considered in detail. In the ideal situation of absence of dissipation, as an effect of the conservation of in-plane wavevector, retarded plasmons on a planar surface represent true stationary states (which are usually called surface plasmon polaritons), whereas retarded plasmons in a spherical nanoparticle, characterized by frequencies that overlap with the transverse electromagnetic continuum, become resonances with a finite radiative broadening. The theory presented constitutes a suitable full quantum framework for the study of nonperturbative and nonlinear effects in plasmonic nanosystems

Semiclassical theory of multisubband plasmons: Nonlocal electrodynamics and radiative effects

Coherent multisubband plasmons in doped semiconductor quantum wells have recently attracted large interest as they allow us to strongly enhance light-matter interaction via collective Coulomb coupling among different intersubband transitions. In this work, we develop a semiclassical theory of intersubband plasmons in quantum wells, on the basis of nonlocal electrodynamics. The nonlocal treatment provides a proper description of collective effects in the electromagnetic response of the system and, in the long-wavelength approximation, it predicts the same resonance frequencies as the quantum mechanical description. The nonlocal formalism is applied to the study of the radiative decay rate of multisubband plasmons and plasmon polaritons, both in the case of an isolated quantum well and of a planar microcavity. We show that subpicosecond radiative lifetimes are to be expected for intersubband plasmons in semiconductor quantum wells, similarly to quantum well excitons. The theory is formulated in the context of the transfer-matrix method and it can be applied in a straightforward way to stratified geometries of any degree of complexity

Effective bichromatic potential for ultra-high Q-factor photonic crystal slab cavities

We introduce a confinement mechanism in photonic crystal slab cavities, which relies on the superposition of two incommensurate one-dimensional lattices in a line-defect waveguide. It is shown that the resulting photonic profile realizes an effective quasi-periodic bichromatic potential for the electromagnetic field confinement yielding extremely high quality (Q) factor nanocavities, while simultaneously keeping the mode volume close to the diffraction limit. We apply these concepts to pillar- and hole-based photonic crystal slab cavities, respectively, and a Q-factor improvement by over an order of magnitude is shown over existing designs, especially in pillar-based structures. Thanks to the generality and easy adaptation of such confinement mechanism to a broad class of cavity designs and photonic lattices, this work opens interesting routes for applications where enhanced light–matter interaction in photonic crystal structures is required

Surface plasmons and strong light-matter coupling in metallic nanoshells

A theory of the interaction between a radiating dipole and the plasmonic excitations of a spherical metallic nanoshell in the quasistatic approximation is formulated. After a derivation of surface plasmon frequencies and a comparison with the corresponding modes of metal spheres and cavities, we introduce an expression for the effective volume for any position of the dipole inside or outside the nanoshell, describing the local electromagnetic field enhancement in analogy to other cavity-QED systems. The modification of the dipole decay rate is calculated as a function of frequency for various geometrical parameters, and it reflects the spectrum of spherelike and cavity-like surface plasmon excitations.We then give a formulation of emission spectra, suitable for describing light-matter interaction beyond perturbation theory, and study the conditions for the strong coupling regime to occur. By suitably tuning the geometrical parameters of the nanoshell and by choosing the order of surface plasmon modes to minimize the effective volume, a vacuum Rabi splitting can occur in emission spectra for dipole oscillator strengths as small as a few units, which can be easily achieved with organic molecules or quantum dots. The most favorable situation for strong coupling is when the dipole is located inside the nanoshell. Surprisingly, this dipole couples with spherelike modes more strongly than with cavity-like ones, if the shell is thin enough. As a conclusion, metallic nanoshells turn out to be a suitable platform in order to investigate the strong-coupling regime of light-matter interaction by exploiting surface plasmon resonances

Data underpinning - Persistence and lifelong fidelity of phase singularities in optical random waves

The data set includes the complex fields (amplitude and phase separately) used for the results presented in the publication titled "Persistence and lifelong fidelity of phase singularities in optical random waves". In addition, the data set contains the raw data of every plot presented in the manuscript