Adriano Cacciola

Plasmon-enhanced optical trapping of gold nanoaggregates with selected optical properties.

We show how light forces can be used to trap gold nanoaggregates of selected structure and optical properties obtained by laser ablation in liquid. We measure the optical trapping forces on nanoaggregates with an average size range 20-750 nm, revealing how the plasmon-enhanced fields play a crucial role in the trapping of metal clusters featuring different extinction properties. Force constants of the order of 10 pN/nmW are detected, the highest measured on a metal nanostructure. Finally, by extending the transition matrix formalism of light scattering theory to the optical trapping of metal nanoaggregates, we show how the plasmon resonances and the fractal structure arising from aggregation are responsible for the increased forces and wider trapping size range with respect to individual metal nanoparticles.

Ultrastrong Coupling of Plasmons and Excitons in a Nanoshell

The strong coupling regime of hybrid plasmonic-molecular systems is a subject of great interest for its potential to control and engineer light-matter interactions at the nanoscale. Recently, the so-called ultrastrong coupling regime, which is achieved when the light-matter coupling rate reaches a considerable fraction of the emitter transition frequency, has been realized in semiconductor and superconducting systems and in organic molecules embedded in planar microcavities or coupled to surface plasmons. Here we explore the possibility to achieve this regime of light-matter interaction at nanoscale dimensions. We demonstrate by accurate scattering calculations that this regime can be reached in nanoshells constituted by a core of organic molecules surrounded by a silver or gold shell. These hybrid nanoparticles can be exploited for the design of all-optical ultrafast plasmonic nanocircuits and -devices.

Spin-Momentum Locking in the Near Field of Metal Nanoparticles

Light carries both spin and momentum. Spin–orbit interactions of light come into play at the subwavelength scale of nano-optics and nanophotonics, where they determine the behavior of light. These phenomena, in which the spin affects and controls the spatial degrees of freedom of light, are attracting rapidly growing interest. Here we present results on the spin-momentum locking in the near field of metal nanostructures supporting localized surface resonances. These systems can confine light to very small dimensions below the diffraction limit, leading to a striking near-field enhancement. In contrast to the propagating evanescent waves of surface plasmon-polariton modes, the electromagnetic near-field of localized surface resonances does not exhibit a definite position-independent momentum or polarization. Close to the particle, the canonical momentum is almost tangential to the particle surface and rotates when moving along the surface. The direction of this rotation can be controlled by the spin of the...

ULTRAVIOLET RADIATION INSIDE INTERSTELLAR GRAIN AGGREGATES. III. FLUFFY GRAINS

We study the problem of light depolarization within fluffy interstellar dust grains, in which coagulation generates interstitial cavities partly filled with icy condensed gas. A significant amount of elliptical polarized ultraviolet radiation may be generated in situ by geometry-induced depolarization within the cavities. The behavior of the depolarization is studied both by changing the orientation of the aggregates and by changing chemical composition and size of the subunits forming the aggregate. We found that a considerable amount of depolarization occurs within the ice mantles of the subunits, provided their thickness is not too large. We discuss the implications of these results for chiral selection in space.

Near-Field Optical Detection of Plasmon Resonance from Gold Nanoparticles: Theoretical and Experimental Evidence

The study of plasmon-induced electromagnetic fields is a very interesting topic for basic research and photonic applications. The plasmon properties depend on many factors, such as composition, size, shape and arrangement of nanoparticles. In this paper, we propose an experimental and theoretical study on the optical properties of gold nanoparticles deposited by pulsed laser ablation and investigated by near-field optical microscopy (SNOM) in a transmission far-field collection scheme. The electromagnetic field properties have been simulated by an exact theoretical analysis based on the multipolar expansion of the fields and on T-matrix approach. The theoretical model almost accurately reproduces the experimental data and makes us confident that the used method is suitable to describe more complex system of metal nanoparticles.

Plasmonic Absorption Enhancement of a Single Quantum Dot

We study analytically the absorption properties of an individual quantum absorber, modeled as a two-level system, in the presence of a metallic nanoparticle. The coupling between the two systems can give rise to a Fano interference effect and to a relevant modification of the absorption cross section. Such effect strongly depends on the angle between the dimer axis and the electromagnetic field polarization. From this analysis, we can conclude that the localized surface plasmons are able to enhance the absorption of nanostructures, thus increasing the efficiency of solar cells based on absorbing nanoparticles.