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.

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.

We investigate experimentally and theoretically optical trapping of metal nanoparticles and aggregates. In particular, we show how light forces can be used to trap individual gold nanoaggregates of controlled size and structure obtained by laser ablation synthesis in solution. Due to their surface charge, no agglomeration of isolated nanoparticles was observed during trapping experiments and reliable optical force measurements of isolated and aggregated nanoparticles was possible through an analysis of the Brownian motion in the trap. We show how the field-enhancement properties of these nanostructures enables surface-enhanced Raman spectroscopy of molecules adsorbed on aggregates optically trapped in a Raman tweezers setup. We finally discuss calculations of extinction and optical forces based on a full electromagnetic scattering theory for aggregated gold nanostructures where the occurrence of plasmon resonances at longer wavelength play a crucial role in the enhancement of the trapping forces

We show, by accurate scattering calculations, that nanostructures obtained from thin films of J-aggregate dyes, despite their insulating behavior, are able to concentrate the electromagnetic field at optical frequencies like metallic nanoparticles. These results promise to widely enlarge the range of plasmonic materials, thus opening new perspectives in nanophotonics. Specifically we investigate ultrathin nanodisks and nanodisk dimers that can be obtained by standard nanolithography and nanopatterning techniques. These molecular aggregates display highly attractive nonlinear optical properties, which can be exploited for the realization of ultracompact devices for switching by light on the nanoscale without the need of additional nonlinear materials. W hen light interacts with metal nanoparticles and nanostructures, it can excite collective oscillations known as localized surface plasmons (LSPs), which provide the opportunity to confine light to very small dimensions below the diffraction limit. 1−4 This high confinement can lead to a striking near-field enhancement, which can significantly enhance weak nonlinear processes 5 and enables a great variety of applications such as optical sensing, 6,7 higher efficiency solar cells, 8 nanophotonics 1,5,9 including ultracompact lasers and amplifiers, 10 and antennas transmitting and receiving light signals at the nanoscale. 4,11,12 The small mode volume of LSP resonances also increases the photonic local density of states (LDOS) close to a plasmonic nanoparticle, enabling the modification of the optical properties (decay rate and quantum efficiency) of emitters placed in its close proximity (see for example ref 13 and Supporting Information Figure S1). The interaction of quantum emitters, as quantum dots or dye molecules, with individual metallic nanostructures carries significant potential for the quantum control of light at the nanoscale. 1,14−22 As first highlighted by Takahara et al., 23 only materials with a negative real part of the dielectric function and moderate losses are able to excite localized surface plasmons and hence to confine light to very small dimensions below the diffraction limit. These collective and confined excitations are efficiently supported, despite dissipative losses, by noble metals where the effective response of the electrons can be described by a Drude−Lorentz dielectric function whose real part is negative for frequencies below the plasma frequency. 2 Also superconductors or graphene has been proposed as a platform for surface plasmon polaritons. 24,25 Collective oscillations of free electrons are not the only way a negative permittivity may arise. It may also occur in the highenergy tail of a strong absorption resonance. For example, it has been shown that lattice vibrations in polar dielectric materials can also originate negative dielectric permittivity in the far-or mid-infrared spectral range, which can support phononpolaritons confined to the surface. 26,27 It has also been shown