Active Plasmonics

Nanophotonic devices have the capability to concentrate light into the nanoscale range and hold high potential for many applications as integrated optics, plasmonic circuits, biosensing and quantum information processing. A promising way to localize optical radiation into a nanometer-sized volume is obtained by exploiting the unique properties of plasmonic nanomaterials. These materials represent an effective bridge between bulk materials and atomic or molecular structures and exhibit a very intense color absent in the bulk material as well as in individual atoms. The physics behind this behavior can be explained by considering the collective oscillation of free conduction electrons that can be induced by an external electromagnetic field interacting with very small metal particles: the so called Localized Surface Plasmon Resonances (LSPR). In fact, the plasmonic coupling, existing between metal nanoparticles (NPs) and light, enables a series of interesting optical phenomena, such as Surface-Enhanced Raman Scattering (SERS), Resonance Light Scattering (RLS) and Surface Plasmon Resonance (SPR). The “active” control of plasmonic resonances is a hot-topic and several possibilities to obtain this result have already been demonstrated. Depending if the surrounding medium of plasmonic subentities is modified, a plasmonic coupling mechanism between them is exploited or collective plasmon resonances are excited, the key control of active plasmonics can be chosen between electric or optical fields, temperature, mechanical strain and exciting light polarization.

Plasmonic coupling

When nanoparticles are very close to each other, near-field coupling leads to concentrated and highly localized electric fields. Our past and ongoing experimental activities demonstrate that influencing the plasmonic coupling at the nanoscale can be as simple as pulling a rubber stripe. The prototype is made of a PDMS sample where a single layer of Au nanoparticles where immobilized by electrostatic interaction (collaboration with Prof. Buergi, Univ. of Geneva). By stretching the sample, the average distance between the nanoparticles becomes larger in the stretching direction and shorter in the perpendicular one, giving rise to a visible change of colour. This technology can be efficiently exploited for deformation sensing. Many other more complex possibilities can be envisioned like nanoscale platforms for biological or biomedical experiments.

Surrounding medium effect

A convenient way to dynamically modify the plasmon resonance frequency of a homogeneous (surface or bulk) distribution of mono-dispersed metal NPs is to vary the dielectric permittivity of the medium surrounding the NPs. Indeed, the optical properties of spherical particle dispersions can be predicted by the Mie theory through the expression of the extinction cross section. Based on this theory, for small and isolated metal particles, the spectral position of the plasmonic absorption peak depends on the refractive index of the surrounding medium. A modification of the dielectric behaviour of the host material corresponds, therefore, to a tuning action of the Plasmon resonance frequency. The outstanding properties of Liquid Crystals (LCs) make them an ideal candidate for this role; indeed, these materials represent an excellent example of reconfigurable medium where the refractive index can be finely controlled by means of external stimuli.

Collective plasmon resonances

Gratings of metal nanoparticles have been investigated for their optical properties since the beginning of the 1990s. Compared to their isolated counterparts, which support localized plasmon modes, those gratings present “lattice plasmon modes” which are dispersive and moreover exhibit in their optical spectra very narrow features which can take the shape of sharp maxima or minima. In regular arrays, these features arise when an “edge- diffraction” occurs for a wavelength close to localized surface plasmon resonances. Edge diffraction happens at so-called Rayleigh wavelengths, when one of the diffracted order changes from radiative (shorter wavelength) to evanescent (longer wavelength). The transition corresponds to grazing propagation of the diffracted order along the grating, and the perpendicular component of the wavevector cancels. In most of the theoretical and experimental configurations, such systems have been investigated at normal incidence, where the Rayleigh wavelengths solely depend on the period of the grating. However, as the edge diffraction depends as well on the incidence angle of the illumination, playing with the direction of the incoming plane wave gives the advantage to tune the position of the associated features. The fact that the lineshapes of spectra around Rayleigh wavelengths can be very narrow makes those kind of systems very suitable for designing plasmon nanolasers, or for biosensing applications as the associated figures of merit are expected to be large. For the same reason, they could be used for monitoring of mechanical stress and deformations.