Plasmon spectroscopy: Theoretical and numerical calculations, and optimization techniques

We present an overview of recent advances in plasmonics, mainly concerning theoretical and numerical tools required for the rigorous determination of the spectral properties of complex-shape nanoparticles exhibiting strong localized surface plasmon resonances (LSPRs). Both quasistatic approaches and full electrodynamic methods are described, providing a thorough comparison of their numerical implementations. Special attention is paid to surface integral equation formulations, giving examples of their performance in complicated nanoparticle shapes of interest for their LSPR spectra. In this regard, complex (single) nanoparticle configurations (nanocrosses and nanorods) yield a hierarchy of multiple-order LSPR s with evidence of a rich symmetric or asymmetric (Fano-like) LSPR line shapes. In addition, means to address the design of complex geometries to retrieve LSPR spectra are commented on, with special interest in biologically inspired algorithms. Thewealth of LSPRbased applications are discussed in two choice examples, single-nanoparticle surface-enhanced Raman scattering (SERS) and optical heating, and multifrequency nanoantennas for fluorescence and nonlinear optics.

Simple modeling of the ratio of fields at a tip and at contacting surface

The proposed concept of Raman probe for nearfield optical microscopy raises the question about the similarity of fields acting on specimens deposited at the tip apex and contacting surface. The signal generated at these two close but different points is defined by local fields, so it is the ratio of the fields at these points, that is the quantity of interest here. This work is concerned with the application of a simple dipole model for the analysis of the ratio of fields at the tip apex and at contacting surface as a function of their separation.

Nonlinear emission from silver-coated 3D hollow nanopillars

High aspect ratio metal nanostructures have been the subject of a number of studies in the past, due to their pronounced resonances in the infrared that can be exploited to enhance vibrational spectroscopies. In this work, we investigate the nonlinear optical response of both individual and closely-packed assemblies of vertical hollow Ag nanopillars upon excitation with ultrafast laser pulses. In particular, the analysis of their nonlinear emission spectra evidences an intense two photon photoluminescence (TPPL) emission and a neat signature of second harmonic generation (SHG). Given the relatively low background, this pronounced nonlinear emission could be employed as a local probe for analytes trapped at the surface of the nanopillar or flowing through it. For this reason, these nanostructures may become appealing building blocks in multi-purpose devices for nonlinear photonics and sensing.

Hybrid silica-gold core-shell nanoparticles for fluorescence enhancement

We demonstrate that SiO

Optical Detection of core-gold nanoshells inside biosystems

Metal nanoshells having a dielectric core with a thin gold layer are generating new interest due to the unique optical, electric and magnetic properties exhibited by the local field enhancement near the metal – dielectric core interface. These nanoshells possess strong, highly tunable local plasmon resonances with frequencies dependent upon the nanoshell shape and core material. These unique characteristics have applications in biosensing, optical communication and medicine. In this paper, we developed a theoretical, numerical and experimental approach based on a scanning near optical microscope to identify nanoshells inside mouse cells. Taking advantage of the characteristic near-infrared transparency window of many biological systems, i.e. the low light absorption coefficient of biological systems between 750−1100 nm, we were able to identify a 100−150 nm diameter barium titanate-gold nanoshell inside the h9c2 mouse cells.

On the SERS depolarization ratio

The Raman depolarization ratio is a quantity that can be easily measured experimentally and offers unique information on the Raman polarizability tensor of molecular vibrations. In Surface Enhanced Raman Scattering (SERS), molecules are near-field coupled with optical nanoantennas and their scattering properties are strongly affected by the radiation patterns of the nanoantenna. The polarization of the SERS photons is consequently modified, affecting, in a non trivial way, the measured value of the SERS depolarization ratio. In this article we elaborate a model that describes how the SERS depolarization ratio is influenced by the nanoantenna re-radiation properties, suggesting how to retrieve information on the Raman polarizability from SERS experiments.

Integrated plasmonic double bowtie / ring grating structure for enhanced electric field confinement

Metallic nanoparticles and nanoantennas have been extensively studied due to their capability to increase electromagnetic field confinement which is essential in numerous applications ranging from optoelectronics to telecommunication and sensing devices. We show that a double bowtie nanoantenna has a higher electric field confinement in its gap compared to a single bowtie nanoantenna, which is expected to give better fluorescence enhancement of a single emitter placed in the gap. We show that the electric field intensity can be further increased by placing the double bowtie inside a ring grating structure where the excitation of surface plasmon-polaritons (SPPs) is achieved. We perform FDTD simulations to characterise the double bowtie nanoantenna and study the effect of its dimensions on the electric field enhancement in the gap. Our proposed integrated structure with gratings is shown to increase the electric field by a factor of 77 due to a double cavity effect. Next steps would be to study the fluorescence enhancement of emitters placed inside our double bowtie / ring grating nanocavity to see if the strong coupling regime can be attained.

Spectrophotometric observations of thiacyanine dye J-aggregation on citrate capped silver nanoparticles

The J-aggregation of anionic cyanine dye, 5,5’ – disulfopropyl-3,3’ – dichloro - thiacyanine sodium salt (TC) in the presence of 2x10-3 M KCl and citrate capped silver nanoparticles (AgNPs, diameter ~10nm) was investigated. The influence of added salt (KCl), as well as AgNPs and TC concentration on the intensity of the absorption bands with the maxima at 464 and 481 nm, characteristic of the J-aggregation of the dye in solution and on AgNPs surface, respectively, was studied. The spectrophotometric and fluorescence spectra confirmed that AgNPs induced the decomposition of J-aggregates formed in solution on the account of their formation on NPs surface. The obtained results enabled the evaluation of number of TC molecules per AgNPs participating in J-aggregate formation. The experimental results yielded about 320 molecules of TC per AgNPs in at least three layers in a slanted orientation.

Liquid Crystals as an Active Medium: Novel Possibilities in Plasmonics

The peculiar properties of Liquid Crystals (LCs) foster new possibilities in plasmonics. The combination of the intrinsic tunability of LCs with the plasmonic properties of metallic nanoparticles (NPs) provides novel and intriguing features of systems commonly identified as active plasmonics. Being LCs, one of the media whose refractive index can be controlled through the application of external stimuli, they represent a convenient host for enabling plasmonic tunability. On the other hand, the localized plasmonic resonance, typical of NPs, can strongly influence and control the behaviour of LCs. In this paper, we overview several systems of NPs combined with LCs arranged in different configurations. The properties of the resulting systems suggest novel, intriguing outcomes in both fundamental and applied research.

Fluorescence imaging of hybrid nanostructures composed of natural photosynthetic complexes and reduced graphene oxide

Herein, we describe the results of fluorescence microscopy imaging of peridinin-chlorophyll-protein (PCP) photosynthetic complex mixed with reduced graphene oxide (rGO). Upon incorporation of rGO the fluorescence image of PCP changes substantially from one characterized by uniform intensity towards a more complex pattern. The isolated bright spots feature up to ten times higher emission intensity compared to the fluorescence of PCP in the reference sample, where no rGO was added. The number of the bright spots increases with increasing rGO concentration. At the same time the fluorescence intensity away from the bright spots in the PCP/rGO hybrid system is quenched in comparison to the PCP – only reference.