Linear and Polygonal Assemblies of Plasmonic Nanoparticles: Incident Light Polarization Dictates Hot Spots

The assemblies of metal nanoparticles, thanks to their intriguing plasmonic properties, have provided numerous opportunities for manipulating light at the nanoscale. Driven by the recent experimental success in using polarization of light as a handle to control plasmonic features, we consider the organization of spherical gold nanoparticles as linear and polygonal assemblies (<i>n</i> = 1–6) and perform a detailed analysis of the optical features as a function of the polarization of the incident light (θ = 0°, 30°, 60°, and 90°) using the finite-difference time-domain (FDTD) method. Our investigations reveal that the extinction features in linear chains show a strong dependence on the state of polarization of the source, whereas those in the polygonal assemblies are polarization-insensitive. However, the hot spot distribution in polygonal assemblies is strongly dependent on the polarization state of the incident light, thereby giving rise to interesting control over hot spot features for surface-enhanced spectroscopy. Finally, we also comment on the role of the wavelength of light, size of the metal particle, and the gap size between the particles in governing the plasmonic properties of the assemblies

Overwhelming Analogies between Plasmon Hybridization Theory and Molecular Orbital Theory Revealed: The Story of Plasmonic Heterodimers

Plasmon hybridization theory (PHT), an analogue of molecular orbital theory (MOT) for plasmonic molecules, has enjoyed tremendous success over the last decade in discerning the optical features of hybrid nanostructures in terms of their constituent monomeric nanostructures. Dimers of metal nanoparticles served as prototypes in elucidating many of the key aspects of plasmon hybridization. Employing quantum two-state model, in conjunction with the quasi-static approximation and the finite-difference time-domain simulations, we demonstrate that the analogy between PHT and MOT can be further propelled by a theoretical estimation of the plasmon-coupling strengths and the relative contributions of the unhybridized monomeric states toward the hybrid dimeric states in plasmonic Ag–Au nanorod heterodimers. The aspect ratio of the constituent nanorods and the gap size between the monomeric nanorods can further be used as handles to tune the relative contributions of (i) the bonding and the antibonding modes to the total extinction and (ii) the monomeric states toward the dimeric states, with meaningful implications for surface-enhanced spectroscopy. The tunability in light absorption properties of heterodimers in the 400–800 nm region arising as a result of broken symmetry is also suggestive of their potential role as plasmonic rulers for measuring distances

Ag@SiO₂ Core–Shell Nanostructures: Distance-Dependent Plasmon Coupling and SERS Investigation

Enhancement of Raman signals of pyrene due to the enhanced electric fields on the surface of silver nanoparticles has been investigated by controlling the thickness of the silica shell. Dimeric nanostructures having well-defined gaps between two silver nanoparticles were prepared, and the gap size (<i>d</i>) was varied from 1.5 to 40 nm. The molecules trapped at the dimeric junctions showed higher Raman signal enhancements when the gap was less than 15 nm due to the presence of amplified electric field, in agreement with our theoretical studies. The experimental Raman enhancement factors at the hot spots follow a 1/<i>d</i>^<i>n</i> dependence, with <i>n</i> = 1.5, in agreement with the recent theoretical studies by Schatz and co-workers. Experimental results presented here on the distance dependence of surface enhanced Raman spectroscopy (SERS) enhancement at the hot spots can provide insight on the design of newer plasmonic nanostructures with optimal nanogaps

Plexcitons: The Role of Oscillator Strengths and Spectral Widths in Determining Strong Coupling

Strong coupling interactions between plasmon and exciton-based excitations have been proposed to be useful in the design of optoelectronic systems. However, the role of various optical parameters dictating the plasmon-exciton (plexciton) interactions is less understood. Herein, we propose an inequality for achieving strong coupling between plasmons and excitons through appropriate variation of their oscillator strengths and spectral widths. These aspects are found to be consistent with experiments on two sets of free-standing plexcitonic systems obtained by (i) linking fluorescein isothiocyanate on Ag nanoparticles of varying sizes through silane coupling and (ii) electrostatic binding of cyanine dyes on polystyrenesulfonate-coated Au nanorods of varying aspect ratios. Being covalently linked on Ag nanoparticles, fluorescein isothiocyanate remains in monomeric state, and its high oscillator strength and narrow spectral width enable us to approach the strong coupling limit. In contrast, in the presence of polystyrenesulfonate, monomeric forms of cyanine dyes exist in equilibrium with their aggregates: Coupling is not observed for monomers and H-aggregates whose optical parameters are unfavorable. The large aggregation number, narrow spectral width, and extremely high oscillator strength of J-aggregates of cyanines permit effective delocalization of excitons along the linear assembly of chromophores, which in turn leads to efficient coupling with the plasmons. Further, the results obtained from experiments and theoretical models are jointly employed to describe the plexcitonic states, estimate the coupling strengths, and rationalize the dispersion curves. The experimental results and the theoretical analysis presented here portray a way forward to the rational design of plexcitonic systems attaining the strong coupling limits