Giant circular dichroism of a molecule in a region of strong plasmon resonances between two neighboring gold nanocrystals

We report on giant circular dichroism (CD) of a molecule inserted into a plasmonic hot spot. Naturally occurring molecules and biomolecules have typically CD signals in the UV range, whereas plasmonic nanocrystals exhibit strong plasmon resonances in the visible spectral interval. Therefore, excitations of chiral molecules and plasmon resonances are typically off-resonant. Nevertheless, we demonstrate theoretically that it is possible to create strongly-enhanced molecular CD utilizing the plasmons. This task is doubly challenging since it requires both creation and enhancement of the molecular CD in the visible region. We demonstrate this effect within the model which incorporates a chiral molecule and a plasmonic dimer. The associated mechanism of plasmonic CD comes from the Coulomb interaction which is greatly amplified in a plasmonic hot spot.Comment: Manuscript: 4+pages, 4 figures; Supplemental_Material: 10 pages, 7 figure

Doubly resonant optical nanoantenna arrays for polarization resolved measurements of surface-enhanced Raman scattering

We report that rhomb-shaped metal nanoantenna arrays support multiple plasmonic resonances, making them favorable bio-sensing substrates. Besides the two localized plasmonic dipole modes associated with the two principle axes of the rhombi, the sample supports an additional grating-induced surface plasmon polariton resonance. The plasmonic properties of all modes are carefully studied by far-field measurements together with numerical and analytical calculations. The sample is then applied to surface-enhanced Raman scattering measurements. It is shown to be highly efficient since two plasmonic resonances of the structure were simultaneously tuned to coincide with the excitation and the emission wave- length in the SERS experiment. The analysis is completed by measuring the impact of the polarization angle on the SERS signal.Comment: 13 pages, 5 figure

Silver Amalgam Nanoparticles and Microparticles: A Novel Plasmonic Platform for Spectroelectrochemistry

Plasmonic nanoparticles from unconventional materials can improve or even bring some novel functionalities into the disciplines inherently related to plasmonics such as photochemistry or (spectro)electrochemistry. They can, for example, catalyze various chemical reactions or act as nanoelectrodes and optical transducers in various applications. Silver amalgam is the perfect example of such an unconventional plasmonic material, albeit it is well-known in the field of electrochemistry for its wide cathodic potential window and strong adsorption affinity of biomolecules to its surface. In this study, we investigate in detail the optical properties of nanoparticles and microparticles made from silver amalgam and correlate their plasmonic resonances with their morphology. We use optical spectroscopy techniques on the ensemble level and electron energy loss spectroscopy on the single-particle level to demonstrate the extremely wide spectral range covered by the silver amalgam localized plasmonic resonances, ranging from ultraviolet all the way to the mid-infrared wavelengths. Our results establish silver amalgam as a suitable material for introduction of plasmonic functionalities into photochemical and spectroelectrochemical systems, where the plasmonic enhancement of electromagnetic fields and light emission processes could synergistically meet with the superior electrochemical characteristics of mercury

Fano resonances in plasmonic core-shell particles and the Purcell effect

Despite a long history, light scattering by particles with size comparable with the light wavelength still unveils surprising optical phenomena, and many of them are related to the Fano effect. Originally described in the context of atomic physics, the Fano resonance in light scattering arises from the interference between a narrow subradiant mode and a spectrally broad radiation line. Here, we present an overview of Fano resonances in coated spherical scatterers within the framework of the Lorenz-Mie theory. We briefly introduce the concept of conventional and unconventional Fano resonances in light scattering. These resonances are associated with the interference between electromagnetic modes excited in the particle with different or the same multipole moment, respectively. In addition, we investigate the modification of the spontaneous-emission rate of an optical emitter at the presence of a plasmonic nanoshell. This modification of decay rate due to electromagnetic environment is referred to as the Purcell effect. We analytically show that the Purcell factor related to a dipole emitter oriented orthogonal or tangential to the spherical surface can exhibit Fano or Lorentzian line shapes in the near field, respectively.Comment: 28 pages, 10 figures; invited book chapter to appear in "Fano Resonances in Optics and Microwaves: Physics and Application", Springer Series in Optical Sciences (2018), edited by E. O. Kamenetskii, A. Sadreev, and A. Miroshnichenk

Modeling Surface-Enhanced Spectroscopy With Perturbation Theory

Theoretical modeling of surface-enhanced Raman scattering (SERS) is of central importance for unraveling the interplay of underlying processes and a predictive design of SERS substrates. In this work we model the plasmonic enhancement mechanism of SERS with perturbation theory. We consider the excitation of plasmonic modes as an integral part of the Raman process and model SERS as higher-order Raman scattering. Additional resonances appear in the Raman cross section which correspond to the excitation of plasmons at the wavelengths of the incident and the Raman-scattered light. The analytic expression for the Raman cross section can be used to explain the outcome of resonance Raman measurements on SERS analytes as we demonstrate by comparison to experimental data. We also implement the theory to calculate the optical absorption cross section of plasmonic nanoparticles. From a comparison to experimental cross sections, we show that the coupling matrix elements need to be renormalized by a factor that accounts for the depolarization by the bound electrons and interband transitions in order to obtain the correct magnitude. With model calculations we demonstrate that interference of different scattering channels is key to understand the excitation energy dependence of the SERS enhancement for enhancement factors below 103