Benchmarking Common Approximations for Determining the Particle-Size Dependence of Adsorbate-Induced Localized Surface Plasmon Resonance Shifts

Anomalies are investigated that exist between many long-standing theoretical models of the optical behavior of sensors based on changes in the localized surface plasmon resonance upon analyte adsorption. In particular, we focus on single metal nanoparticles which represent the core building-block of many recent sensing devices. Theoretical approaches include the Retarded Mie theory, the Non-Retarded quasi-static-dipole approximation, and two radiative corrections to the Non-Retarded case (radiative damping and radiative damping + depolarization). We find that the most accurate Non-Retarded approximation to the Retarded Mie theory varies strongly on a case by case basis; anyway, for particle radii beyond a few tens of nanometers, none of the considered approximations represents properly the adsorbate induced plasmon shift. We also find that the size-dependent peak shift has a complex dependence on the metal dielectric function. Accordingly, the trend of the adsorbate-induced plasmon peak shift as a function of the particle radius reveals an unexpected nonmonotonic behavior. We eventually identify an interesting range of particle radii over which the adsorbate-induced plasmon shift is unaffected by the particle size. Moreover, we give examples where nanoparticle batches with large size dispersion provide higher sensor reproducibility than monodisperse samples. On the other hand, in light of our findings, single particle measurements are pivotal to disclose the exact structure of the peak shift trend as a function of the particle radius

Anisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable Magneto-Optical Activity

We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show how coherent diffractive far-field coupling between elliptical nickel nanoantennas is governed by the two in-plane, orthogonal and spectrally detuned plasmonic responses of the individual building block, one directly induced by the incident radiation and the other induced by the application of an external magnetic field. The consequent excitation of magnetic field-induced Fano lattice surface modes leads to highly tunable and amplified magneto-optical effects as compared to a continuous film or metasurfaces made of disordered noninteracting magnetoplasmonic anisotropic nanoantennas. The concepts presented here can be exploited to design novel magnetoplasmonic sensors based on coupled localized plasmonic resonances, and nanoscale metamaterials for precise control and magnetically driven tunability of light polarization states