Post-doc, University of Chicago
Control of the spatial polarization of light allows tailoring the electromagnetic response of plasmonic nanostructures. In this work, we show that focused cylindrical vector beams (CVBs) can be used to efficiently excite dark plasmon modes in highly symmetric gold nanoparticle (AuNP) dimers. Specifically, we use single particle spectroscopy and FDTD simulations to study the response of AuNP dimers excited by linearly (LP), azimuthally (AP), and radially (RP) polarized beams. Under LP excitation, the resonances correspond to in-phase coupling of the dipolar moments of the particles, with dipolar moments parallel or perpendicular to the dimer axis. These resonances are known as bright modes, as they can easily couple to light. By contrast, the field distribution of focused AP or RP beams indicates that the fields acting on the AuNPs is primarily perpendicular or parallel to the dimer’s axis, but with opposite directions at each particle. Therefore, the resonances here are out of plane coupling of dipolar moments, or so called “dark modes”. In addition, multipolar expansion of the fields associated with each scattering spectrum shows that the resonances excited by LP beams are dominated by electric dipole modes. By contrast, CVB excitation causes new modes, such as magnetic dipole and electric quadrupole modes, to be driven and that they even dominate the scattering spectra. This work opens new opportunities for spectroscopic investigation of dark modes and Fano resonances in plasmonic nanostructures, which are difficult or impossible to be excited by conventionally polarized light.
Abstract: Gold plasmonic nanostructures with several different adhesion layers have been studied with monochromated electron energy loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and with surface-enhanced Raman spectroscopy (SERS). Compared to samples with no adhesion layer, those with 2 nm of Cr or Ti show broadened, lower intensity plasmon peaks as measured with EELS. This broadening is observed in both optically active (“bright”) and inactive (“dark”) plasmon modes. When the former are probed with SERS, the signal enhancement factor is lower for samples with Cr or Ti, another indication of reduced plasmon resonance. This work illustrates the capability of STEM-EELS to provide direct near-field measurement of changes in plasmon excitation probability with nanoscale spatial resolution. Additionally, it demonstrates that applications requiring high SERS enhancement, such as biomarker detection and cancer diagnostics, can be improved by avoiding the use of a metallic adhesion layer.
Pub.: 13 Jan '17, Pinned: 28 Jun '17
Abstract: Orbital angular momentum of light has recently been recognized as a new degree of freedom to encode information in quantum communication using light pulses. Methods to extract this information include reversing the process by which such twisted light was created in the first place or interference with other beams. Here we propose an alternative new way to directly read out the extra information encoded in twisted light using plasmonic nanoantennas by converting the information about the orbital angular momentum of light into spectral information using bright and dark modes. Exemplarily considering rotation-symmetric nanorod nanoantennas, we show that their scattering cross section is sensitive to the value of the orbital angular momentum combined with the polarization of an incident twisted light beam. Explaining the twist dependence of the excited modes with a new analytical model, our results pave the way to twisted light nanoplasmonics, which is of central importance for future on-chip communication using orbital angular momentum of light.
Pub.: 02 Mar '17, Pinned: 28 Jun '17
Abstract: Geometric phases have attracted considerable attention in recent years, due to their capability of arbitrary beam shaping in a most efficient and compact way, while traditional geometric phases are usually limited to handling single-structured beams and lack the capability of parallel manipulation. Here, we propose a digitalized geometric phase enabling parallel optical spin and orbital angular momentum encoding. The concept is demonstrated in inhomogeneous anisotropic media by imprinting a particularly designed binary phase into a space-variant geometric phase. We theoretically analyze its spin–orbit interaction of light and experimentally created higher-order Poincaré sphere beam lattices, the order number and symmetry of which can be flexibly manipulated. Special lattices of cylindrical vector beams and orbital angular momentum modes with square and hexagonal symmetry are presented. This work discloses a new insight in programming geometric phases for tailoring the optical field and inspires various photonics applications.
Pub.: 15 May '17, Pinned: 28 Jun '17