Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes

Abstract

Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-on-plane, NPOP) are investigated using two complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), we detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. To our knowledge, this is the first time that both far- and near-field spectroscopy are carried out for identical individual nanostructures interacting via a subnanometer gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.

Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths

“…We suggest that the observed field enhancement occurs in wedge-like features at the substrate-particle interface, especially when these are conductively connected. This is in contrast to other situations [2,13,31] where there is a thin insulating barrier between particle and metallic substrate. The existence of an enhanced field in the region of the substrate-particle interface is verified by electromagnetic simulations using two different methods.…”

Direct imaging of the near field and dynamics of surface plasmon resonance on gold nanostructures using photoemission electron microscopy

“…[30][31][32][33][34][35] Photoemission electron microscopy records the electrons emitted from a sample in response to the absorption of incident photons. Conventional PEEM uses ultraviolet (UV) light or X-ray radiation as the excitation source and has been demonstrated as a powerful imaging and characterization tool in the fields of surface physics/chemistry, material growth and magnetic materials.…”

“…Therefore, the fundamental frequency of Ti:sapphire femtosecond laser pulses should be more suitable for exciting such LSPR modes. However, only few reports have described investigations of the LSPR using MP-PEEM with femtosecond pulses from an 800-nm excitation source, [33][34][35] likely because of the difficulty in achieving higher nonlinear photoemission. Furthermore, no reports have yet described the direct observation of the dynamics of LSPR on Au nanoparticles using TR-MP-PEEM at this wavelength.…”

Laser intensity effects in carrier-envelope phase-tagged time of flight-photoemission electron microscopy

“…We performed first CEP-tagged PEEM experiment on gold nanoparticles on gold plane (NPOP) with a subnanometer gap [46][47][48] via a multiphoton photoemission process. A very thin layer of the organic molecule cysteamine serves as a spacer or gap between these gold nanoparticles (~90 nm average diameter) and a 50-nm-thick gold film.…”