Photoemission Electron Microscopy for Direct Observation of Photonic and Plasmonic Phenomena

Abstract

Photoemission electron microscopy (PEEM) is a high-resolution microscopy technique that collects photoemitted electrons from the sample surface to form an image. PEEM offers a non-scanning imaging method with a spatial resolution in the range of 5-100nm by combining the advantages of light excitation and electron imaging. Our work looks at PEEM as an analysis tool for photonic and plasmonic phenomena. Photonic wave guiding structures exhibiting a strong dispersion relation have attracted considerable attention for applications in integrated optics, communications and sensing devices. Line defects in a photonic crystal (PC) slab offer a highly efficient way to create light with group velocities much smaller than is achievable in uniform materials. Slow light is needed for numerous device applications involving non-linearity in absorption, transmission and reflection, and optical buffers. Propagation velocities in PC waveguide structures are typically measured with interferometric methods involving the outcoupling of the slow light into an optical fiber and comparing its phase delay to a reference light wave. Direct imaging of the modes in the defect, however, is more challenging. Here we present a new approach for direct imaging of defect modes in PEEM. Metallic nanoparticles are another area of interest due to their plasmonic properties. They exhibit localized surface plasmon resonances (LSP) at particular excitation frequencies depending on their size, shape, and material and are used in a wide variety of applications in medical tagging, sensing, solar cells, and optical coupling. As we move toward nanoscale devices it becomes critically important to be able to understand and measure not only bulk properties, but also the resonances of individual and small groups of nanoparticles. PEEM is a promising candidate for evaluating such systems due to its noninvasive measuring mechanism and nonlinear response to surface plasmon excitation. Here we report observation of avoided level crossing due the hybridization of the dipolar dimer mode and the single nanoparticle substrate mediated mode in 45nm silver nanoparticle dimers on an indium tin oxide (ITO) substrate with interparticle gaps ranging from 1nm-10nm. By changing the polarization and direction of incidence of the exciting beam the strong coupling can be switched on and off and different resonant modes of the dipole excited. If the substrate is changed to be conductive charge can flow from one particle to the other and an additional charge transfer plasmon (CTP) mode can be excited. These results demonstrate the importance of substrate effects on plasmonic nano-systems and that interparticle