Hrvoje Petek Professor, Physics and Astronomy
Research
Ultrafast microscopy of coherent optical excitations on the nanometer scale
Keywords: Time-resolved photoemission microscopy, plasmonics
By combining ultrafast two-photon photoemission excitation with photoemission electron microscopy (PEEM) we study coherent optical excitations in nanostructured metal films on <50 nm spatial and <10 fs temporal scales. Using identical phase-related pump and probe pulses we excite electromagnetic fields and probe them coherently through the nonlinear photoemission process. Pump-probe delay scanning with interferometric control enables recording of movies of coherent optical fields propagating on metal surfaces with 300 attosecond (1/4 of an optical cycle) increment between frames. The imaging of the spatial distribution of photoelectron emission with electron microscope optics enables us to capture electromagnetic fields propagating at the local speed of light (roughly 300 nm/fs) with sub-optical-wavelength spatial resolution. With this technique we can study localized and propagating surface plasmons polaritons at sliver-vacuum interfaces. We perform quantitative studies of surface plasmon polariton generation, diffraction, interference, and focusing by fabrication of suitable coupling structures in Ag films. Focused ion beam milling of Si substrates onto which we deposit silver, or directly in silver, generates discontinuities, which are abrupt on the optical wavelength scale, and therefore can act as a source of momentum to couple the external light pulses into surface plasmon polariton wave packets. The physical nanometer scale morphology of the coupling structures defines the initial spatial and temporal phase, and therefore the subsequent spatiotemporal evolution of the plasmon fields. By analysis of PEEM images we can model accurately the surface electromagnetic fields, which is a prerequisite for rational design of nanoplasmonic optical structures. The time-resolved PEEM method is applicable to other coherent excitations, and in the future will be upgraded to achieve <10 nanometer resolution.
