Imaging nanostructures using a photoemission electron microscope

Abstract

Plasmonics is an emerging field due to its attractive applications such as the fabrication of novel materials, plasmonic waveguides, and light-driven nano-electronics. A collective electron motion driven by a laser field at a metallic surface is a main subject of the study in plamonics. The collective electron motion excites surface plasmon polaritons that propagates along the metal-dielectric surface, or localized surface plasmons in a nano-size metallic structure. The optical property of the nanostructure is often counterintuitive due to these plasmons, showing unusually high transmittance through a nano-gap, or the strong field enhancement effect at a sharp edge. These phenomena have become hot topics in attosecond physics in which ultrafast electron dynamics is studied with an attosecond temporal resolution. Since a localized surface plasmon absorbs and emits a light field, an electromagnetic field near the nanostructure can be modified. It is important to characterize the near field for studying the ultrafast electron dynamics near the nanostructure. In this work, we investigate the near field distribution in space and time for different types of nanostructure using finite-difference time-domain (FDTD) method. The FDTD method is a Maxwell’s equation solver (partially for Ampere’s and Faraday’s laws) that directly finds a solution in the time domain with discretized forms. A highly localized and enhanced near-field in the vicinity of the nanostructure is examined. The plasmonic effects such as resonant absorption for the polarization directions are analyzed near the apex of the nanostructure. It is found that the near field spectra and the absorption resonance frequency greatly depend on the design and dimension of the gold structure. The ultrafast electron dynamics near nanostructures can be studied using a photoemission electron microscope (PEEM). Since an electron can tunnel out from the surface of the nanostructure by the near field, the photoelectron created by the near field provides information on the near field. In order to study the photoelectron, a PEEM is installed and tested. The spatial resolution of the PEEM is estimated to be 40-nm using a UV source and a femtosecond pulsed laser. Images of gold structures obtained using the two different light sources are compared. The image clearly shows ‘hotspots’ due to multiphoton emission when the femtosecond laser pulse is used. The PEEM will be used for the space and time-resolved near field measurement in future experiments.