Photoemission Electron Microscopy as a Tool for the Investigation
of Optical Near Fields
M. Cinchetti, A. Gloskovskii, S. A. Nepjiko and G. Schönhense
Johannes Gutenberg-Universität, Institut für Physik, 55099 Mainz, Germany
H. Rochholz and M. Kreiter
Max Planck Institut für Polymerforschung, 55128 Mainz, Germany
The intensity of optical ﬁelds may be largely enhanced (by orders of magnitude) in the vicinity of nanoscopically structured metal objects. In analogy to antenna used for radiation of lower frequency, small metal objects can be regarded as antenna for the optical regime. These antenna are characterised by an overall optical resonance similar to the well-known plasmon resonance of spherical metal particles. Extreme local ﬁeld enhancement is believed to be responsible for the increase of the Raman cross section of organic molecules by up to a factor of 1014  in the vicinity of stochastically roughened silver ﬁlms. Fluorescence as well is drastically altered by an enhanced optical near ﬁeld which was shown to improve the performance of chromophores and semiconductor quantum dots signiﬁcantly.
In this project we use Photoemission electron microscopy (PEEM) to image the electrons being photoemitted from specially tailored Ag nanoparticles (e.g. crescents) deposited on a SiOx substrate, see Fig. 1. Photoemission was induced by illumination with a Ti:sapphire femtosecond laser (hv = 3.1 eV, λ = 400 nm, pulse width below 200 fs). While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the two-photon photoemission is localized at tips of the structure (top right image). This is interpreted as a signature of the local electrical ﬁeld thereby providing a tool to map the optical near ﬁeld of the crescent. The lateral resolution (20 nm) is far below the optical wavelength because it is defined by the electron microscope. A simulation reproduces the essential features (lower panels). The two-photon photoemission process from plasmon-resonant nanoparticles reveals an unusual spectral behaviour, see papers by Cinchetti et al.
 S. Nie and S. R. Emory, Science 275, 1102 (1997)
Funded by BMBF (03 N 6500 „Nanocentre“) and by Stiftung Rheinland-Pfalz für Innovation (project 535).
Fig. 1 Top: Structure (left) and femtosecond laser-excited PEEM-image at hv = 3.1eV (right, same field of view!) of a Ag crescent on SiOx. Bottom: Local magnitude of the electric ﬁeld, calculated for a 2D geometry of a silver structure in vacuum for λ=250nm (left) and for the resonance frequency of 750 THz (λvac = 400nm). The scale bar indicates the enhancement factor of the squared ﬁeld amplitude.
M. Cinchetti, A. Gloskovskii, S. A. Nepjiko, G. Schönhense, H. Rochholz, M. Kreiter
Photoemission Electron Microscopy as a tool for the investigation of optical near fields
Phys. Rev. Lett. 95 (2005) 047601
M. Cinchetti and G. Schönhense
Two-photon photoemission spectromicroscopy of noble metal clusters on surfaces studied using time-of-flight photoemission electron microscopy
J. Phys.: Condens. Matter 17 (2005) S1319-1328
M. Cinchetti, A. Oelsner, G.H. Fecher, H.J. Elmers, G. Schönhense
Observation of Cu surface inhomogeneities by multiphoton photoemission spectromicroscopy
Appl. Phys. Lett. 83 (2003) 1503-1505