Regensburg 2007 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 23: Symposium: Ultrafast Nanooptics III

O 23.4: Vortrag

Dienstag, 27. März 2007, 12:00–12:15, H38

Multiphoton-Photoemission Microspectroscopy of Polystyrene Microspheres — •Gerhard Lilienkamp, Florian Lindla, Christoph Senft, and Winfried Daum — Instuitut für Physik und Physikalische Technologien, TU Clausthal, Leibnizstraße 4, 38678 Clausthal-Zellerfeld

Previous multiphoton-photoemission microscopy (PEEM) studies have focused on metallic and magnetic nanostructures. We have extended the application of multiphoton-PEEM to a dielectric system. Polystyrene (PS) microspheres with diameters between 300 nm and 1000 nm deposited on an oxidized Pt substrate have been imaged using a photoelectron microscope with 400 nm (3.1 eV) femtosecond laser excitation. Despite of a minimum energy of 7.6 eV required to excite electrons from the HOMO of PS to the vacuum level [1], strong photoemission from the microspheres is observed upon illumination with 3.1 eV laser photons. Analysis of the dependence of the photoemission on the power density of the laser pulses yields a nearly pure quadratic dependence for the Pt substrate consistent with two-photon photoemission while the emission from PS contains both linear and quadratic contributions. This finding can be rationalized in terms of a population of an unoccupied state in PS by two-photon excitation with a lifetime that is large compared to the time between two consecutive laser pulses (12.5 ns). Using energy-resolved PEEM, we observed laser-induced photoconductivity of PS which is consistent with a highly filled intermediate state.

[1] C.B. Duke et al. Phys. Rev. B 187 (1978) 571