Developments in Femtosecond Nanoelectronics

Ultrafast Emission and Control of Electrons in Optical Near-Fields

by Georg Herink
Doctoral thesis
Date of Examination:2014-12-16
Date of issue:2015-01-21
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Tim Salditt
Referee:Prof. Dr. Bert Hecht
Persistent Address: http://dx.doi.org/10.53846/goediss-4874

 

 

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Abstract

English

Coupling light to metal surfaces can break the diffraction limit and concentrate optical fields at the nanoscale. The studies presented in this cumulative thesis investigate the influence of optical field localization onto strong-field photoemission from metallic nanotips in a spectral range from near- to far-infrared frequencies (1 - 400 Terahertz). Intense infrared excitation is demonstrated to provide access to the strong-field photoemission regime and to the ponderomotive scaling of the electron kinetic energy. Specifically, at long wavelengths, conditions are reached, in which the quiver amplitude of the photoelectrons can exceed the dimension of the localized driving field. As a result, the light-electron interaction is confined to a fraction of the optical half-cycle, enabling quasi-instantaneous field-driven electron acceleration. A spatial adiabaticity parameter is introduced to classify the experimentally observed transition in the electron energy scaling and to distinguish the oscillatory and the sub-cycle acceleration regimes. The adaptation of a two-step model of strong-field photoemission for near-fields allows for the quantitative numerical analysis of the spatio-temporal dynamics and the reproduction of the experimental findings. In another set of experiments, employing a two-color streaking scheme with near-infrared and Terahertz (THz) pulses, the local THz-waveform at the emission site is temporally mapped and the sub-cycle acceleration dynamics are resolved in-phase. Moreover, electron dynamics at the transition from oscillatory to sub-cycle acceleration are studied via streaking experiments and numerical simulations. The findings demonstrate new schemes for the ultrafast manipulation of electron trajectories during their transit in time-varying near-fields. Direct applications arise, e.g., in the compression of electron pulses, and are sketched at the end of this work. Finally, the high field enhancement at long wavelengths is demonstrated to enable ultrafast cold field emission with THz-radiation. Peaked at high energies, the observed energy spectra reveal the characteristic signatures of emission and acceleration in the sub-cycle regime. Moreover, the nonlinearity of the field emission process is employed to temporally track the picosecond carrier cooling at the nanotip apex after intense near-infrared excitation. The findings indicate reduced cooling-rates due to the nanoscopic heat confinement and establish an optical-pump / field-emission-probe scheme for the local access to carrier-dynamics in nanostructures.
Keywords: Nonlinear Photoemission; Strong-field Physics; Mid-Infrared spectroscopy; Photoemission; Field emission; Terahertz spectroscopy; Nanostructures; Nano-optics; Streaking spectroscopy; Nanoelectronics; Laser; Near-fields optics; Nanotip; Ultrafast spectroscopy; AC-tunneling
 
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