Probing Light-Matter Interactions in Plasmonic Nanotips
Benjamin Schröder
Doctoral thesis
Date of Examination:
2020-07-14
Date of issue:
2020-09-03
Advisor:
Ropers, Claus Prof. Dr.
Referee:
Ropers, Claus Prof. Dr.
Referee:
Wodtke, Alec M. Prof. Dr.
Referee:
Wenderoth, Martin Dr.
Referee:
Volkert, Cynthia Prof. Dr.
Referee:
Moshnyaga, Vasily Prof. Dr.
Referee:
Tilgner, Andreas Prof. Dr.
Persistent Address: http://hdl.handle.net/21.11130/00-1735-0000-0005-1473-3
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Abstract
English
The exploration of light-matter interaction has inspired some of the most essential theories and applications in fundamental science and technology. Cutting-edge research, such as ultrafast time-resolved and super-resolution experiments, relies on the in-depth knowledge of the interaction of photons and electrons. In these research fields, the controlled guiding, concentration, and conversion of electromagnetic energy are the central requirements of any technological advance. This thesis explores the remarkable properties of nanotips featuring waveguiding, field enhancement, and energy localization of electromagnetic surface waves which arise from collective electron oscillations – called surface plasmons (SPs). Nanotips are key components in ultrafast time-resolved electron-beam and scanning probe techniques. However, a complete physical picture of the complex SP evolution is not yet established. Here, we provide detailed contributions to the understanding of surface plasmons in metal nanotips studied in three experiments. In particular, we analyze the SP mode propagation along the tip shaft and its behavior when approaching the tip end by means of electron energy loss spectroscopy (EELS). EELS allows for spatially and spectrally resolved SP measurements. We find characteristic standing wave patterns in the SP maps and implement a semianalytical model that identifies SP back-reflection from the apex as the main reason for the observed standing waves. Our analysis reveals a reflection efficiency of nearly 100% for sufficiently small opening angles. In a subsequent experiment, we exploit the near-field enhancement of SPs to investigate nonlinear photoelectron emission from gold tips. The SP modes are excited via direct apex illumination or via grating couplers milled into the shaft several tens of micrometers away from the tip end. We demonstrate efficient remote multiphoton photoemission driven by grating-coupled plasmons by inserting the tips into a field emitter assembly enabling the control of the active emission sites along the tip structure. The final experiment explores the excitation of photoelectrons in a sub-nanometer gap between a gold tip and a flat metal substrate. For this purpose, the challenging combination of scanning tunneling microscopy with pulsed femtosecond-laser excitation is realized. Based on a one-dimensional transport model, electrons in the tip are found to absorb energy from the enhanced SP near-field in the gap. This results in a non-equilibrium charge distribution with electrons populating high-energy states and transferring to the sample. We use the locality of the photocurrent for a sophisticated imaging mode with nanometer precision.
Keywords: Nanoplasmonics; Nanotip; Scanning Tunneling Microscopy; Multiphoton Photoemission; Electron Transport; Electron Emitter; Electron Energy Loss Spectroscopy; Grating Coupling; Collective Electron Oscillations; Ultrafast Electron Excitation; Femtosecond Laser Excitation; Nonlinear Photoemission; Electron Gun; Local Excitation Microscopy
