- 413 views
- 458 downloads
Imaging Ultrafast Dynamics on the Atomic Scale with a Terahertz Scanning Tunneling Microscope
-
- Author / Creator
- Jelic, Vedran
-
The combination of ultrafast laser pulses with atomic resolution microscopy has been actively pursued for the last few decades. Its achievement unlocks a new age in modern science where the fundamental interactions between individual atoms and molecules can be investigated at their intrinsic operating timescales. Conventional scanning probe techniques have demonstrated unprecedented imaging of atoms and molecules on surfaces, but they struggle to resolve any events that occur faster than a few hundred microseconds. Conversely, state-of-the-art lasers can generate sub-femtosecond pulses, but diffraction prevents their localization to nanoscale dimensions. In this thesis, ultrafast time-resolved imaging and tunneling spectroscopy is demonstrated by coupling free-space-propagating single-cycle terahertz pulses to the metallic tip of a scanning tunneling microscope (THz-STM). For the surfaces that were imaged here, a spatial resolution of 0.3 nm is achieved together with a temporal resolution of 500 fs. A thorough review of the THz-STM is presented from its early beginnings in ambient conditions, to atomic-scale imaging of semiconductor surfaces in ultrahigh vacuum. At first, the performance of the THz-STM is assessed in ambient air by imaging the picosecond dynamics of photoexcited InAs nanodots on a GaAs surface. Then, the THz-STM system is re-designed for an ultrahigh vacuum environment that enables the highly reactive Si(111)-(7x7) surface to be investigated. THz-STM imaging of this surface reveals that ultrafast THz pulses can coherently tunnel several hundred electrons across the STM junction within a few hundred femtoseconds in a manner that preserves the ultimate spatial resolution of the microscope. The THz pulses generate extreme transient current densities across the tunnel junction in excess of 10^11 A/cm^2 due to a fundamentally different terahertz tunneling conductance compared to the steady-state. This unique tunneling conductance is due to reduced screening of the THz pulse electric field at the silicon surface, which leads to subsurface THz-pulse-induced band bending and non-equilibrium charging of the poorly conducting Si(111)-(7x7) surface. Next, the ultrafast carrier dynamics of an optically excited GaAs(110) surface are imaged with the time-resolved THz-STM. The local transient surface photovoltage is found to be strongly modulated near charged defects on the sample surface. Two different regimes of the THz pulse electric field amplitude are identified. In the nearly-linear regime, the photoexcited transient surface photovoltage directly samples the THz pulse near-field waveform at the tip apex. However, at high THz pulse electric fields, ultrafast non-equilibrium charging of the GaAs subsurface dominates the response of the transient tunnel current. The high THz pulse electric field can also induce significant structural modifications to the tip apex and sample surface, but the onset of these effects is strongly dependent on the sample temperature.
-
- Graduation date
- Spring 2019
-
- Type of Item
- Thesis
-
- Degree
- Doctor of Philosophy
-
- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.