Photoemission Electron Microscopy for Ultrafast Nano-Optics - Femtoseconds to Attoseconds

Sammanfattning: Ultrafast nano-optics is a new and quickly evolving research field centred around the control, manipulation, and application of light on a nanometre and femtosecond scale. This can lead to improved electro-optical devices, more sensitive spectroscopy, and real-time control of chemical reactions. However, understanding the simultaneous nanometre and femtosecond evolution of nano-optical fields requires characterization methods with ultrahigh spatiotemporal resolution. A method that during the past 15 years has shown great promise for such studies is photoemission electron microscopy (PEEM) in combination with ultrashort laser pulses. Both PEEM, nanostructure fabrication methods, and a large variety of pulsed light sources are under rapid parallel development, leading also to quickly increasing possibilities of nanometre and femtosecond characterization. This thesis explores the combination of PEEM with various state-of-the-art lab-based sources of femtosecond and attosecond pulses with wavelengths spanning from 30 nm to 1.55 µm for studies of ultrafast nano-optics. It is based on experiments carried out with five different laser systems, studying light interaction with tailored metallic and semiconducting nanostructures. The work comprises construction of new experimental setups, PEEM measurements, development of data analysis tools, and complementary investigations using techniques such as scanning electron microscopy, X-ray photoelectron spectroscopy, and scanning tunnelling microscopy. Using few-cycle pulses from an ultra-broadband Ti:sapphire oscillator, localized surface plasmons in metallic nanostructures were studied with a temporal resolution down to a few femtoseconds. Metallic structures were also studied with PEEM using femtosecond pulses in the telecommunication wavelength regime. Other light sources employed include an optical parametric chirped pulse amplification system, with which anisotropy effects in semiconductor nanowires were studied. Finally, the thesis explores the use of extreme ultraviolet attosecond pulse trains produced by high-order harmonic generation (HHG) as light source for PEEM. Working with 1 kHz repetition rate, the spatial resolution was found to be limited by space charge effects to a few hundred nanometres. However, with a new HHG system working at 200 kHz repetition rate, the resolution was improved by a factor of 2—3, along with a reduction in acquisition time by an order of magnitude. Novel high-repetition rate attosecond light sources are therefore expected to play a key role in pushing the temporal resolution of PEEM into the attosecond regime.