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Ultrafast phenomena in nanoparticle excitations

Final Report Summary - UPNEX (Ultrafast phenomena in nanoparticle excitations)

The proposed research aimed at the investigation of ultrafast nanoparticle excitations, especially in the sub-10-fs domain. To this end, state-of-the-art controlled optical waveforms were used with pulse durations comparable to the oscillation cycle of light. These sources have been enabling breakthrough achievements in ultrafast science in recent years and the extension of their use in nanoplasmonics also promises novel results. I investigated spatially and spectrally resolved photoelectron emission from nanostructured metal samples. Due to the original scientist-in-charge leaving the host institute during the middle of the project, I had the chance to embark on new research with the new scientist-in-charge, Prof. Ferenc Krausz in the last 9 months of the project. This involved femtosecond light source development and, most importantly, the investigation of optically induced currents in dielectric samples.

Related to the first project objective, first I constructed a retarding field imaging spectrometer to investigate photoemission and other strong-field processes from plasmonic nanoparticles. After initial experiments, this device was transferred to the University of Erlangen where I and other colleagues continued to use it as a standard tool. Currently it is used to observe signatures of the carrier-envelope phase of ultrashort laser pulses imprinted on photoelectron trajectories. We develop this device further by changing the detector readout and imaging parts since I found the carrier-envelope phase contrast much weaker than expected. The project helped to establish a long-term collaboration in this field that will go well beyond the project end date. Related to the first project objective, I also established a new collaboration with the University of Exeter where THz generation from plasmonic nanoparticles was scrutinized. We published results of this research recently by demonstrating the correlation of the plasmonic resonance of nanostructured samples and the THz signal generated from the sample. We found that for low intensities it is rather optical rectification type processes that contribute to the observed, surprisingly high THz signal, whereas for higher intensities it is plasmonic photoemission being responsible. By clarifying these mechanisms, we opened the pathway toward the exploitation of related phenomena in the field of laser-driven surface-integrated THz sources.

As for the second project objective, I set up another a vacuum chamber in which two opposing nanotips can be controllably investigated. Instead of having samples with coupled plasmonic nanoparticles, we opted for this solution since having two nanotips on nanopositioners enables fine control over the coupling of these nanosystems. I devised new ways for approaching the tips down to a distance of some 100-150 nm reproducibly by two alternative, complementary methods. After moving this chamber to the University of Erlangen as well, it was used successfully for demonstrating a nanoscale vacuum-tube diode consisting of two metal nano-tips as an ultrafast electronic device employing pulsed electrons emitted by few-cycle photoemission. With further tip approach below 100 nm we expect to be able to directionally control currents between the tips with optical waves, paving the way toward nanosized ultrafast switches and devices.

The realization of the third project objective was hindered by delays in the development of the 1800 nm source and the subsequent transfer of the laboratory to the University of Erlangen. Therefore, instead of working in this direction, I joined the group of the new scientist-in-charge at the host institute (Prof. Ferenc Krausz) and performed the investigation of optically induced currents in dielectrics as well as light source development tasks and, also offering a potential in research toward PHz electronics.

It was recently demonstrated that optically induced currents in dielectric media can be efficiently controlled with intense ultrashort laser pulses. This opens a pathway to the development of integrated optical components with an unprecedented speed of switching. Developments toward potential ultrafast PHz electronics architectures offers tremendous societal impact, as well. In addition to the initial research of currents in dielectrics, strong-field photoelectron emission induced by propagating surface plasmons, plasmonic nanoparticles and from (non-plasmonic) nanotips was recently demonstrated and this highly nonlinear interaction can also serve as a basis for a light-controlled ultrafast current switch. As the interactions serving as a basis for the desired switching effects take place at high laser intensity, plasmonic field enhancement in various environments will help to reach the desired switching effects with relatively low-intensity sculpted light bringing the current generation and switching within reach with more simple lasers as well. Therefore, ideal schemes will bring together metallic and dielectric structures and junctions for PHz electronics that will be necessary in later real-life applications of the architectures under scrutiny.

As first steps in this direction, we investigated the bandgap dependence of optically induced currents in dielectrics based on the recent discovery of this phenomenon (Schiffrin et al., Nature 493, 70 (2013)). For investigating the influence of the band gap three crystalline dielectrics with the same space group were chosen: CaF2 (12.1 eV bandgap), BaF2 (9.1 eV bandgap) and MgO (7.8 eV bandgap). We found that the critical field strength at which the optically induced current signal sets in increases with band gap. Alternative sample geometries were also employed to investigate the dependence of the current on crystallographic orientation of the sample, gap size, focusing conditions etc. The project is still ongoing and it served as the basis of a future long-term collaboration between the Marie Curie host institute and my new host institute. In the framework of this, we will investigate optically induced currents in semiconductor materials which can be appealing for future optoelectronics applications.

Since these experiments require state-of-the-art femtosecond light sources, I also launched a side-project investigating a pulse compression scheme of so-called long-cavity Ti:sapphire oscillators. With the help of this, we discovered a fundamentally interesting phenomenon related to the conversion of chirp upon the pulse compression process. Results of this work have been already published in Optics Letters.

In summary, the project served as an ideal training for various laboratory skills including electron spectroscopy, femtosecond light sources, nanofabrication and light-matter interaction experiments. I had the chance to work in two different groups and acquired a sound scientific knowledge in a leading institute. Some results of the work are already published in high-profile journals and some publications are being prepared right now. Long-term collaborations were also established that reach well beyond the original project duration and objectives and that wil strengthen European research.
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