Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs

Time-Resolved Photoemission Electron Microscopy: Development and Applications

Time-resolved photoemission electron microscopy (TR-PEEM) belongs to a class of experimental techniquescombining the spatial resolution of electron-based microscopy with the time resolution of ultrafast opticalspectroscopy. This combination provides insight into fundamental processes on the nanometer spatial andfemto/picosecond time scale, such as charge carrier transport in semiconductors or collective excitations ofconduction band electrons at metal surfaces. The high spatiotemporal resolution also offers a detailed view of therelationship between local structure and ultrafast photoexcitation dynamics in nanostructures and nanostructuredmaterials, which is beneficial in exploring new materials and applications in opto-electronics and nano-optics.This thesis describes the investigation of ultrafast photoexcitation dynamics in metal- and III-V semiconductornanostructures using TR-PEEM. We investigate hot carrier cooling in individual InAs nanowires where we findevidence that electron-hole scattering strongly contributes to the intra-band energy relaxation of photoexcitedelectrons on a sub-picosecond time scale and we observe ultrafast hot electron transport towards the nanowiresurface due to an in-built electric field. We demonstrate the combination of TR-PEEM with optical time-domainspectroscopy to enable time- and excitation frequency-resolved PEEM imaging. The technique is applied to GaAssubstrates and nanowires. TR-PEEM is further used to investigate localized and propagating surface plasmonpolaritons. We explore the optical properties of disordered, porous gold nano-particles (nanosponges). Using TRPEEM,we can resolve several plasmonic hotspots with different resonance frequencies and lifetimes within singlenanosponges. We also explore excitation and temporal control of surface plasmon polaritons by means of singlelayeredcrystals of the transition metal dichalcogenide WSe2.In addition, this thesis includes developments in ultrafast optics, aiming to expand the capabilities of the TR-PEEMsetup. We present a setup for generating tunable broadband ultraviolet (UV) laser pulses via achromatic secondharmonic generation. The setup is suitable for operation at high repetition rates and low pulse energies due to its highconversion efficiency. Further, we describe a transmission grating-based interferometer for the generation of stable,phase-locked pulse pairs. Pulse shaping based on liquid crystal technology allows accurate control over the temporalshape of femtosecond laser pulses. We characterize Fabry-Perot interferences affecting the accuracy of such pulseshapers, and we demonstrate a calibration scheme to compensate for these interference effects

Micro-spectroscopy of Buried Short-Range Surface Plasmon Polaritons Supported by Thin Polycrystalline Gold Films

The dispersive properties of short-range surface plasmon polaritons are investigated at the buried interfaces in vacuum/Au/fused silica and vacuum/Au/SiO2/Si multilayer systems for different gold film thicknesses of up to 50 nm using two-photon photoemission electron microscopy. The experimental data agrees excellently with results of transfer matrix method simulations, emphasizing the sensitivity of the plasmonic wave vector to the thickness of the gold film and an ultrathin native substrate oxide layer. The results furthermore illustrate the exceptional qualification of low-energy electron photoemission techniques in studying electronic excitations at buried interfaces

A quantitative comparison of time-of-flight momentum microscopes and hemispherical analyzers for time- and angle-resolved photoemission spectroscopy experiments

Time-of-flight-based momentum microscopy has a growing presence in photoemission studies, as it enables parallel energy- and momentum-resolved acquisition of the full photoelectron distribution. Here, we report table-top extreme ultraviolet (XUV) time- and angle-resolved photoemission spectroscopy (trARPES) featuring both a hemispherical analyzer and a momentum microscope within the same setup. We present a systematic comparison of the two detection schemes and quantify experimentally relevant parameters, including pump- and probe-induced space-charge effects, detection efficiency, photoelectron count rates, and depth of focus. We highlight the advantages and limitations of both instruments based on exemplary trARPES measurements of bulk WSe2. Our analysis demonstrates the complementary nature of the two spectrometers for time-resolved ARPES experiments. Their combination in a single experimental apparatus allows us to address a broad range of scientific questions with trARPES.Comment: 19 pages, 9 figures. The following article has been submitted to Review of Scientific Instruments / AIP Publishing. After it is published, it will be found at https://aip.scitation.org/journal/rs

Time-resolved plasmonics in designed nanostructures

A metal nanoparticle can be considered as consisting of a base of positive ion cores and a sea of free electrons. When the free electrons are displaced, for example, by an incident electric field, a restoring force acts on the electrons. The electrons may then oscillate back and forth until equilibrium is reached. This oscillation occurs at the natural frequency, or eigenfrequency, of the system. By matching the driving frequency with this frequency, the amplitude (the maximum electron displacement) can be made large - the system is in resonance. This resonance mode is a plasmon. The separation of charge on that small length scale will result in a large field in the vicinity of the nanoparticle. This large field, often oscillating at optical frequencies, on the spatial scale of nanometers, has many potential applications, such as high-resolution microscopy, photo-voltaics, light emission and coherent control. Because of the interest in manipulating light on the nanoscale, particles having their resonances in the optical domain are often used. The collective electron oscillation, when resonantly excited, therefore occurs on the femtosecond timescale. Due to this ultrashort timescale, the dynamics are difficult to follow in time. The spatial confinement of the oscillation to the nanometer scale makes it challenging to also image them. This thesis explores ways of studying the ultrafast dynamics of plasmons spatially and temporally, simultaneously. Two types of experiments are discussed. The first is autocorrelation experiments where the induced and enhanced field is autocorrelated with itself. For one of these experiments, bowtie nanoantennas were manufactured, using the focused ion beam technique. In the second kind of experiment an infrared laser pulse is used to excite the plasmon, and a short attosecond pulse probes it. The work described in this thesis deals with the fabrication of nanostructures and the implementation of attosecond pulse generation schemes suitable for this purpose

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

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