Session S05: Ultrafast Processes in Solids and Surfaces

Live

Time resolving photo-induced structural changes in matter, requires the tracking of initial electronic excitations and their further connection and impact on the local structure. This is a challenging endeavour, as traditional techniques can only separately address either the electronic dynamics or the changes on the atomic structure. Thus, several different methods are typically combined to gain some understanding of the physics. One has however to be very careful when combining, e.g., frequency domain with time-domain methods as it is by no means guaranteed that the same state of the system is measured. Core-level K-shell X-ray absorption near edge structure spectroscopy (XANES) is a well-established method capable to extract information on the electronic and lattice structure of a material with state selectivity. Combining its capabilities with the temporal resolution provided by attosecond soft X-ray pulses produced via high-harmonic generation (HHG) could thus address this problem. Here, we present such first measurement on graphite which provides simultaneous electronic and structural information with real-time resolution provided by using 165 as soft X-ray pulses at the K-edge of carbon at 284 eV.   

Live

Charge separation at an interface between two materials is a fundamental process in electronic components. It determines how fast signals can be transmitted in transistors and how efficiently power is generated in solar cells. Novel material combinations of stacked two-dimensional materials, so-called heterostructures, allow such interfaces to be tailored on the atomic scale. In order to investigate how fast charge transfer takes place at such an interface, we have epitaxially grown graphene, on top of the doped semiconductor silicon carbide (SiC). Such an interface is known as Schottky contact. We show that charge transfer across the graphene-SiC solid-state interface can take place within 300 attoseconds (1as = 10$^{-18}$\,s), representing the fastest charge transfer across a solid-state interface. To reveal the charge transfer time, we have developed a new method called Chameleon: We apply femtosecond laser pulses and use saturable absorption in graphene as an intrinsic clock to determine the lifetime of a photoexcited electron prior to charge transfer into SiC or inelastic scattering. $[1]$ Heide, C., Hauck, M., Higuchi, T. et al. Attosecond-fast internal photoemission. Nat. Photonics (2020) https://doi.org/10.1038/s41566-019-0580-6   

Live

Topological properties of solids can have a huge influence on the generation of high-harmonics. Such effects were first observed in one-dimensional linear chains using TDDFT simulations [1,2]. The harmonic yield of the topological and trivial phase differ by many orders of magnitude for energies below the band gap. The same difference is observed using a tight-binding approach [3]. We use this simplified tight-binding description - which is both computationally cheaper and more accessible to analytical treatments - to simulate one-dimensional chains and topological, graphene-like systems in laser fields. The bulk-boundary correspondence asserts that a nonvanishing topological invariant of the bulk results in topologically protected edge states in the corresponding finite system. The edge states play an important role in the high-harmonic generation process. As edge states are absent with periodic boundary conditions (bulk), differences in the spectra with and without periodic boundary conditions are presented and explained.\newline\newline [1] D. Bauer and K. K. Hansen, Phys. Rev. Lett. \textbf{120}, 177401 (2018)\newline [2] H. Dr\"{u}eke and D. Bauer, Phys. Rev. A \textbf{99}, 053402 (2019)\newline [3] C. J\"{u}r{\ss} and D. Bauer, Phys. Rev. B \textbf{99}, 195428 (2019)   

Live

Photoinduced charge generation plays a central role in a broad range of physical, chemical, and biological processes that underlie natural and engineered photocatalytic and photovoltaic systems. The inherent complexity of most of these systems, however, makes a direct determination of the fundamental photon-to-charge conversion mechanisms on their natural time- and length-scales challenging. We employ femtosecond time-resolved X-ray photoelectron spectroscopy (TRXPS) at the FLASH free electron laser to study the photoinduced emergence of free charge carriers in a planar heterojunction consisting of a copper-phthalocyanine (CuPc) donor and a C60 acceptor phase. The elemental and chemical sensitivity of TRXPS enables a direct quantification of the timescale and efficiency for the separation of interfacial charge transfer (ICT) states into separate charges, as well as the competing loss channel of ICT recombination in a single, interfacial site-specific measurement. Implications for the achievable light harvesting efficiency of the specific heterojunction and, more generally, for evaluating such efficiencies by complementary techniques are discussed.   

Live

The use of plasmonic metal nanoparticles (NPs) to improve the solar light-harvesting efficiencies of semiconductor (SC) systems is an attractive approach towards renewable energy technologies, such as the use of photoelectrochemical (PEC) water splitting to produce storable fuels. Improving the economic viability and efficiency of these systems requires a more complete understanding of underlying photo-induced electron transfer processes. We have used picosecond time-resolved X-ray photoemission spectroscopy (tr-XPS) to study photo-induced electron dynamics in a nanoporous TiO2 film sensitized with 20 nm gold NPs. This technique enables quantitative probing of transient charge distributions from the elementally specific perspectives of donor (AuNP) or acceptor (TiO2) sites. An injection efficiency of \textasciitilde 2 electrons per NP, corresponding to a photon-to-charge conversion efficiency of \textasciitilde 0.1{\%} has been observed, lower than previously reported for similar systems. Back electron transfer was also observed, proceeding through a predominant, fast recombination channel on a 60 \textpm 10 ps timescale, followed by a less prominent pathway, which is completed after approximately 1 ns. Implications for the design of nanoplasmonic light-harvesting systems will be discussed as well as an outlook toward studying PEC water splitting dynamics by in situ tr-XPS.   

Live

Laser-induced molecular reactions on aerosolized nanoparticles can lead to exotic chemical reactions. The formation of the trihydrogen cation H$_{3}^{+}$ from H$_{2}$O molecules attached to the surfaces of nanoparticles is one case that requires hydrogen migration and over the surface roaming mechanisms. In this presentation, we show that high-repetition rate reaction nanoscopy permits the investigation of the formation of H$_{3}^{+}$ cations on the surfaces of aerosolized nanoparticles. We use a novel high-power fiber-based laser system (Active Fiber) at a central wavelength of 1030 nm, repetition rate of 150 kHz, pulse durations of around 40 fs, and pulse energies reaching 6.2 $\mu$J. The laser pulses were tightly focused onto a jet of aerosolized nanoparticles by a spherical silver mirror (f=10 cm). The aerosol source is operated with SiO$_{2}$ nanoparticles in water. The studies show remarkably different energies between the H$_{3}^{+}$ cations created on the surface of the nanoparticles and those created from the dissociation of isolated ethanol molecules.   

Live

Strong-field ionization of non-centrosymmetric gas molecules with femtosecond lasers typically displays preferential emission directions in molecular frame photoelectron angular distributions (MF-PADs), depending whether the permanent dipole moment is parallel or anti-parallel to the laser field. So far, there was no direct analogy of such behavior in condensed matter. Two-color laser-induced ablation of ferroelectric lithium niobate (LiNbO$_{\mathrm{3}})$ was realized experimentally. Providing a macroscopic observable with large contrast, the ablated area of the material modulates by 35{\%} when scanning the relative phase between an 1800nm field and its second harmonic. Rotating the crystal by 180 degrees around the laser propagation axis, the modulation is $\pi $-shifted, although the band structure of the material remains unchanged. Numerical simulations based on a two-band model reveal that the ionization process is sensitive to the field polarity due to the microscopic spontaneous polarization of LiNbO$_{\mathrm{3}}$. Demonstrating for the first time how this fundamental property of ferroelectric materials impacts on the ionization rate, the results open new perspectives for the direct control of ionization dynamics in solids.   

Live

We report the observation of optical precursors through double-resonance cold atoms (39K). We present how double-resonances affect the optical precursor patterns, especially for the case of weakly dispersive narrow resonance. The separation of double resonance associated with the excited state energy splitting causes fast modulation patterns at a constant period of 17 ns in addition to a tunable modulation due to the carrier frequency detuning that appeared in a single-resonance Lorentz dielectric. We obtain analytic expression showing the modulation at the excited state splitting of 39K D1 transition (58MHz), by assuming interference between optical precursors originated from each peak of the double resonance. The analytic expression agrees well with our experimental data. The fixed modulation pattern will reversely be used to probe a single- or double-resonance Lorentz dielectric. Most of the resonant optical precursor research has been performed for a single-resonance because of complex analysis in double-resonance. Our work can be extended to the multi-resonance optical precursor study for the application of optical communication or biomedical imaging requiring deep penetration through multi-resonance dielectric media.   

On Demand

Modern light generation technology offers extraordinary capabilities for sculpting light pulses, with full control over individual electric field oscillations within each laser cycle. These capabilities are at the core of lightwave electronics -- the dream of ultrafast lightwave control over electron dynamics in solids, on a few-cycle to sub-cycle timescale, aiming at information processing at tera-Hertz to peta-Hertz rates. At the same time, quantum materials have opened the way to dissipationless electron transport and to the possibility to harness extra electronic degrees of freedom, such as the valley pseudospin, that can be used as additional information carriers. Im this talk, I will merge these two fields, and show a robust and general approach to ultrafast, valley-selective electron excitations in two-dimensional materials by controlling the sub-cycle structure of non-resonant driving fields at a few-femtosecond timescale. Bringing the frequency-domain concept of topological Floquet systems to the few-femtosecond time domain, I will demonstrate a transparent control mechanism in real space to induce and control topological properties on topologically-trivial monolayers, and an all-optical, non-element-specific method to coherently write, manipulate and read selective valley excitations using fields carried in a wide range of frequencies, on timescales orders of magnitude shorter than valley lifetime, crucial for implementation of valleytronic devices.