Session R05: Ultrafast Dynamics in Solid-State Systems

Imaging the electron motion dynamics in graphene

In this work, we utilized the recently demonstrated attosecond electron microscope "Attomicroscopy" to image the electron motion dynamics in graphene. In a strong field, the electron is moving in the reciprocal space around the Dirac point following the waveform of the driver field. The attosecond electron diffraction experiment allowed us to extract the electron density distribution in the reciprocal space at different time instants and connect it with the electron motion in real space. This work allows us to see and control the light field-induced electron motion in graphene and opens the door to developing ultrafast lightwave graphene-based electronics.   

Electric field-resolved nonlinear optical spectroscopy in hexagonal boron nitride

We present measurements of the femtosecond electric field emitted from a third-order nonlinear optical interaction in hexagonal boron nitride (hBN). Using a sensitive interferometric technique known as TADPOLE, we measure the complete femtosecond electric field of the weak nonlinear signal generated using a degenerate four wave mixing (DFWM) scheme. In this scheme, three femtosecond pulses of the same wavelength (800 nm) interact with free-standing multi-layer hBN mounted on a 300-micron diameter aperture from which a nonlinear signal pulse is emitted. Two of the pulses are time delayed with respect to the third which allows us to extract dynamical information about the nonlinear interaction. Our work shows that both the amplitude and phase of the signal provide insights into nonlinear interactions occurring on ultrafast time scales. We then extend these measurements to strong-field ionized molecules in the gas-phase.   

Attosecond transient metallization in silica and diamond probed with inner-shell spectroscopy

Inner-shell attosecond transient absorption (ATA) is a promising technique for measuring ultrafast processes in solids, but experiments often hinge on simulation for relating spectra to underlying dynamics. In this talk, we present a first principles approach to simulating ATA in solids using bulk-mimicking clusters and real-time time-dependent density functional theory along with tuned range-separated hybrid functionals and Gaussian basis sets. This method provides good agreement with experimental data for the breakdown threshold of silica and diamond. This calculated breakdown voltage corresponds to a Keldysh parameter of approximately one in both cases, and thus corresponds to a transition to a tunneling regime. The calculated extreme ultraviolet ATA spectra also compare well with experiment, and in both materials the transient population in the conduction band causes a decrease in the optical density at the corresponding spectral peaks. First-principles approaches such as this are valuable for interpreting the complicated modulations in a spectrum, and for guiding future attosecond experiments on solids. Time permitting, generalizations to molecular X-ray pump/X-ray probe will also be discussed.   

Lightwave controlled band engineering of quantum materials

Stacking and twisting atom-thin structures with matching symmetry creates new superlattice structures with emergent quantum properties [1]. In parallel, coherent electron motion in solid can also be manipulated with strong light fields, leading to potential applications in quantum electronics for ultrafast switches of quantum properties at room temperature [2].

We have implemented a tailored lightwave-driven analogue to twisted layer stacking in a hexagonal boron nitride monolayer [3, 4]. We tailored the symmetry of the light waveform to that of the crystal lattice (C₃), which allowed for sub-femtosecond control over time-reversal symmetry breaking and thereby band structure engineering. In this way, for the first time, we have realized the light-analogue of the topological Haldane model [5] in an insulating material, controlling its parameters with sub-femtosecond precision. Twisting the lightwave relative to the lattice orientation enables switching between band configurations, providing unprecedented control over the magnitude and location of the band gap, and curvature. This also allows us to establish a new regime of valleytronics [6] that uses non-resonant light fields and allows valley polarization control on femtosecond timescales.   

Ultrafast Electron Dynamics in a Heterogeneous Plasmonic Photocatalyst Studied by Time-Resolved X-ray Photoelectron Spectroscopy

Heterogeneous light harvesting systems consisting of metal nanoparticles interfaced with transition metal semiconductors are among the most intensely studied platforms for solar photon based approaches to sustainable energy supplies and climate change mitigation. Yet, it remains challenging to disentangle the fundamental electronic dynamics and mechanisms that drive the photocatalytic activity, such as solar fuels production or CO₂ reduction. To address this challenge, we translate the atomic-scale sensitivity of X-ray photoemission spectroscopy (XPS) to interfacial electronic and chemical configurations into the ultrafast time-domain. Utilizing femtosecond and picosecond time-resolved XPS (TRXPS) at the FLASH Free Electron Laser and the Advanced Light Source (ALS) synchrotron, respectively, we study photoinduced charge transfer dynamics in gold nanoparticle sensitized TiO₂ under ultrahigh vacuum conditions as well as under exposure to water. Pronounced, systematic differences between dynamics at dry and H₂O-exposed interfaces indicate that conditions for photocatalytic activity of the interface improve substantially with the introduction of the reactant. We will discuss the experimental findings in light of first results of ab initio calculations aimed at revealing the underlying physical mechanisms.   

Theoretical Calculations of Ultrafast Field-resolved Four-Wave Mixing Spectroscopy in the Solid State

We develop a theoretical model to calculate the Degenerate Four-Wave mixing (DFWM) signal in large band gap materials like MgO. Using time-dependent perturbation theory, we evaluate the third order term of the Dyson series to find the microscopic current in the solids. This current arises from the perturbation of Bloch electrons in the presence of femtosecond laser pulses, which is directly related to the output electric field signal. We further investigate various properties of the DFWM signal electric field, including peak intensity and chirp, as a function of the time delay between DFWM pulses. Furthermore, we modified the current expression to include the laser-induced band modification effect. Presenting the results of the numerically calculated electric field of the signal under the approximation of constant effective mass, we elucidate how the current expression explains the essential characteristics observed in the experimental data.   

Multi-Modal Measurements of X-Ray Induced Surface Reactions in Nano-Scale Systems at XFELs

In recent years, nano-scale materials have emerged as effective catalysts for light-to-chemical energy conversion, with their nano-dimensionality notably enhancing catalytic potential, reaction rates, and yield. These materials are pivotal in clean energy science, where photoexcitation controls surface catalytic reactions through rapid electron excitation and charge carrier generation, initiating reactions on the nanosurface. Understanding the interaction between charge, energy migration, and chemical reactions is vital for advancing light harvesting systems. Here we present a multi-modal method for investigating the chemical and charge dynamics induced through the X-ray ionization of nanoparticles. This approach enables the simultaneous capture of single-shot 3D momentum-resolved ion emission spectra, X-ray photoelectron spectroscopy, and far-field coherent diffraction imaging (CDI) patterns from individual nanoparticles. Results from the experiment conducted using the SQS instrument at EuXFEL demonstrate the feasibility of capturing single-shot diffractive images that can be used to reconstruct the morphology of nanoparticles, while also offering insights into surface chemical reactions induced by X-ray irradiation. The study sheds light on the active participation of generated photoelectrons in catalytic processes, either through direct interaction with surface adsorbates or by generating secondary electrons and radicals that drive the reactions.