The Speakers

Tentative title of the talk:

Beyond ultrafast: what happens on the trajectory to a topologically ordered state?

Floquet Valleytronics in 2H-WSe2

Driving quantum materials out-of-equilibrium allows generating states of matter that are not accessible using standard equilibrium tuning methods. Upon periodic coherent driving of electrons using electromagnetic fields, Floquet–Bloch states emerge and enable the creation of exotic quantum phases. In transition metal dichalcogenides (TMDCs), broken inversion symmetry within each monolayer results in a non-zero Berry curvature at K and K' valley extrema, leading to chiroptical selection rules, which are at the heart of valleytronics. Here, we bridge the gap between Floquet engineering and valleytronics. Using time- and polarization-resolved extreme ultraviolet momentum microscopy, we demonstrate the formation of valley-polarized Floquet-Bloch states in 2H-WSe2 upon below bandgap coherent driving of electrons using chiral light pulses. We investigate quantum path interference between Floquet-Bloch and Volkov states, which is shown to be valley- and light-helicity-dependent. Lastly, we performed advanced characterization of valley-polarized Floquet-Bloch states through circular dichroism in photoelectron angular distributions (CDAD). Our results shed light on the emergence of a novel type of engineered out-of-equilibrium phase of matter upon breaking time-reversal symmetry by coherent dressing of winding Bloch electrons with chiral light.

Ultrafast dynamics at metal-oxide interfaces: traps everywhere?!

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO2 in terms of charge separation efficiency, and the physical origin of this deficiency is still under debate.

In my talk, I will firstly show in how far ZnO is special, with particular focus on its susceptibility to defects and how these can significantly alter the transition metal oxide’s surface electronic structure. [1,2] Secondly, I will focus on how charge separation is hindered at ZnO-organic hybrid interfaces: By femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations we identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. We demonstrate that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large timeframe for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.

[1] L. Gierster et al., Nat. Commun. 12 978 (2021)

[2] L. Gierster et al., Faraday Disc. DOI:10.1039/D2FD00036A (2022)

[3] L. Gierster, et al., Adv. Sci. 2403765 DOI:10.1002/advs.202403765 (2024)

Tentative title of the talk:

Vibronic Excitation Beyond Single Molecules

Direct observation of ultrafast currents in black phosphorus

Black phosphorus has been identified as an auspicious candidate for optoelectronic applications due to its direct thickness-dependent bandgap (0.3-2 eV), high carrier mobility and compatibility with existing methods of large-scale production The strongly anisotropic crystal structure of BP leads to broadband optical absorption dependent on polarization and tunable electro-optical light polarization conversion. During the talk, I will focus on time- and angle- resolved photoemission spectroscopy investigations of electron dynamics in the whole surface Brillouin zone of black phosphorus. We observed a transient carrier population imbalance in the side valleys which we attribute to generation of ultrafast currents. The experimental results will be supplemented with simulations based on the ab-initio non-equilibrium Green’s function theory. 

Tentative title of the talk:

The photoexcitation induced quasiparticle dynamics in solids - a perspective from first principles calculations

Tentative title of the talk:

Ultrafast dynamics of non-equilibrium electron distributions in laser-excited metals 

Exotic transversal plasmons in 2D semiconductors

In the first part of the presentation, I will focus to explain some new theoretical achievements and formulations for describing the electromagnetic properties of 2D crystals from the first principles. I will demonstrate that the heterostructures of 2D crystals such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN) or layered molecular crystals (such as fulerite C 60) support robust transverse polarization mode. This trapped photon mode, even results of strong coupling between interband electron-hole excitations and transversal s(TE) photons, is fundamentally distinct from extensively studied plasmon-polaritons, exciton-polaritons, as well as photon-cavity modes. It can be viewed as a transverse counterpart to the longitudinal 2D Dirac plasmon mode (DPP) found in doped graphene, characterized by coupling between intraband electron-hole excitations and longitudinal p(TM) photons. It will be demonstrated that trapped photon has much greater oscillatory strength than DPP. Furthermore, the trapped photon is easily tunable by adjusting the number of semiconducting layers, allowing for enhanced photon-electron-hole binding strength. In addition, we suggest how to achieve strong or even ultra-strong binding between excitons in some layered crystals and cavity photons.

Tentative title of the talk:

Do we understand Dynamics of Hydrogen Scattering from Surfaces?

Substrate and coverage effects in the photodesorption of CO from Pd(111)

Two pulse correlation measurements are commonly used in laser-induced femtochemistry to discern whether a certain photo-reaction is either driven by the electrons the laser excites or by phonons that are concomitantly excited by those electrons. In the case of CO photodesorption from Pd(111), the half widths at half maximum that are measured for surface coverages of 0.24 and 0.75 monolayers (ML) suggest that the process more likely is electron-driven in the former but phonon-driven in the latter. Motivated by these experiments, we constructed a multicoverage potential energy surface based on the embedded atom neural network method. The photodesorption process was simulated by combining the two temperature model–to describe the laser-excited electrons and phonons– and Langevin dynamics with electronic time-dependent temperature for both the adsorbates and the surface atoms–to model the coupling of the nuclei degree of freedom with the laser-excited electrons–. Our simulations, which can separately include the effect of either the excited electrons or excited phonons, show that phonons dominate the photodesorption process at both coverages, 0.33 and 0.75 ML. In spite of it, the 2PC measurements are qualitatively well reproduced, but only when the dependence of the palladium thermodynamic constants on the electronic temperature is adequately included the two temperature model. It is the peculiar electronic structure of Pd(111), with the d-band edge located nearby the Fermi level, that indirectly enhances the minor differences between both coverages at the short delays.

Beyond-dipole light-matter interactions in the self-consistent Maxwell-TDDFT framework: nanoplasmonics and magneto-optical effects

To better understand the quantum electrodynamics of molecular and nanoscale systems in electromagnetic environments, several semiclassical methods have been developed to make atomistic simulations more feasible. These approaches treat the radiated field classically using Maxwell's equations, with the matter system coupled self-consistently to the induced fields. These advancements have successfully captured phenomena such as radiative lifetimes and Lamb shifts, although the coupling is typically done at the electric-dipole (ED) level or, in some cases, includes higher-order interactions like magnetic dipole and electric quadrupole as done in the Maxwell-TDDFT method. While the ED approximation is suitable for many scenarios due to the relatively long wavelength of the incoming field compared to the system size, beyond-dipole effects become significant in certain contexts, such as core-level spectroscopy, nanoplasmonic systems, and strong-field phenomena. Additionally, retardation and phase-matching effects are important in self-consistently coupled light-matter simulations and should be considered.

In this talk, we present the theory and applications of a multiscale framework that enables the treatment of light-matter interactions in electromagnetic environments without relying on multipolar truncations. Specifically, we discuss the extension of the Maxwell-TDDFT method using full minimal coupling, as implemented in the open-source Octopus code. We demonstrate the effectiveness of this method in self-consistently describing phenomena such as Cherenkov radiation from an electronic wavepacket and the frequency and phase shifts of plasmonic modes in nanoparticle dimers. Furthermore, we show that beyond-dipole interactions can induce magneto-optical effects in non-chiral molecules subjected to non-chiral light, depending on the angle of incident. Finally, we will discuss the potential applications of this new framework to strong-field phenomena driven by light with orbital angular momentum, as well as the investigation of nondipole signatures in X-ray spectra.

Subcycle terahertz nanoscopy of ultrafast electron dynamics in semiconducting quantum materials

To establish a causal link between nanoscopic dynamics and macroscopic functionalities, it is essential to advance optical microscopy to the shortest length- and timescales. Here, we will discuss two recent breakthroughs in tracing ultrafast nanoscale charge carrier dynamics.

First, ultrafast terahertz nanoscopy unravels the interplay between structure, composition and carrier dynamics in individual grains of lead halide perovskite films. Phonon fingerprinting allows us to discern nano-grains with different crystallographic phases and chemical compositions. Tracing deep-subcycle shifts of the near-field waveforms upon photoexcitation, we present a way to extract the out-of-plane charge carrier diffusion on the nanoscale and find a surprising robustness against structural and chemical variations.

Secondly, we present "Near-field Optical Tunnelling Emission" (NOTE) microscopy - a novel technique that promotes near-field microscopy to the atomic scale while retaining subcycle temporal resolution for the first time. We show how it can image packing defects on gold surfaces and trace the electron flow between a scanning tip and a semiconducting van der Waals trilayer directly in the time domain. Compatible with insulating samples, NOTE provides direct access to atomic scale light-matter interaction and the subcycle quantum flow of matter.

A momentum-resolved view on ultrafast and coherent phenomena in two-dimensional materials

Transition metal dichalcogenides (TMDs) are an exciting model system to study ultrafast energy dissipation pathways, and to create and tailor emergent quantum phases. The versatility of TMDs results from the confinement of optical excitations in two-dimensions and the concomitant strong Coulomb interaction that leads to excitonic quasiparticles with binding energies in the range of several 100 meV. In TMD stacks consisting of at least two layers, the interlayer interaction can be precisely controlled by manipulating the twist angle: The misalignment of the crystallographic directions leads to a momentum mismatch between the high symmetry points of the hexagonal Brillouin zones. This strongly impacts the interlayer wavefunction hybridization, and, moreover, adds an additional moiré potential. Crucially, in this emergent energy landscape, dark intra- and interlayer excitons dominate the energy dissipation pathways. While these dark excitonic features are hard to access in all-optical experiments, time-resolved momentum microscopy [1] can provide unprecedented insight on these quasiparticles [2-5].

In my talk, I will present our recent results on the ultrafast formation dynamics of interlayer excitons in twisted WSe2/MoS2 heterostructures. First, I will report on the identification of a hallmark signature of the moiré superlattice that is imprinted onto the momentum-resolved interlayer exciton photoemission signal. With this data, we reconstruct the electronic part of the exciton wavefunction, and relate its extension to the moiré wavelength of the heterostructure. Second, I will show that interlayer excitons are effectively formed via exciton-phonon scattering, and subsequent interlayer tunneling at the interlayer hybridized Σ valleys on the sub-50 fs timescale. Finally, I will discuss our recent efforts to monitor the interlayer exciton formation dynamics with spatiotemporal resolution using femtosecond photoelectron dark-field microscopy.

Finally, I will provide an overview on how coherent light-fields can be used to create Floquet-Bloch states in monolayer graphene [6].

References

[1] Keunecke et al., Rev. Sci. Ins. 91, 063905 (2020).

[2] Schmitt et al., Nature 608, 499 (2022).

[3] Bange et al., 2D Materials 10, 035039 (2023).

[4] Bange et al., Science Advances 10, eadi1323 (2024).

[5] Schmitt et al., arXiv:2305.18908 (2023).

[6] Merboldt et al., arXiv:2404.12791 (2024).

Simulating ultrafast molecular photodynamics: from initial conditions to observables

Nonadiabatic molecular dynamics offers a powerful tool to study the excited-state dynamics of molecular systems beyond the Born-Oppenheimer approximation. The key to any nonadiabatic dynamics simulation is the definition of the initial conditions, ideally representing the initial molecular quantum state of the system of interest. We provide a detailed analysis of how initial conditions may influence the calculation of experimental observables, focusing on a range of molecular systems from small atomopheric molecules [1,2] to nucleobases in water [3]. We show how the choice of initial conditions critically affects photoabsorption cross-sections and other key observables obtained from the dynamics.

[1] A. Prlj, E. Marsili, L. Hutton, D. Hollas, D. Shchepanovska, D. R. Glowacki, P. Slavíček and B. F. E. Curchod, ACS Earth Space Chem 2022, 6, 207-217.

[2] A. Prlj, D. Hollas and B. F. E. Curchod, J. Phys. Chem A 2023, 127, 7400-7409.

[3] A. Prlj, L. Grisanti, in preparation

Charge trapping and exciton dynamics in CVD-grown atomically thin two-dimensional semiconductors

Defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) greatly influence their electronic and optical properties by introducing localized in-gap states. Using different non-invasive techniques, we have investigated the spatial distribution of intrinsic defects in as-grown chemical vapor deposition (CVD) MoS2 monolayers and correlated the results with the growth temperature of the sample. We have shown that increasing the CVD growth temperature reduces the concentration of defects, as well as their spatial distribution and type change, influencing the sample's electronic and optical properties.

In this study, we have correlated CVD synthesis parameters of as-grown single crystal MoS2 samples with their electronic and optical properties. using a combination of microscopic techniques: KPFM, temperature-dependent PL measurements, and room-temperature ultrafast transient absorption (TA) microscopy. With these non-destructive techniques, we were able to correlate the synthesis parameters with the spatial distribution of defects that are intrinsic to CVD-grown materials. It was observed that higher growth temperature results in a sample with homogeneous electronic and optical properties, with longer-lived charge carriers, due to lower defect concentration. On the other hand, the sample synthesised at a lower growth temperature has position-dependent electronic and optical properties, and its charge carriers have shorter lifetimes due to increased defect concentration.

Tentative title of the talk:

Photo-induced superconductivity: an overview of experimental and theoretical efforts

Selective electron-phonon coupling strength from nonequilbrium optical spectroscopy: the case of MgB2

The coupling between quasiparticles and bosonic excitations rules the energy transfer pathways in strongly correlated systems. How the strength of each coupling can be deduced from the characteristic time scale in the energy transfer process is debated. In this talk, I will discuss this topic on the exemplary case of MgB2, which hosts strong electron coupling with specific E2g phonon modes. By means of broadband time-resolved optical spectroscopy, we show that this selective electron-phonon coupling dictates the nonequilibrium optical response of MgB2 at early times (< 100 fs) following the photoexcitation. We quantify the selective electron-phonon coupling strength through effective-temperature modelling of the nonequilibrium optical response, and show that this is consistent with the value obtained from the dispersion renormalization of the coupled σ electronic bands measured by ARPES at equilibrium. Time-resolved optical spectroscopy performed on the isostructural compound AlB2 shows that the relaxation dynamics evolves on different pathways due to the lack of strongly coupled phonon modes. Our findings demonstrate the possibility to resolve and quantify selective electron-phonon coupling from nonequilbrium optical spectroscopy.

Towards atomic-scale imaging with attosecond electron pulses

I will review our recent efforts and progress towards time-resolved atomic-scale imaging using attosecond electron beams.

Tentative title of the talk:

Ultrafast spectroscopy and microscopy of organic microcavity polaritons