We explore how attosecond x-ray and extreme ultraviolet pulses can be employed for imaging electron dynamics in real time. In the first part of my talk, I will review our theoretical developments to describe pump-probe experiments. We describe ultrafast imaging by means of photoelectron momentum microscopy with extreme ultraviolet pulses. I will talk about attosecond momentum-resolved resonant x-ray scattering that is another imaging technique to follow electron dynamics in materials. We develop an ab initio scheme based on the Bethe-Salpeter equation and the full-potential linearized augmented-plane-wave method to treat optical-pump -- resonant x-ray probe-techniques and will demonstrate our results for x-ray absorption spectroscopy of optically excited excitons in 4H-SiC. In the second part of my talk, I will present our ab initio description of experiments involving wave mixing of optical and x-ray pulses, namely, attosecond x-ray diffraction and absorption spectroscopy from materials during the time they are optically driven by light.

Group-VII transition metal dichalcogenides like ReS₂ holds novel in-plane anisotropic excitons, owing to their reduced lattice symmetry. Despite their potential, the coherent dynamics of these anisotropic excitons remain unexplored yet. To address their coherent properties, here, we perform polarization sensitive, ultrafast non-linear optical measurements on ReS₂. By implementing four-wave mixing spectroscopy along with spectral heterodyning detection at the microscopic limit, we measure the ultrafast coherence and population dynamics of the anisotropic excitonic system in layered ReS₂. We attribute their dephasing times (T2) and radiative lifetime (T1) in a sub-picosecond range for both the anisotropic excitons. We observed the robustness of the homogeneous broadening with respect to the flake thickness, excitation powers, and temperatures. Such homogeneous broadening features suggest a low excitonic disorder level, setting a unique characteristic of ReS₂ among two-dimensional semiconductor systems. Additionally, the layer-independent measured lifetime and exciton coherence times can be fundamental for understanding the exact electronic band structure of ReS₂.

Multidimensional spectroscopy techniques with high spectral and temporal resolution are instrumental for the experimental access to many-body interactions and energy migration pathways in functional materials and condensed matter systems. Here, we reveal the distinct many-body effects caused by excitons and free charge carriers in bulk semiconductors enabled by accessing the excitation spectral information. In addition, we precisely track the thermalization, cooling, and exciton formation of excited charge carriers at liquid helium temperature as a function of excess energy.

Hybridization between inter- and intralayer excitons can occur in Transition Metal Dichalcogenide (TMD) bilayers, giving rise to dipolar excitons with high oscillator strength. Such excitons can be exploited to achieve high optical nonlinearities, when TMDs are strongly coupled to light confined in optical microcavities. However, observations of TMD polaritons ultrafast temporal dynamics and their exploitation remain elusive. We performed pump-probe spectroscopy experiments at 8K in a custom-made microscope to study hBN-encapsulated monolayers and bilayers of MoS2 placed in optical microcavities. We probe the ultrafast dynamics of exciton-polaritons in such systems by resonantly exciting the cavities with femtosecond pulses and measuring the transient differential reflectivity. Our experiments revealed an ultrafast sub-picosecond switching from strong to weak coupling regime with a fast reversible recovery, and we demonstrated its high frequency operation (250 GHz) as an optical switch. The rich dynamics of TMD polaritons explored in our work give access to extreme nonlinear phenomena in TMD systems on ultrafast time scales for future optical logic gates.

Two-dimensional semiconductors offer a compelling platform for excitons with robust interaction with light, owing to their confined nature and their numerous manipulable degrees of freedom. In bilayers, interlayer excitons (IX) combine these degrees of freedom with high interactions due to their out-of-plane alignment. However, their oscillator strength is often negligible. Interlayer hybridization provides IX with a significant oscillator strength. Here, we examine the ultrafast dynamics of these hybrid IX in bilayer and trilayer MoSe₂. We find that IX are particularly strong in trilayers. These unexplored excitonic species exhibit fundamentally different dynamics from IX in bilayers, with delayed rise times of over 2 ps and significantly longer lifetimes. We attribute this to the origin of this excitonic species and confirm it with theory. Our findings offer insights into high oscillator strength, long-living interlayer excitons in trilayers, superior to their bilayer counterparts.

Using time-resolved magneto-optics, we explore the dynamics of spins and electrons in thin Fe3O4 films, through the metal-insulator transition associated to Verwey temperature. In particular, we show a clear signature of this transition in the out-of-equilibrium behavior of the different degrees of freedom. Our results are a first step towards all optical control of properties of such transitional material at ultimate timescales.

Controlling magnetization without using magnetic fields is a technology-driven strong motivation in the quest for new electronic devices allowing for fast control with low energy consumption. A lot of results exist for ultrafast demagnetization in pure material (as pure Ni for example) but it is essential to understand this phenomenon in alloys or hetero structures since these systems present the highest potential for applications. We describe how we are using the chemical selectivity and the optical properties of the high harmonics to study the demagnetization induced by femtosecond laser pulses in thin nickel cobalt films presenting weak stripes magnetic domains.

Traditionally, magnetic solids are divided into two major classes – ferro and antiferromagnets. Recently, it was realized that this division is incomplete and needs to be complemented with the third class called altermagnets. Owing to their unique properties, combining antiferromagnetic order with phenomena typical for ferromagnets, altermagnets are believed to hold a great potential for spintronics and magnonics. Here we demonstrate a new functionality of altermagnets for magnonics operating at THz clock-rates.

Ultrafast electron microscopy provides unique access to dynamics in heterogeneous nanomaterials by implementing laser-pump electron-probe spectroscopy, diffraction, and imaging. In particular, tailored optical interactions promise the coherent control of free electrons and material excitations. Here, I will discuss new opportunities in Ultrafast Transmission Electron Microscopy (UTEM) featuring coherent electron pulses. Optical phase modulation of nanometer-focussed electron beams enables mode-resolved analysis of plasmonic nanocavities, and consecutive mixing with a phase-locked reference gives access to attosecond field-driven dynamics in condensed matter systems. Furthermore, by employing high-Q integrated photonic microresonators, electron-photon coupling is realized down to the single particle level. This enables flexible and highly efficient electron beam structuring, e.g., at the driving optical frequency or deep sub-harmonics using optical beat notes or soliton pulses.

In this work, we introduce a technical approach to harness liquids for highly stable and efficient Supercontinuum Generation (SCG) at up to few hundreds of kHz. Using a differential pressure scheme, the velocity at which the liquid interacting with the laser is exchanged, is optimized to achieve pump source limited stability. This approach is validated by generating a SC in water pumped at the Fundamental Wavelength (FW) and the Second Harmonic (SH) of a Yb:KGW laser amplifier at 50 kHz and 100 kHz. In addition to its high stability, the resulting SC signal is more broadband and has a higher spectral intensity compared to the signals obtained with the established crystals Yttrium Aluminum Garnat (YAG) and sapphire.

We present a new referencing scheme for visible and near infrared ultrafast Transient Absorption (TA) measurements using a signal and a referencing beam. Instead of using the traditional ratiometric approach, where the absorption changes are referenced at each wavelength independently, we use a method used in the infrared range that fully utilized the spectral correlation between the reference and signal. We applied this method on a setup that produces an ultrabroad white light (WL) probe beam spanning the 600-1700nm range. This WL is noisy as it is generated from the Idler of an optical parametric amplifier in a YAG crystal. The new referencing scheme allowed to completely suppress the noise introduced by the WL generation. We will present the method, the setup and its application to some new organic semiconducting materials used or photovoltaic applications that benefited a lot from the broad spectral coverage and the low noise of our TA setup.

We detect macroscopic currents driven by intense light fields in a photoconductive antenna, which we switch on using ultrafast vacuum ultraviolet light pulses. By comparing these currents with the vector potential of the incident light, we can follow nonequilibrium inter- and intra-band carrier dynamics with attosecond resolution.

Strong coupling in cavity system provides the formation of hybrid polariton states with mixed excitonic and photonic nature. Recently, cavity systems comprised of donor-acceptor organic semiconductors have shown long-range energy transfer between excitons up to a distance of few micrometers, overcoming the limit imposed by Förtser theory. Here, we exploit two-dimensional electronic spectroscopy to study 2micron distanced j-aggregated semiconductors embedded in a microcavity. The high temporal/spectral resolution provided by this technique and the balanced photonic-excitonic nature of the polaritons produce an ultrafast energy delocalization among the entire system by promoting a quasi-instantaneous energy transfer from the energetically higher polariton to all the other states. Our findings manifest the ability of polaritons to connect different excitonic species over mesoscopic distances and exploit the cavity design to engineer new optoelectronic devices.

We use femtosecond UV-Vis absorption spectroscopy to investigate the photoreaction mechanism of a recently synthesized oxindole-based molecular switch showing a large C=C double bond photoisiomerization quantum yield (50%), and promising applications e.g. in photopharmacology. Due to an electron-donating phenol moeity, the molecular switch exhibits a push-pull electronic effect which affects its photophysical properties. In solvents of various polarities and hydrogen bonding capabilities, we observe a faster (sub-ps) photoisomerization dynamics of the deprotonated phenolate form of the compound, where the push-pull effect is enhanced. This work aims at unraveling the synthetic design strategies towards optimizing the photoreaction dynamics and quantum yiled of such molecular switches.

The excited-state dynamics of the aminoazobenzene derivative, Metanil Yellow (MY), were studied by ultrafast Transient Absorption (TA) spectroscopy and state-of-the-art XUV tTme-Resolved Photoelectron Spectroscopy (TRPES). Experiments were carried out with two different excitation wavelengths, λ=370 nm and λ=490 nm, to investigate the non-hydrated and hydrated forms of the molecule and reveal differences in their dynamics. The dynamics were also studied in two solvents, water and ethanol, to investigate the effect of hydrogen bonding with the solvent. In TRPES experiments the dynamics were studied in water solution, using a λ=400 nm pump, thus exciting both forms. The timescales from the TRPES experiments are in good agreement with the results from the TAS measurements. Based on quantum chemical calculations the dynamics are tentatively assigned to the S2→S1 conversion followed by relaxation to a long-lived state, the nature of which (possibly a twisted intramolecular charge transfer – TICT – state) remains to be confirmed.

Decoherence or dephasing of the exciton is a central characteristic of a Quantum Dot (QD) that determines the minimum width of the exciton emission line and the purity of indistinguishable photon emission during exciton recombination. Here, we analyze exciton dephasing in colloidal InP/ZnSe QDs using transient four-wave mixing spectroscopy. We obtain a dephasing time of 23ps at a temperature of 5K, which agrees with the smallest linewidth of 50ueV we measure for the exciton emission of single InP/ZnSe QDs at 5K. By determining the dephasing time as a function of temperature, we find that exciton decoherence can be described as a phonon-induced, thermally activated process. The deduced activation energy of 0.32meV corresponds to the small splitting within the nearly isotropic bright exciton triplet of InP/ZnSe QDs, suggesting that the dephasing is dominated by phonon-induced scattering within the bright exciton triplet.

We perform ultrafast Faraday holographic imaging to track the magnetization dynamics of perovskites in time and space. This interferometric imaging technique, based on off-axis holography, has the advantage of being shot-noise limited and allows us to get access to both amplitude and phase information of the measured signal. As a result, we can directly retrieve and disentangle the angular momentum and the spin components of the total magnetic moment inside the material. Here, we present our results on Methylammonium Lead Tribromide (MAPbBr3), a prototypical hybrid metal halide perovskite with captivating magnetic properties for future opto-spintronic applications.

We have studied ultrafast structural transformations in sub-picosecond laser-excited metals - thin Fe and Pd layers. The temporal evolution of the samples’ state was characterized using the x-ray diffraction technique at XFEL facility. The application of the ultrashort (fs) x-ray pulses allowed to direct probe the atomic structure of the sample with an unprecedentedly high temporal resolution of ~500 fs (relevant for the ultrafast rates of studied processes). The experimental results were compared with molecular dynamics simulations. The proposed experimental approach is matching the timescales of experimental and computational studies of structural transformations. It enabled new insight into the atomic-level mechanisms and kinetics of ultrafast phase transitions.

A Study of THz spin dynamics was performed in a single crystal of antiferromagnetic TbFeO3. Terbium orthoferrite exhibits magnetic phase transition of the Jahn-Teller type resulting in simultaneous rotation of both iron spins and terbium orbital moments and even leading to the emergence of a multiferroic state. A single-cycle THz pulse, generated in the LiNbO3 crystal, is used as a driven torque. The temperature-dependent measurements, across the phase transition region, revealed, that apart from the expected coexistence of two well-distinguished modes of antiferromagnetic resonance at 650 GHz and 450 GHz, near the phase transition temperature, the lower frequency mode bandwidth widens significantly with a subsequent increase of the spectral weight. The widening effect, revealed near the transition temperature, is due to the strong interaction between Tb-Fe sublattices. The interaction is increasing at lower temperatures so that the dynamics, detected in the Fe-sublattice, are mainly governed by the Tb-sublattice. Surprisingly, near the transition point, even lover frequency modes (~150 GHz), assigned to the impurity modes, were observed.

It is well known that the Gilbert relaxation time of a magnetic moment scales inversely with the magnitude of the externally applied field, H, and the Gilbert damping, α. Therefore, in ultrashort optical pulses, where H can temporarily be large, the Gilbert relaxation time can momentarily be extremely short, reaching even picosecond timescales. Here we show that for typical ultrashort pulses, the magnetization can respond within the optical cycle such that the optical control of the magnetization emerges by merely considering the optical magnetic field in the Landau-Lifshitz-Gilbert (LLG) equation. Interestingly, when circularly polarized optical pulses are introduced to the LLG equation, an optically induced helicity-dependent torque result. We find that the strength of the interaction is determined by η=αγH/f_opt, where f_opt and γ are the optical frequency and gyromagnetic ratio. Our results illustrate the generality of the LLG equation to the optical limit and the pivotal role of the Gilbert damping in the general interaction between optical magnetic fields and spins in solids.

Graphene nanostructures, such as Graphene Quantum Dots (G-QDs), Graphene Nanoribbons (G-NRs) and Carbon Nanotubes (C-NTs), combine the unique mechanical and electronical transport properties of sp2- hybridized carbon materials and the optical properties of direct semiconductors provided by the optical gap resulting from the reduction of dimensionally. Here we use transient absorption of 30 fs temporal resolution with polarization-controlled configuration to probe the hot exciton relaxation (internal conversion, Sn→S1) in rectangular G-QDs of various lateral lengths. We selectively excite the different samples at the second optically active electronic transition and, thought the appearance of a photo-induced emission signal at the energy corresponding to the bandedge and red-shifted vibrational replica (i.e. at the position of the steady-state photoluminescence peaks), the dynamics of relaxation were unveiled. The resulting relaxation times range from 100 fs to 175 fs. These results allowed to discuss the mechanism of relaxation, with the effect of the length of the graphene nanoflakes and of the fluence excitation.

Using ultrafast techniques, the dynamics of electrons in bulk and two-dimensional condensed matter systems can be studied with femtosecond time resolution. The spatial resolution is however limited by the diffraction limit and is therefore insufficient for investigating single nanostructures or molecules with atomic precision. As a first step to overcome this limitation, we propose to exploit the electric field of near-infrared single-cycle laser transients to coherently drive electron tunnelling across the junction of a Scanning Tunnelling Microscope (STM). We sweep the carrierenvelope phase of the laser pulses while acquiring the laser-induced current, showing optical control of the tunnelling process. This is a first step to implement femtosecond time resolution and nanometric spatial resolution within the same experimental setup.

We investigate the ultrafast photoinjection process initiated by a few-femtosecond optical pulse in monocrystalline undoped germanium with attosecond transient reflectivity spectroscopy. By comparison with theoretical calculations, we decouple several distinct but concurring physical phenomena that are found to exhibit different timing within the pump envelope. As a result of their complex interplay, we found that intra-band motion hinders charge injection, in contrast with what has previously been observed in other semiconductors.

This conference presentation was prepared for SPIE Photonics Europe, 2024

Bismuth vanadate (BiVO₄) is a key prototypical photocatalyst for water splitting, due to efficient collection of sunlight with an absorption onset at 2.5 eV, close to the maximum flux of the solar spectrum, and high solar to hydrogen conversion efficiency of up to 9.2%. Despite these promising characteristics, the fundamental nature and dynamics of photoexcitations in BiVO₄ remain unclear. We now use advances in x-ray pump-probe techniques at sub-picosecond timescales to study the interactions of photo-excitations with the crystal lattice and connected changes of the atomic valency state in BiVO₄ thin films. We measure pump-probe X-Ray Diffraction (XRD), X-ray Diffuse Scattering (XDS) and X-ray Absorption Near-Edge (XANES) at EuXFEL and APS to resolve structural and electronic dynamics. We find an unexpected ultrafast photoinduced structure change from monoclinic to tetragonal phases. From dynamics of related electronic valency changes and lattice strain fields, we draw up a detailed mechanistic model of our observations.

In this work, we utilize time-resolved fluorescence spectroscopy to investigate the exciton diffusion properties of polymeric, organic nanoparticles loaded with fluorescent dyes, which mimic the role of natural light-harvesting complexes found in photosynthetic organisms. We employ polarization-resolved fluorescence up-conversion spectroscopy to track the kinetics of fluorescence anisotropy decay, unravelling the timescales of homo- Exciton Energy Transfer (EET). Additionally, we employ photoluminescence spectroscopy to study the fluence-dependent population decay kinetics, uncovering the Singlet-Singlet exciton Annihilation (SSA) mechanism. Moreover, we explore the population kinetics of donor dyes when co-encapsulated with a fluorescent acceptor at low concentrations within the ONPs. From the measured parameters, we deduce a diffusion constant of ~0.5 nm^2/ps, resulting in a diffusion length as large as 70 nm, i.e. twice as large as the ONP diameter.

Archaerhodopsin-3 (AR-3) is a light-driven proton pump found in Halorubrum sodomense. AR-3 was put forward as a possible candidate for optogenetic investigations. Also, multiple mutants then emerged, with fluorescence quantum yields reaching up to 1.2% which is a 100-fold increase with respect to the wild-type protein. To understand this exceptionally strong effect of the mutations in detail, we studied changes in the electrostatic interactions of the protein cavity containing retinal chromophore induced in the double mutant DETC and the quintuple mutant Arch-5. Multiple ultrafast optical techniques, such as Impulsive Vibrational, Transient Absorption and Fluorescence Up-conversion spectroscopies together with temperature-dependent measurements were used to study excited state dynamics.

Excitons play an essential role in the optical response of two-dimensional materials. These are bound states due to correlations in many-body systems and are conceived as quasiparticles formed by an electron and a hole. We develop a numerical approach to simulate the electron dynamics induced by laser pulse interactions. The real-time simulations enable us to simulate the coherent dynamics of excitons and calculate the observables that can be measured in ultrafast experiments currently available in HHG laser-based laboratories. An exciting venue is the modeling of the interaction of attosecond pulses with 2D materials and the manipulation of excitons in valleytronics schemes. These simulations allow us to explore ultrafast electronics and valleytronics adding time as a control knob and exploiting electron coherence at the early times of excitation.

In the Floquet engineering picture, time periodic optical fields perturbatively replicate states shifted by photon energy quanta, and cause field-dependent Autler-Townes splitting. As the field intensifies, light matter interaction shows more non-perturbative nature. Here we reveal the onset of non-perturbative responses in multiphoton photoemission (mPP) process for a driven two-level system of Cu(111) surface states. With strong enough driving, Floquet side bands form avoided crossing gaps, and thus lead to Landau-Zener (LZ) non-adiabatic tunneling within subcycle time scale. We further simulate the population dynamics with Instantaneous Floquet State (IFS) formalism, and successfully reproduce experimental mPP features. Interpretation of the mPP process by Floquet-LZ theory elaborates the importance of non-adiabatic dynamics in strong field regime.

The formation of local strain fields is a key aspect in understanding light-induced processes in semiconductors: For instance, electric conductivity is influenced by the formation of polarons, quasiparticles that evolve from the interaction of a charge carrier with the lattice. We performed pump-probe experiments with an X-ray Free-Electron Laser (XFEL) to measure the photoinduced X-ray scattering dynamics of epitaxial BiVO4 with femtosecond time resolution. We then compared this data to simulations of different localized strain fields in a regular quadratic lattice. While the material shows little diffuse scattering, comparison with simulations of an acoustic strain wave indicates that the material is contracting in a concerted motion.

Making use of non-degenerate time-resolved pump-probe ultrafast spectroscopy, we investigate the photocarrier dynamics in a polar ZnO/Zn0.85Mg0.15O Quantum Well (QW) using a pump pulse at 266 nm, creating photocarriers in the ZnMgO barrier, and a super-continuum probing the differential reflectance of the sample in the 345-400 nm spectral range. We evidence an ultrafast capture of carriers inside the QW followed by an efficient radiative recombination. A low energy companion of the exciton is identified as an excitonic complex (trion or biexciton) while the high sensitivity our experiment enabled us to observe a signature at 60 meV higher than the fundamental exciton that we attribute to the first excited state in the QW.

The X-ray Free Electron Lasers (XFELs) are nowadays used to study the structure and dynamics in matter with unprecedented temporal and spatial resolutions. With the current X=x-ray methods, however, the access to attosecond domain, such as x-ray spectroscopy and x-ray diffraction, remains elusive. In this work we report on a new experimental approach to study sub-femtosecond processes in matter. Based on the x-ray chronoscopy concept, it explores the time distribution of ultra-short x-ray pulses before and after interaction with a sample. The pulse time structure can be measured using the state-of-the-art terahertz streaking cameras at XFELs arranged in the camera-sample-camera sequence.