Advances in Ultrafast Condensed Phase Physics IV

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.

From the outset of research into femtomagnetism, the field in which spins are manipulated by light on femtosecond or faster time scales, several questions have arisen and remain highly debated: How does the light interact with spin moments? How is the angular momentum conserved between the nuclei, spin, and angular momentum during this interaction? What causes the ultrafast optical switching of magnetic structures? What is the ultimate time limit on the speed of spin manipulation? What is the impact of nuclear dynamics on the light-spin interaction? In my talk I will advocate a parameter free ab-initio approach to treating ultrafast light-matter interactions, and discuss how this approach has led both to new answers to these old questions but also to the uncovering of novel and hitherto unsuspected spin dynamics phenomena [1,2]. In particular I will highlight 3 aspects of spin dynamics: (a) Femto- phono- magnetism: an extra degree of control over spin dynamics can be obtained by selective excitation of phonon modes [1,2,3]. (b) Pulse design: control of spin- valley- tronics via pulse shaping [4,5]. (c) Spin vacuum switching: full reversible switching using spin-currents. References [1] Dewhust et al., Nano Lett. 18, 1842 (2018). [2] Siegrist et al. Nature 571, 240 (2019) [3] Sharma et al. Sci. Advs. 8, eabq2021 (2022) [4] Sharma et al. Optica 9 (8), 947-952 (2022) [5] Sharma et al. Sci. Advs. 9, eadf3673 (2023)

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.

The quest for manipulation of magnetization on ultrafast timescales faces many technological challenges. Successful achievement thereof could shed light on novel fundamental phenomena, such as inertial magnetization dynamics, as well as accelerate technological advancements towards higher information processing rates. One of the recent approaches towards this end concerns excitation of magnetization dynamics via laser-induced picosecond acoustic pulses, which has given birth to the field of ultrafast magneto-acoustics. Considerable progress has been made in the field from an experimental point of view, as well as from the perspective of theoretical modelling. In this talk, we aim to review some of the aforementioned progress, as well as introduce the figure of merit for ultrafast magneto-acoustics.

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 this 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.

This work explores hybrid nanostructures responsive to external stimuli like temperature, humidity, and electrical fields. The investigation, guided by theoretical simulations, employs pump and probe transient reflectivity experiments to examine acoustic phonon dynamics (5-500 GHz) in materials such as mesoporous SiO2/TiO2 multilayers, GaAs/AlAs DBRs with mesoporous SiO2, YBCO/STO multilayers, and vanadium oxide. Findings reveal the interplay between nanostructures and stimuli, revealing their tunability and responsiveness, with potential applications in nanoacoustic sensing technologies and reconfigurable optoacoustic nanodevices.

Detection of the energy conversion pathways between photons, charge carriers, and the lattice is of fundamental importance to understand fundamental physics and to advance materials and devices. Yet, such insight remains incomplete due to experimental challenges in disentangling the various signatures on overlapping timescales. Here, we show that attosecond core-level x-ray absorption fine-structure spectroscopy (XANES) meets this challenge by providing an unambiguous and simultaneous view on the temporal evolution of the photon-carrier-phonon system. We provide surprising new results by applying the method to graphite, a seemingly well-studied system whose investigation is complicated by a variety of mechanisms occurring across a wide range of temporal scales. The simultaneous real-time measurement of electrons and holes reveals disparate scattering mechanisms for infrared excitation close to the Fermi energy.

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.

Optical cavities with nonlinear elements and delayed self-coupling are widely explored candidates for photonic reservoir computing (RC). For time series prediction applications that appear in many real-world problems, energy efficiency, robustness and performance are key indicators. With this contribution I want to clarify the role of internal dynamic coupling and timescales on the performance of a photonic RC system and discuss routes for optimization. By numerically comparing various delay-based RC systems e.g., quantum-dot lasers, spin-VCSEL (vertically emitting semiconductor lasers), and semiconductor amplifiers regarding their performance on different time series prediction tasks, to messages are emphasized: First, a concise understanding of the nonlinear dynamic response (bifurcation structure) of the chosen dynamical system is necessary in order to use its full potential for RC and prevent operation with unsuitable parameters. Second, the input scheme (optical injection, current modulation etc.) crucially changes the outcome as it changes the direction of the perturbation and therewith the nonlinearity. The input can be further utilized to externally add a memory timescale that is needed for the chosen task and thus offers an easy tunability of RC systems.

Programmable photonic circuits manipulate the flow of light on a chip by electrically controlling a set of tunable analog gates connected by optical waveguides. Light is distributed and spatially rerouted to implement various linear functions by interfering signals along different paths. A general-purpose photonic processor can be built by integrating this flexible hardware in a technology stack comprising an electronic monitoring and controlling layer and a software layer for resource control and programming. This processor can leverage the unique properties of photonics in terms of ultra-high bandwidth, high-speed operation, and low power consumption while operating in a complementary and synergistic way with electronic processors. This talk will review the recent advances in the field and it will also delve into the potential application fields for this technology including, communications, 6G systems, interconnections, switching for data centers and computing.

Ultrafast photoreactions are governed by multidimensional excited-state potential energy surfaces (PESs), which describe how the molecular potential varies with the nuclear coordinates. Nature has tailored electronically excited PESs, in which the molecular geometry is specifically modified from the ground state equilibrium configuration to efficiently convert the absorbed light energy into specific nuclear rearrangements, driving the system photochemistry. This can be rationalized by the displacements between two different PESs, crucial quantities that are encoded in the Franck-Condon overlap integrals. Conventional spectroscopic approaches probe transition amplitudes, only accessing the absolute value of nuclear displacements; herein we introduce an experimental technique, based on broadband impulsive Raman response to directly measure the magnitude and the sign of excited state displacements, revealing the first steps of photoreaction processes.

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 time-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 results. 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.

This presentation introduces a novel approach to achieve ultrafast conductivity control in LaVS3, a misfit-layered compound with an incommensurate crystal structure. By employing near-infrared light pulses, we can swiftly reduce LaVS3's conductivity within hundreds of femtoseconds. This effect results from electron promotion into localized energy states within vanadium clusters, triggered by the crystal structure's incommensurability. Our explanation combines theoretical tools, including DFT calculations, rate equations, and the two-temperature model. This research highlights the potential for ultrafast control of conductivity properties in solid materials using localized states within incommensurate systems.

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 [Quistrebert et al., in preparation].

The progress in DFT-based description of the electron-phonon scattering allowed to describe the relaxation dynamics of hot or photoexcited electrons in several materials, in very good agreement with time-resolved spec- troscopy experiments [1-3]. As hot carriers also start to attract attention in the context of emerging concepts for energy conversion, here we present our first results related to the coupling of ab initio data with device-oriented Monte Carlo simulation methods [4]. We show that DFT-based description of the electron-phonon intervalley scattering in GaAs, coupled with stochastic Monte Carlo method, allows to describe the energy transfer from electrons to phonons in transient regime, in good agreement with previous time-resolved photoemission experiments. [1] J. Sjakste et al, J. Phys: Cond. Mat. 30, 353001 (2018). [2] Chen, Sjakste et al, PNAS 117, 21962-21967 (2020). [3] H. Tanimura et al, Phys. Rev. B 100, 035201 (2019). [4] R. Sen, N. Vast, J. Sjakste, Appl. Phys. Lett. 120, 082101 (2022).

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.

We report on high harmonic generation in solids driven by a high energy fiber laser system operating around 1550 nm. The driving laser source consists in an erbium-doped fiber chirped pulse amplifier combined with a post-compression stage featuring a hollow-core photonic crystal fiber (HC-PCF) filled with noble gases. The nonlinear self-compression occurring in the HC-PCF enable the generation of ultrashort pulses with 50 fs duration and 0.91 μJ at 660 kHz repetition rate. Perturbative and non-perturbative harmonics up to H7 were observed when focusing the laser into small bandgap materials such as Zinc Oxide (ZnO). Subsequently, the system was enhanced to measure high harmonics in the extreme ultraviolet (XUV) range, with harmonics up to H25 observed using large bandgap material (magnesium oxide (MgO)). To the best of our knowledge, this represents the first solid-state HHG source driven by a high-energy few-cycle fiber laser in the telecom region.

We employed attosecond transient reflectivity spectroscopy in the extreme-ultraviolet energy range to investigate ultrafast electron dynamics in single-crystalline diamond induced by few-femtosecond light pulses. Combining experimental results with numerical time-dependent density functional theory calculations, we are able to distinguish the intricate role of inter- vs intra-band transitions which contribute to the total, sub-femtosecond optical response of the material to the external light field. Further decomposing the different transient features in terms of individual electronic transitions allowed us to understand the influence of the material band structure in defining the ultrafast response of the electronic system. Our results widen our knowledge of light-matter interaction in dielectrics at the attosecond time resolution, thereby moving one step forward towards future applications in the field of petahertz optoelectronics.

High-order harmonics were generated from mono- and polycrystaline molybdenum disulfide (MoS2) monolayers with an infrared femtosecond pulse. We control the orbital angular momentum (OAM) and spatial polarization distribution of the generation beam by using a liquid crystal Q-plate. We then measure the OAM and the full polarization map of the emitted harmonics. We observe that monocrystaline MoS2 behaves as a polarization converter, while polycrystaline MoS2 may be used as a phase mask.

We study the impact of the pulse duration on high harmonics’ spectra in wide band gap dielectrics (Al2O3, MgO, CaF2). We compress a Ti:S pulse in a hollow core fibre, allowing us to tune the pulse duration from 30 fs to 4.5 fs. We performed scans varying both intensity and pulse duration to examine macroscopic scaling laws for harmonics. We measure the flux, the cut-off energy, the divergence and the wavelength of emission. For the same intensity, the pulse duration affects the process of emission, impacting the spectro-spacial features of the harmonics.

I will discuss high-harmonic generation in a model that realizes a transition from a broken time-reversal symmetry Weyl-semimetal to a semi-Dirac regime, i.e. a gapless semimetal with dispersion that is parabolic in one direction and conical in the other two. The induced anomalous high harmonics (i.e. in the current in a direction perpendicular to the electric field) are high in particular in the semi-Dirac regime. For Weyl semimetals, I will show that anomalous high harmonics are due to excitations at momenta where the dispersion is not strictly linear and that in the linearized low energy theory the anomalous response is harmonic only.

Sub-nm scale magnetic memory devices with controllable magnetic properties at THz frequencies are required to improve data processing and spintronics technologies. An ambitious objective lies in achieving optical control of magnetic anisotropy in 2D magnets within ultrashort time scales. Toward that goal, we conduct a comprehensive investigation of sub-picosecond magnetic order and charges dynamics in FePS3 van der Waals antiferromagnet, while the magnetic phase transition is crossed at 120 K.

Within the scope of energy transition and more specifically hydrogen production, organo-metallic photosensitizers received highest attention. Initially based on noble metals, the low amount of the latters oriented research towards earth-abundant metals, such as iron. Yet, because of its smaller 3d orbitals and reduced ligand field splitting, iron does not possess its noble counterpart photophysical properties, such as the long-lived metal-to-ligand charge transfer (MLCT) state. The latter condition is necessary for an effective electron transfer to a photocatalytic moiety. Therefore, research in this field is focused on increasing the MLCT lifetime using rational ligand design for example. Here we present new quinoline-based iron(II) bidentate complexes. Analysis of transient absorption spectroscopy and time-resolved luminescence measurements data revealed a two-state parallel decay, one with a lifetime of about 100 ps. The addition of Spectro-Electro-Chemical measurements tends to point towards a long-lived MLCT state.

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 [1] with femtosecond time resolution. We then compared this data to simulations of different localized strain fields [2] in a regular quadratic lattice. While the material shows little diffuse scattering, comparison with simulations of an acoustic strain wave [3] indicates that the material is contracting in a concerted motion. [1] V. F. Kunzelmann, I. D. Sharp, Journal of Materials Chemistry A 2022, 10, 12026-12034. [2] B. Guzelturk, A. M. Lindenberg, Nature Materials 2021, 20, 618-623. [3] C. Mariette, M. Cammarata, Nature Communications 2021, 12.

Lanthanide ion Ho3+ has been co-doped together with Bi3+ into Cs2AgInCl6 double perovskite nanocrystals (DPNCs) in order to improve their excitation energy, crystallinity and PLQY. Spectral analysis reveals that varying the doping amount of Ho3+ with a fixed trace amount of bismuth (set at 1% doping) induces a dual emission phenomenon, where self-trapped excitons (STEs) are emitted in addition to characteristic emissions from Ho3+ at around 658 nm upon excitation with 368 nm energy. The temperature-dependent photoluminescence (PL) and time-resolved PL of these nanocrystals (NCs) indicated that the presence of Bi3+ ions resulted in a reduction of the excitation (absorption) energy requirements, introduced a novel avenue for absorption, and augmented the rate of energy transfer to Ho3+ ions. Transient absorption spectroscopy (TAS) findings confirmed the presence of nonradiative energy transfer from STE states. We conducted an in-depth analysis of TAS data to elucidate the emergence of energy transfer mechanisms and pathways from STEs to the excited states of holmium.

Ab-initio DFT methods and atomistic models have allowed the evaluation of macroscopic frequency-dependent responses, both in the linear and beyond-linear field regime. However, these response functions are found after a self-consistent calculation of the band structure, and therefore do not include many-body correlations beyond mean field. In some scenarios, as in gapped materials, including electron-hole correlations in the theory is a must to correctly match with experiments, even in a qualitative picture. Here, we show our on-going implementation of electron-hole interactions in the second-order AC response to a uniform electric field. We set a workflow to (i) set a quasiparticle band structure in the whole BZ, (ii) solve the exciton many-body states using the BSE equation with an effective kernel interaction and (iii) evaluate the shift-current DC response as a first calculation. Our approach is planned to work by using any Hamiltonian projected in a local orbital basis as the starting point; those include tight-binding models, Wannier functions-based interpolations or first-principles DFT codes using atomic orbitals basis sets.

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) [1] are nowadays used to study the structure and dynamics in matter with unprecedented temporal and spatial resolutions. With the current 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 [2], 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 [3] arranged in the camera-sample-camera sequence. [1] B. W. J. McNeil, N. R. Thompson, Nature Photonics 4 (2010) 814. [2] D. J. Bradley et al., Optics Communications 15 (1975) 231. [3] U. Frühling et al., Nature Photonics 3 (2009) 523.

Optophononic resonators based on GaAs/AlAs multilayer structures can confine near-infrared photons and subterahertz phonons. The confinement of phonons can be studied through coherent phonon generation and detection using time-domain Brillouin scattering technique. The optimal generation condition occurs when the laser is in resonance with the cavity mode, maximizing the electromagnetic field inside the cavity. At the same time, the most sensitive position for detection is at the slope of the cavity mode. Therefore, it is challenging to make the two processes efficient simultaneously when there exists only one optical cavity mode. In this work, we experimentally demonstrate that by shaping a micropillar with an elliptical cross-section, the two non-degenerate optical modes enable the enhancement of generation and detection of acoustic phonons at the same wavelength. We report a significant improvement in the phonon signal when comparing the results from elliptical and circular micropillars. This study contributes to advancing the development of an efficient platform for exciting and sensing acoustic phonons.

Various methods have been developed to retrieve the full temporal characteristics on femtosecond laser pulses in IR. We present here a new technique suitable to any XUV femtosecond to picosecond source. It combines a frequency resolved cross correlation of XUV high harmonics and IR laser pulses, with a strong transient multiphoton absorption in gases. This allows getting various temporal information on the IR laser pulses. But more importantly, we have measured temporal intensity profiles and precise pulse durations of the XUV pulses all together with spectra. This gives access to phase information using iterative phase retrieval algorithms.

We have studied the Hubbard model on a dimer under the effect of an intense ultra-short laser pulse using the Dynamical Projective Operatorial Approach (DPOA) [1, 2]. This shows how DPOA, stemming from the Composite Operator Method [3-5], can effectively tackle strongly correlated systems and real materials [6]. To study the effect of the pump pulse on the anti-ferromagnetic-like order, we have analyzed the behavior of the double occupancy. It turns out that the pulse, in a specific range of its parameters, can drive the system in and out of the otherwise very robust anti-ferromagnetic-like phase that naturally emerges at half-filling and low temperatures. In such a situation, the system undergoes Rabi-like oscillations. [1] Eskandari-Asl, A. & Avella, A., DPOA for out-of-equilibrium systems and its application to TR-ARPES, arXiv:2307.01244 (2023) [2] Eskandari-Asl, A. & Avella, A., Generalized Linear Response Theory for Pumped Systems and ..., arXiv:2404.10768 (2024) [3] Mancini, F. & Avella, A., Adv. Phys. 53 (2004) 537 [4] Avella, A., Adv. Cond. Matt. Phys. 2014 (2014) 515698 [5] Avella, A., Eur. Phys. J. B 87 (2014) 45 [6] Inzani, G. et al., Nature Photonics 17 (2023) 1059

Recent experimental advancements in ultrafast pump-probe setups drive a growing need for efficient and insightful theoretical methods. This study will delve into the internals of our newly developed method, the Dynamical Projective Operatorial Approach (DPOA). DPOA stands out as a highly efficient model-Hamiltonian method capable of calculating a wide range of single/multi-time single/multi-particle properties, such as excitation populations, out-of-equilibrium Green's functions, TR-ARPES signals, and various response functions. When dealing with transient optical properties, utilizing the DPOA formulation and capitalizing on the weak intensity of the probe pulse, we efficiently compute the linear response of the pumped system to a generic probe pulse, significantly accelerating calculations. Alongside the theoretical framework, we present noteworthy results that experimentalists can utilize to establish connections between observed effects and their underlying physical phenomena.

3D laser nanoprinting based on multi-photon absorption (or multi-step absorption) has become an established commercially available and widespread technology. Here, we focus on recent progress concerning increasing print speed, improving the accessible spatial resolution beyond the diffraction limit, increasing the palette of available materials, and reducing instrument cost.

Biological discovery is a driving force of biomedical progress. With rapidly advancing technology to collect and analyze information from cells and tissues, we generate biomedical knowledge at rates never before attainable to science. Nevertheless, conversion of this knowledge to patient benefits remains a slow process. To accelerate the process of reaching solutions for healthcare, it would be important to complement this culture of discovery with a culture of problem-solving in healthcare. The talk focuses on recent progress with optical and optoacoustic technologies, as well as computational methods, which open new paths for solutions in biology and medicine. Particular attention is given on the use of these technologies for early detection and monitoring of disease evolution. The talk further shows new classes of imaging systems and sensors for assessing biochemical and pathophysiological parameters of systemic diseases, complement knowledge from –omic analytics and drive integrated solutions for improving healthcare.

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.

High-harmonic spectroscopy is an all-optical nonlinear technique with inherent attosecond temporal resolution. It has been applied to a variety of systems in the gas phase and solid state. Here we extend its use to liquid samples. By studying high-harmonic generation over a broad range of wavelengths and intensities, we show that the cut-off energy is independent of the wavelength beyond a threshold intensity and that it is a characteristic property of the studied liquid. We explain these observations with a semi-classical model based on electron trajectories that are limited by the electron scattering. This is further confirmed by measurements performed with elliptically polarized light and with ab-initio time-dependent density functional theory calculations. Our results propose high-harmonic spectroscopy as an all-optical approach for determining the effective mean free paths of slow electrons in liquids.

High-harmonic generation (HHG) is a light up-conversion process occurring in a strong laser field, leading to coherent attosecond bursts of extreme broadband radiation. As a new paradigm, attosecond electronic or photonic processes such as HHG can potentially generate non-classical states of light well before the decoherence of the system occurs. Here, we display first evidences of quantum properties in bipartite system of HHG generated from various semiconductors such as silicon, gallium arsenide or zinc oxide. Among the results, supported by theory, we report super-poissonian and intensity dependent photon statistics as well as a violation of the Cauchy-Schwarz inequality. Those results are a first step towards proof of multipartite broadband entanglement or multimode squeezing of high harmonics generated light promising a high impact for applications in quantum communication and metrology.

The valley degree of freedom of electrons in materials promises routes toward energy- efficient information storage with enticing prospects towards quantum information processing. Current challenges in utilizing valley polarization are symmetry conditions that require monolayer structures or specific material engineering, non-resonant optical control to avoid energy dissipation, and the ability to switch valley polarization at optical speed. We demonstrate all-optical and non-resonant control over valley polarization using bulk MoS2, a centrosymmetric material with zero Berry curvature at the valleys. Our universal method utilizes spin-angular momentum-shaped tri-foil optical control pulses to switch the material's electronic topology and induce valley polarization by transiently breaking time and space inversion symmetry through a simple phase rotation. The dependence of the generation of the second harmonic of an optical probe pulse on the phase rotation directly demonstrates the efficacy of valley polarization.

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 enables 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 allows 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.