Fused silica is an indispensable material in emergent photonic applications due to its unique optical, mechanical, and thermal properties, especially when it is nano-structured by an ultrashort laser pulse. The precision of the laser-induced modifications relies heavily on the control of the electron excitations and transient optical properties during the laser pulse. In this work we explored the evolution of fused silica bandgap at high densities of excited electrons, using Finite-Temperature Fractional Occupation Density Functional Theory (FT-DFT). Using a molecular-level approach, a molecular cluster based on (SiO4)4- tetrahedra was shown to reproduce accurately the physical properties of amorphous silica. The proposed theoretical approach (FT-DFT) correctly describes electronic and spatial structure both at the ground state and photoexcitation-induced thermalized hot states. Under electron-matrix nonequilibrium conditions, a bandgap narrowing by 2 eV and more is shown. This is explained by a pure geometry relaxation driven by the electron redistribution during the strong laser-induced excitation. The reason for the bandgap decrease is atomic rearrangement resulting in weakening of the bonds. Such behaviour of the system under excitation has a significative impact on its stability even if changes in geometry are limited to 7.5% bond elongation before the loss of integrity of the system. According to experimental data this atomic rearrangement can be expected on the femtosecond timescale. Defect formation in fused silica due to bond breaking is finally expected to occur for electronic temperatures above 2.8 eV.

Ultrafast photoinduced isomerization is a fundamental process governing molecular dynamics both in biologically relevant chromophores and in functional materials, e.g. those based on molecular switches and motors. It is widely accepted that the efficiency of isomerization is governed by the dynamics through conical intersections, the regions of the potential energy landscape, where two potential energy surfaces cross. Recent developments in computational chemistry can help identifying conical intersections in isolated chromophores molecules and describe the relaxation dynamics. However, the complex environments hosting the chromophores have a profound influence on the dynamics through conical intersection making direct application of these methods to “real life” problems a very challenging task and underscoring the importance of experimental investigations. Most all-optical time-resolved spectroscopic techniques cannot directly capture the dynamics at conical intersections both because it is very fast and because the gap between the two electronic states vanishes at the intersection. However, the XUV-based spectroscopic techniques and, in particular, XUV time-resolved photoelectron spectroscopy (TRPES) give promise in delivering detectable signals from the regions of conical intersections. TRPES of molecular chromophores requires application of photoemission methods to the liquid phase samples (molecular solutions). Our group has recently become the first to demonstrate liquid phase TRPES of organic molecules by combining an ultrafast tunable XUV source with a microliquid jet sample delivery method and time-of-flight photoelectron detection. In this contribution we will report on the recent results applying this method to the prototypical molecules, Methyl Orange and Metanil Yellow. The experimental results are complemented by high-level time-dependent density functional theory (TDDFT) surface hopping calculations to reveal electronic state involved in ultrafast relaxation of the molecules. We will further show preliminary results for several bio-mimetic chromophores and will discuss the experimental challenges of the techniques, when working with samples of low concentration and using different solvents.

Size dependence of the photoinduced phase transition in Ti3O5 nanocrystals was recorded at SPIE Photonics Europe held in Strasbourg, France 2022.

Materials can be controlled via physical parameters like pressure or temperature. With the advent of ultrashort lasers pulses (typically femtosecond), light excitation has been added to the panel of available techniques for materials control. Two of the major aspects in so-called photo-induced phase transitions are reversibility, meaning the ability to switch back and forth between two states, and efficiency, corresponding to the ratio of the amount of transformed material on the quantity of provided light. One possible approach to amplify the photoresponse is based on the material elastic properties. In light-sensitive and volume-changing Spin Crossover (SCO) materials, the sudden generation (via a laser pulse) of a high enough fraction of photo-excited molecules (switched from Low spin to High spin state) creates local negative pressure. This drives lattice expansion that can induce additional switching of neighboring molecules through positive feedback effect. This cooperative effect is associated as expected with a well-defined threshold mechanism. To unambiguously discriminate between spin state conversion and volume change, we performed time-resolved x-ray diffraction study at ESRF synchrotron. The diffraction patterns measured from ps to µs time delays on nanocrystals powder films give a direct signature of the ultrafast volume expansion. Quantitative analysis of the powder spectra at ps time scale allows going deeper into the understanding of the cooperative aspects of the photo-induced spin conversion in these molecular materials. On the other hand, these observations were supplemented by Monte-Carlo simulations from a mechano-elastic model. These simulations corroborate the positive feedback effect of volume dilation on the spin conversion efficiency at nanosecond timescale.

New methods have emerged during the last decade for studying and controlling molecular conductors, and more generally correlated materials, with the use of external dynamical control parameters such as electric field or light pulse. It allows acting dynamically on materials and thus explore what lies behind the “static” picture. So far, those studies relying on ultrashort pulses to disturb and monitor the system during its out-of- equilibrium journey were restricted to the sole use of temperature as control parameter to depart from point in a phase diagram. Nevertheless, other control parameters such as hydrostatic pressure are mandatory to explore the wealth of phase space and may tune fundamental quantities such as electronic correlations. Recent publications have demonstrated the interest of coupling the control of temperature and pressure for time resolved studies. We have developed a robust setup allowing to perform P/T tunable time-resolved experiment. Indeed, it grants access to a broad phase space spanning 0-7 kbars and 300-10 K, well suited for studies of soft correlated molecular crystals.

In recent years, 2D materials attracted substantial scientific interest from a fundamental perspective, and for their potential technological applications. In this context, transient changes of the optical properties, in particular the A exciton resonance (AX) were investigated, which displays complex dynamics (linewidth broadening, blue, and red shifts) depending on substrate, excitation density, and photon energy. However, the particular interplay of the dielectric screening of the substrate and the dynamic screening of quasi-free carriers and excitons on exciton binding energy and free carrier band gap remains elusive. We access the time-dependent dielectric function of the A exciton in monolayer WS2 on different substrates using reflectivity and transmittivity contrast experiments and applying the 2D linearized [1] and the Fresnel transfer matrix model, tracking peak shift and broadening of the resonance upon photoexcitation [2]. These are then described by a simple two/three-level model based on the time evolution of the photo-excited carrier and exciton populations. We determine exciton formation and recombination times, the Auger recombination rate, broadening parameters of quasiparticle scattering events, the free carrier band gap, and its renormalization parameter in good agreement with literature values where available. This allows the disentanglement of the thermal, exciton, and free carrier contribution to the XA peak shift and broadening as well as to conclude that (i) linewidth broadening is dominated by exciton scattering, (ii) exciton screening acts mainly on the exciton binding energy, while (iii) quasi-free carriers are largely responsible for band gap renormalization. These findings constitute a comprehensive picture of the non-equilibrium dynamics in monolayer WS2 and will pave the path towards the application and control also of other 2D materials’ transient properties. [1] Y. Li et al. 2D Mater. 5, 025021 (2018) [2] S. Calati et al. PCCP 23, 22640 (2021) [3] S. Calati, Q. Li, X.-Y. Zhu, J. Stähler, submitted (2022)

Ab initio calculations of the ultrafast relaxation dynamics and electronic transport properties in semiconductors was recorded at SPIE Photonics Europe held in Strasbourg, France 2022.

Doping of organic semiconductor films enhances their conductivity for applications in organic electronics, thermoelectrics and bioelectronics. However, much remains to be learnt about the properties of the conductive charges in order to optimize the design of the materials. Electrochemical doping is not only the fundamental mechanism in organic electrochemical transistors (OECTs), used in biomedical sensors, but it also represents an ideal playground for fundamental studies. Benefits of investigating doping mechanisms via electrochemistry include controllable doping levels, reversibility and high achievable carrier densities. We introduced here a new technique, applying in-situ terahertz (THz) spectroscopy directly to an electrochemically doped polymer in combination with time-resolved spectro-electrochemistry and chronoamperometry. We evaluate the intrinsic short-range transport properties of the polymer (without the effects of long-range disorder, grain boundaries and contacts), while precisely tuning the doping level via the applied oxidation voltage. We find that polarons and bipolarons need to co-exist in an optimal ratio to reach high THz conductivity for in aqueous KPF6 electrolyte. Moreover, the analysis of the spectro-electrochemistry data shows that those two species are generated at different rates in amorphous and crystalline regions of the film. Overall, the study allows a precise understanding of the electrochemical doping mechanism of the most common semi-crystalline conjugated polymer P3HT.

Dye-Sensitized Solar Cells (DSSCs) belong to the next generation of photovoltaic technologies. They are advantageous, because in DSSCs both photon harvesting and charge separation and collection occur in the active area of the cell. Additionally, the dye wavelength-selective absorption flexibility extends up the near-IR, driving towards transparent applications [1]. We have already introduced and characterized using the fs-Transient Absorption technique, the near-IR cyanine sensitizer VG20 that strongly absorbs at 830 nm (ε=154’000 M-1cm-1) when adsorbed on TiO2 [2]. After optimization and use of the de-aggregating agent CDCA, as well as electrolyte additives to tune the SC energy, VG20 exhibited the record, by the time, product of PCE efficiency (3.1%) and Average Visible Transmission (AVT) of 75% with injection occurring in 2-4 ps. The temporal competition of energy transfer (ET) from monomers to aggregates limits the injection efficiency to 30%. Herein, we report two novel pyrrolopyrrole cyanine dyes (TB207, TB423) designed to strongly absorb in the near-IR and hinder the aggregate formation upon sensitization. Both absorb at ~760 nm (ε~138’000 M-1cm-1) in solution and when incorporated in real DSSCs cover the 650-850 nm region. By the successful combination of fs-fluorescence and fs-transient absorption set-ups in solution, in non-injecting Al2O3 DSSCs and injecting TiO2 DSSCs, it was identified for TB207 that both the injection and ET occur in a distribution of times (2-23 ps) due to the inhomogeneity of the system. The total PCE reached 4.2% for TB207, with AVT 74% and injection efficiency to be 40-60%, while for TB423 the injection efficiency reached 80%. Interestingly, for the latter no pronounced dependence on the aggregate concentrations was recorded indicating possible secondary high yield injection from aggregates. Finally, the next experiments are focusing on new promising dyes designed to overcome limitation issues with target the optimization towards transparent and colourless devices. [1] F. Grifoni et al., Adv. Energy Mat., 2021, 2101598, 1-47 [2] W. Naim et al., JACS Au, 2021, 2021, 1, 4, 409–426

In strong coupling light-matter coupling, optical transitions in a material are resonantly coupled to the electromagnetic mode confined within a cavity. During the past decade, strong coupling in molecular system is gaining increasing interest, which is driven by recent observations that the hybridization of the molecular wavefunctions with the photonic ones can be used for modifying chemical reactions and material properties. Until today, research in this evolving new field, which is known as polaritonic chemistry, had focused on the internal degrees of freedom of molecules, involving the coupling of either electronic or vibrational transitions to optical cavities in the visible or mid-IR spectral region. Here, I will present our recent results, demonstrating for the first time vibrational strong coupling with intermolecular THz vibrations in organic materials. In many cases, these excitations correspond to a collective, oscillatory motion of the molecules with respect to one another and, as such, they govern the structural and mechanical properties of organic materials. First, I will discuss THz strong coupling in a tunable cavity containing α-lactose crystallites. By analyzing the field exiting the cavity in the time-domain, we directly observed vacuum Rabi-oscillations with a period of ~15psec, corresponding to the reversible, quantum-coherent energy exchange between the intermolecular bonds in the organic crystal network and the cavity. These results open a new path toward controlling mesoscale and bulk properties of materials and take polaritonic chemistry into a new class of material processes, for which collective, spatially extended degrees of freedom participate in the dynamics. Finally, I will discuss our recent demonstration of self-hybridization in a monolithic, 1D-photonic crystal made from α-lactose: in such a system, the intermolecular vibrations are strongly coupled to the electromagnetic modes confined by the α-lactose structure itself, without a cavity or any other external photonic structure.

The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be elucidated. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques will be presented. We show that it is possible to modulate at the picosecond time scale the ferroic order by playing with the out-of-plane and in-plane light-induced strains. These new results provide new insights for the understanding of the physics of photo-induced strain, in relation with the light-induced ferroelectric modulation in nanostructured ferroic compounds and could be the first step towards their use as on-purpose ferroic architectures in devices like actuators or modulators with ultra-short light pulses.

High order harmonic generation (HHG) is the most typical extreme nonlinear optical phenomenon, where integer multiples of driving laser photon energy are emitted, and gives us the fundamental insight of non-perturbative light-matter interaction in solids. So far, HHG have been mainly studied in semiconductors which can be described with single electron approximation. However, little has been elucidated about HHG in strongly correlated electron systems such as Mott-insulators, where electron-electron correlation plays a dominant role. Here, we firstly report HHG from the Mott-insulating phase of Ca2RuO4. By changing sample temperature, we controlled the gap energy of Ca2RuO4 and investigated the relation between HHG emission properties and gap energy. We found that yields of high harmonics are well scaled by the gap energy: exponential increase of HHG yields with an increase of gap energy for the harmonic order greater than third [1]. We also study HHG theoretically in Mott-insulators and conclude that this anomalous behavior should originate from a strong coupling between charge and other degree of freedom (spin, orbital, etc…) with the cooperation of the thermal ensemble, and the resulting strongly temperature-dependent coherence between charge carriers [2]. Our results demonstrate that correlations between different degrees of freedom, which are a characteristic feature of strongly correlated solids, have significant and nontrivial effects on nonlinear optical responses. [1] K. Uchida et al., arXiv:2106.15478 (PRL accepted). [2] Y. Murakami et al., arXiv:2203.01029.

Our aim is to better understand and observe the many events occurring during the interaction of an intense and ultrashort pulse with a dielectric material. It is a challenging task, due to the competition between many different elementary physical mechanism, all occurring at sub-picosecond or femtosecond time scale: electron-phonon interaction, elastic and inelastic electron-electron scattering - including impact ionization, formation of transient or permanent defect states, exciton self-trapping, exciton-exciton interaction, etc. The direct observation of these processes is beyond the capacity of traditional time resolved femtosecond experiment due to a lack of temporal resolution. To encompass this intrinsic difficulty, we have carried out experiments with a double exciting pulse scheme. Under appropriate conditions, we can control independently the two key parameters: plasma density and temperature. Then, using time resolved interferometry as a probe, we could directly observe for the first time an electronic avalanche induced by a laser pulse in a solid, namely crystalline SiO2 [1]. A complete modeling, using multiple rate equation and taking into account the laser propagation, allows to fully describe the experimental results. In materials where exciton self-trapping does not occur, no evidence of electronic avalanche is found, and the lifetime of excited carrier is much longer [2]. Our investigations on a large set of different materials (NaCl, KBr, CaF2, etc) provide a strong indication that a link exists between two apparently opposite relaxation mechanisms, exciton self-trapping and laser induced impact ionization and avalanche.

Last decade, understanding of transient excitation of electrons in solids has brought important developments for several classes of materials, at the level of both fundamentals and applications. While laser-excitation of dielectrics induces measurable ultrafast currents in the PHz regime, employing few-cycle laser pulses with controlled carrier envelope phase enables a coherent control of the electron dynamics with reduced crystal damage probability. Notably, the possibility of transiently closing the band-gap of solids during their irradiation by linearly polarized ultrashort laser pulses was evidenced and attributed to the light-induced Zener tunneling. The corresponding ultrafast modification of the band structure induced by laser dressing of electronic states can be measured experimentally and analyzed theoretically using the Floquet formalism. Despite its simplicity and limitations, it is applicable to several classes of materials and enables to study the effects of light coupling with electrons in solids for a wide range of experimental conditions. In this work, after preparing the electronic band structures of two metals (Au and Mo), a semiconductor (Si) and a dielectric (α-SiO2) using the density functional theory (DFT), the effects of dressing by a polarized laser light on the corresponding electronic band structures were investigated using the Floquet formalism. While a selective excitation of the electrons can be achieved via a choice of laser wavelength and field strength, the Floquet simulations illustrate how the change in crystal orientation affects the electron dynamics in solids. Overall, the proposed approach outlines promising ways for selecting materials and laser parameters, via a computer-aided manner, broadening perspectives in ultrafast photonics.

We study the electron-hole dynamics of c-plane GaN/AlN quantum dots (QDs) emitting above the bulk GaN bandgap [1] by means of time-resolved photoluminescence (TRPL) [2]. The PL dynamics displays a bi-exponential decay with a short time-constant (0.3 ns), independent of photon energy, and a longer one that follows the expected evolution of the radiative lifetime with the QD size. We attribute the fast dynamics to a non-radiative energy transfer to deeper levels, which have sufficiently low density and long lifetime such that their population can be easily saturated. A very long component, showing up as a constant background in our experiment, can thus be explained by the corresponding reverse process: carriers having experienced an ionization and/or trapping event will remain in that state during a long time period before slowly relaxing to their original state. The intrinsic radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, due to the presence of the large built-in electric field typical of such polar quantum heterostructures [3]. By analytically modeling TRPL spectra, we show that the QD luminescence undergoes a linear redshift with increasing time-delay accompanied by a decrease of its linewidth, while the initial Gaussian lineshape is conserved. From this analysis, we not only demonstrate that the experiment is performed in such excitation conditions such that the dynamical screening of the built-in field by free carriers is negligible, but we also determine the energy-dependence of the QD radiative lifetime with an improved precision when compared to the usual procedure that consists in fitting the PL decays for different QD energies. Beyond the confirmation that the dependence of the effective PL lifetime with emission energy in GaN/AlN QDs is dominated by the built-in electric field [3,4], we also show, using variational calculation, that changes in the QD emission energy by several hundreds of meV within a given QD sample can be understood through fluctuations in the lateral dot dimensions within a given QD subset corresponding to a well-defined dot height. Numerical simulations, that show good qualitative agreement with experimental data over an energy range of 1 eV and six orders of magnitude for the effective PL lifetime when assuming an electric field of 7 MV/cm, enable to evaluate the dot height in our sample to be distributed around a mean value of 1.5 nm [2]. Bibliography: [1] S. Tamariz, G. Callsen, and N. Grandjean, Applied Physics Letters 114, 082101 (2019). [2] M. Hrytsaienko, M. Gallart, M. Ziegler, O. Crégut, S. Tamariz, R. Butté, N. Grandjean B. Honerlage and P. Gilliot, Journal of Applied Physics 129, 054301 (2021) [3] T. Bretagnon, P. Lefebvre, P. Valvin, R. Bardoux, T. Guillet, T. Taliercio, B. Gil, N. Grandjean, F. Semond, B. Damilano, A. Dussaigne, and J. Massies, Phys. Rev. B 73, 113304 (2006). [4] S. Kako, M. Miyamura, K. Tachibana, K. Hoshino, and Y. Arakawa, Applied Physics Letters 83, 984 (2003)