This PDF file contains the front matter associated with SPIE Proceedings Volume 11346, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

X-ray absorption fine-structure (XAFS) spectroscopy is a well-established technique capable of extracting information about a material’s electronic and lattice structure with atomic resolution. While the near-edge region (XANES) of a XAFS spectrum provides information about the electronic configuration, structural information is extracted from the extended XAFS (EXAFS) spectrum, consisting of several hundreds of eV above the absorption edge. With the advent of high harmonic sources, reaching photon energies in soft x-ray (SXR) region, it now becomes possible to connect the spectroscopic capabilities of XAFS to the unprecedented attosecond temporal resolution of a high harmonic source allowing the observation of electronic and lattice dynamics in real time [1,2]. Layered materials, such the transition-metal dichalcogenide TiS2 or graphite, are an emerging class of materials with attractive structural and electronic properties as they can be thinned to a single atomic layer with electron mobilities resembling that of a metal, semiconductor, or semi-metal. In this work, we utilized broadband water-window-covering attosecond SXR pulses (300 as, ranging from 200 - 550 eV) capable of accessing orbital-specific K- and L-edges of such layered materials to perform transient XAFS with attosecond time resolution [3,4]. [1] Teichmann, S. et al, "0.5-keV soft x-ray attosecond continua", Nat. Commun. 7, 11493 (2016). [2] Cousin S. et al, "Attosecond streaking in the water window: a new regime of attosecond pulse characterization", Phys. Rev. X, 7, 041030 (2017). [3] Buades, B. et al., “Dispersive soft x-ray absorption fine-structure spectroscopy in graphite with an attosecond pulse”, Optica 5 (5), 502 (2018). [4] Buades, B. et al., “Attosecond-resolved petahertz carrier motion in semi-metallic TiS2”, arXiv: 1808.06493 (2018).

Noble metal nanostructures allow to enhance and tune light absorption to efficiently produce plasmonic excitations, which couple strongly to two subsystems of excitations in the semiconductor: hot carriers and phonons. These excitations relax following complex pathways, unlocking numerous nanoplasmonic applications ranging from photocatalysis to photovoltaic. In this work, we distinguish charge carrier and phonon dynamics in a plasmonic metal/semiconductor heterostructure, with the combined use of time- and angle-resolved photoemission spectroscopy (trARPES) and femtosecond electron diffraction (FED). We use trARPES to detect the non-equilibrium charge-carrier population, while with analysis of FED diffraction patterns we single out phonon dynamics. The heterostructure is composed of Au nanoislands, grown epitaxially on single-crystalline, bulk WSe2. The epitaxial relationship between the atoms of the metal and the semiconductor is reflected on the electronic band structure and the diffraction pattern and it allows material-resolved trARPES and FED measurements. Exploiting the surface sensitivity of electron-based techniques, we restrict the probed area to the active interface, and by choosing different pump wavelengths we control the excitation of the semiconductor. Surface decoration of WSe2 with Au is found to cause a significant shortening of the excitons’ lifetime and accelerated lattice heating, which is a strong indication of charge-transfer towards Au. Moreover, Au sensitizes WSe2 to sub-band-gap photons, allowing to observe non-equilibrium phonon populations in WSe2 when the pump wavelength is longer than the semiconductor absorption threshold. The corresponding lattice heating follows a nonlinear relationship with the incident laser fluence, which can be attributed to plasmonically enhanced phonon excitation.

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization because it is active during the optical excitation and does not rely on secondary mechanisms to dissipate angular momentum [1]. The experimental detection of the optically induced spin transfer (OISTR) is challenging, as it requires access to the element-specific transient density of states around the Fermi energy. In our joint theoretical and experimental work, we show that OISTR from Pt to Co governs the ultrafast demagnetization dynamics of a CoPt alloy [2]. Furthermore, we demonstrate that the analysis of the transient helicity dependent absorption at the resonant M2,3 transition of Co in the extreme ultraviolet spectral range reveals detailed information on the transient spin-split density of states, which we can directly compare to our theoretical simulations employing time-dependent density functional theory. The comparison between the theoretical and experimental data allows us to conclude that the laser-driven spin current originates from the available minority states above the Fermi level in conjunction with the electric field of the laser pulse, making this a general phenomenon in all multi-component magnetic systems. A further all-optical study on different 3d transition metal alloys corroborates that the number of available states above the Fermi level drives OISTR, opening the possibility to control demagnetization on the fastest time scale by the engineering of the density of states. References [1] J. K. Dewhurst et al., Nano Lett., 18 (2018) [2] F. Willems et al., submitted (2019)

Since the discovery of high-Tc superconductors, understanding Mott insulating phases and their insulator to metal transitions has become increasingly important [1]. As opposed to the Mott-insulating ground state found in 3d-electron compounds, a metallic ground state is expected to be found in strontium iridates, due to the extended 5d electronic orbitals of the Ir ions. However Sr2IrO4, shows a non metallic behavior[2]. Its insulating ground state arises mainly from the cooperative action of the onsite Coulomb interaction and strong spin-orbit coupling, leading to a novel Jeff=1/2 Mott-insulating ground state [3].While the insulating ground state of Sr2IrO4 below TN = 240K is stabilized by a Mott-Slatter mecanism, the origin of the high temperature insulating ground state remains under controversy. The presence of magnetic fluctuations may also give rise to a possibly Mott-Slatter hybrid scenario in which pseudo-spins long range correlations may cooperate with spin-orbit and onsite Coulomb interaction[5-6]. A possible way to disentangle magnetic fluctuations effects from Mott physics signatures is realized by photo-exciting strontium iridate single crystals with femtosecond light pulses. Following this approach, earlier pump-probe studies [7,8] have pointed out strong similarities between iridates and cuprates electron dynamics such as a two-time scale dynamics along with the formation of in-gap states. In order to uncover short time (< 100fs) electron dynamics we present a combined super-continuum based time resolved reflectivity along with a HHG based time resolved photoemission study of Sr2IrO4 at room temperature. Our data reveal for the first time, crucial information about the time and energy resolved dynamics of the short lived in-gap states forming in the first 50fs after the photo-excitation. The origin of these in-gap states seems to be consistent with the framework of photo-doping of Mott insulators[9] in which a photo-induced Mott gap renormalization occur. [1] M. Imada, A. Fujimori et al., Rev. Mod. Phys., 70, 1039 (1998) [2] D. A. Zocco, J. J. Hamlin et al., J. Phys.: Condens. Matter, 26, 255603 (2014) [3] B. J. Kim, Hosub Jin et al., Phys. Rev. Lett., 101, 076402 (2008) [4] F. Ye, S. Chiet et al., Phys. Rev. B, 87, 140406R (2013) [5] D. Hsieh, F. Mahmood et al., Phys. Rev. B, 86, 035128 (2012) [6] R. Arita, J. Kuneš et al., JPS Conf. Proc., 3, 013023 (2014) [7] D. Hsieh, F. Mahmood et al., Phys. Rev. B, 86, 035128 (2012) [8] C. Piovera, V. Brouet, Phys. Rev. B, 93, 241114(R) (2016) [9] P. Werner and M. Eckstein, Struc. Dyn., 3, 023603 (2016)

Understanding the complex interactions associated with charge, spin, lattice and orbital degrees of freedom is fundamental for emerging applications of quantum materials. In this context, ultrafast optical spectroscopy systems are promising tools to study the origin of complex orders. Here, an intense optical pulse brings the system out-of-equilibrium, providing an excellent opportunity to distinguish the dynamics of each subsystem. Using ultrafast techniques, we investigated charge density wave (CDW) behavior in transition-metal dichalcogenides (TMDs) after photo-excitation and during the relaxation time. To unravel the mechanisms underlying the correlations in CDW systems, we combined time resolved re ectivity (TRR) and time and angle resolved photoemission spectroscopy (TARPES). Our approach provides clear evidence of the phononic contribution to CDW phenomena in 1T-TiSe₂.

The recent demonstration of gate tuneable third harmonic generation (THG) in single layer graphene (SLG) has sparked renewed interest in the study of the nonlinear optical properties of 2D materials and Dirac semimetals. Interestingly, our results also suggest new routes towards the realization of on-chip integrated nonlinear devices based on SLG, such as broadband gate tuneable nonlinear optical switches. However, a major limiting factor in this regard, which heavily suppresses the efficiency of such devices, is the increase of the SLG electronic temperature that follows from interaction with ultrashort (fs-ps) pulses. The study of the ultrafast hot electron dynamics in SLG is thus crucial both for the understanding of its nonlinear optical properties and for the realization of nonlinear optical devices such as frequency converters and saturable absorbers. In this talk I will discuss our current understanding of the interplay between hot electrons and nonlinear optics in SLG, focusing in particular on the process of THG. In SLG, the THG intensity can be tuned by over one order of magnitude by externally applied gate voltages. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear optical conductivity arising from multiphoton resonant transitions. However, both the THG intensity and its power dependence are heavily affected by an increase in the electronic temperature. I will demonstrate that hot electrons are responsible of a two-orders of magnitude reduction of the THG intensity and of a stark deviation from the cubic power law expected for THG. Finally, I will discuss possible configurations to control the hot electron recombination dynamics. These include interlayer electron-phonon interactions and gate tuning of the SLG chemical potential due to phase-space suppression of the hot electron scattering with optical phonons.

Research on dye-sensitized solar cells (DSSCs) with metal-free organic sensitizers have recently received a new impetus from the perspective to create semi-transparent DSSCs [1,2]. We report here the first femtosecond transient absorption (TAS) study of the cyanine derivative VG20 dye. In ethanol, VG20 dye is characterized by a strong absorption band peaking at 827 nm (= 245’000 M-1cm-1), and covering 700-880nm. When grafted on TiO2, aggregation is observed, which can be minimised to some extent upon the addition of CDCA [3]. After in depth optimization of the sensitizing conditions and electrolyte composition, NIR-DSSCs based on VG20 dye achieved a power conversion efficiencies (PCE) of 3.2-3.6% [3]. In order to quantify the characteristic lifetimes for self-quenching (SQ) competing with charge injection, we prepared DSSC’s either with TiO2 as an injecting material and ZrO2 as a non-injecting semi-conductor. For the cell prototypes, the photo-anodes are immersed in a highly concentrated solution of VG20 and 50 mM of CDCA for several hours, leading to a peak optical density ≈0.4 at 850 nm. The devices are then infiltrated by a liquid electrolyte, as used in the best performing dye solar cells. We conducted intensive broadband transient absorption experiments with a time resolution of 80 fs testing the sample homogeneity and cell-to-cell variability with 5-6 TiO2 DSSCs prepared under similar conditions. The differences in excited state lifetime and ground state recovery are very significant and larger than variances observed for the TiO2 cells only due to sample homogeneity and cell-to-cell variability. Indeed, the TiO2-based devices are characterised by a ≈1 ps component (30-40%) followed by a 40-50 ps decay component, while the ZrO2 devices only display the latter. Most importantly, long-lived charge injection is observed in TiO2 since the ground state bleach signal does not recover fully (≈10% time zero bleach), on the 2-ns time window probed. Next experiments, focusing on the effects of CDCA and electrolyte composition will verify to which degree the design-parameter performances correlate with the ultrafast molecular processes. [1] T. Geiger, I. Schoger, D. Rentsch, A. C. Véron, F. Oswald, T. Meyer, and F. Nüesch, Int. J. Photoenergy, 258984 (2014) [2] S. Kuster, F. Sauvage, MK Nazeeruddin, M. Grätzel, FA Nüesch, T. Geiger, Dyes and Pigments 87, 30 (2010). [3] Vittoria Novelli, PhD thesis, Université de Picardie Jules Verne Amiens, (2017)

The electron current dynamics occurring during the interaction of an intense laser pulse with a dielectric or a semiconductor material generates radiation up to the extreme ultraviolet (XUV) regime [1-3]. This phenomenon, called high-order harmonic generation (HHG), is a coherent process and is a result of the laser driven electron current oscillating at petahertz frequencies emitting photons upon the recollision of the electrons with the atom cores [4]. Since the excursion takes place within a crystal the harmonics are strongly influenced by the crystal structure and can exhibit fundamentally different behaviours compared to the well-known HHG in noble gases. In an theoretical approach this can be understood in a classical three step model where, during the interaction with a strong electric field, the electron is first tunnelled in the conduction band, is accelerated there and recombines after a certain time. Based on this concept two HHG driving mechanisms can be identified: The intraband HHG due to the oscillation of the electron in the Brillouin zone and the interband HHG due to the recombination to the valence band [5, 6]. In order to understand these contributions and fully characterize the electron trajectory a measurement of the attosecond dynamics is a crucial step. This information can be extracted from the HHG spectral phase, accessible using the RABBIT (Reconstruction of attosecond beating by interference of two photon transitions) technique. It is using two photon ionisation of a noble gas by one XUV and one fundamental photon. Since this process allows two channels for the same resulting electron energy, a beating signal at the even order harmonic positions is generated when scanning the XUV pulse with the fundamental pulse [7, 8]. The recorded photoelectron signal allows the reconstruction of the spectral phase and, therefore, direct experimental insight into the temporal attosecond structure of the XUV pulse train. The RABBIT technology is well known and established for the characterization of HHG in noble gases. As a RABBIT target commonly also noble gases are used providing a lowest possible ionisation potential of 12.13eV for Xenon. Since we achieved HHG up to 25eV in magnesium oxide a RABBIT can work in a similar fashion as for HHG from noble gases. However, the contribution of inter- and intraband harmonics is different for each energy. This generates the need to reduce the Ionisation energy of the RABBIT target to observe the effects of both contributions, as well as, enabling studies on materials with lower HHG energy cut-offs (below 12eV typically) such as zinc oxide or gallium arsenide. For these reasons, we will implement a novel RABBIT setup, replacing the noble gas with alkali metals such as potassium (Ip=4.34eV), allowing a full characterization of the spectral phase down to the UV spectral range. Ideally, we would like to resolve the electron dynamics involved in many semiconductors and dielectrics during strong field interactions. This knowledge is relevant for applications of attosecond pulses e.g. for the metrology of optoelectronic switches operating in the petahertz regime. In addition, exact knowledge hopefully allows the control of the generated pulses in time and space by enhancing or discriminating inter- or intraband processes or modulating the atto-phase of the pulse. References [1] Ghimire, S. et al., Nature Physics 7, 138 (2011) [2] Luu, T. T. et al., Nature 521, 498 (2015) [3] Ndabashimiye, G. et al., Nature 534, 520 (2016) [4] You et al., Nature Physics 13, 345 (2017) [5] Ghimire et al., Phys. Rev. A 85, 043836 (2016) [6] Vampa et al., Phys. Rev. Lett. 113, 073901 (2014) [7] P. M. Paul et al., Science 292, 5522 (2001) [8] Y. Mairesse et al., Science 302, 5650 (2003)

SnSe₂ based layered 2-dimensional alloys have drawn much attention due to their excellent optical and electronic properties in which substitutional doping is a well-defined route to modulate functionalities of devices made up of these novel materials. Herein, by using the first-principles method based density functional theory in conjunction with Boltzmann transport equation, we systematically investigate the stability, electronic structure and transport properties of the monolayer of SnSe_2(1−x)O_2x and SnSe_2(1−x)S_2x alloys at 300 K. Our results reveal that oxygen and sulphur behave as iso-electronic dopants and they do not alter the intrinsic nature of the pristine monolayer SnSe₂. The calculated indirect band gap of monolayer pristine SnSe₂, SnSe_2(1−x)O_2x, and SnSe_2(1−x)S2x is 0.79 eV, 0.78 eV, and 0.77 eV, respectively, at the compositional proportion x (0 ≤ x ≤ 0.125). These dopants create energy states near the band edge of the monolayer SnSe₂. Moreover, the corresponding effective masses have been calculated at the curvature of the band edge using the least square fitting. Electrical conductivity (σ/τ ), Seebeck coefficient (S) and thermoelectric power factor (PF/τ ) are calculated as a function of chemical potential. At the studied chemical potential range, for the pristine monolayer SnSe₂ the achieved maximum electrical conductivity, Seebeck coefficient, and thermoelectric power factor are 6.0×1019 Ω⁻¹m⁻¹s⁻¹, 1.25 mV/K and 1.28×10¹¹ Wm⁻¹K−2s⁻¹ respectively. Moreover, for SnSe₂(_1-x)O_2x and SnSe2(_1−x) S_2x, the achieved electrical conductivity, Seebeck coefficient, and thermoelectric power factor are 4.5×10¹⁹ and 5.8×10¹⁹ Ω⁻¹m−1s⁻¹, 1.15 and 1.32 mV/K, and 7.2×10¹⁰ and 1.23×10¹¹ Wm⁻¹K⁻²s⁻¹, respectively. This contribution provides an effective way to understand and improve the transport properties of 2D ternary semiconductor SnSe2_(1−x)X_2x alloys based devices which have potential applications in next-generation integrated optoelectronics due to their flexible and tunable band gap.