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The increasing popularity of time-resolved X-ray absorption measurements for understanding dynamics in molecular and material systems has led to many advances in table-top sources for pulsed X-rays. We report on a table top laser-produced plasma (LPP) source that can perform soft X-ray (SXR), near edge X-ray absorption fine structure (NEXAFS) measurements using a laser source with 23 ps pulse duration. The spectrometer's key specifications, such as brilliance, resolution, and stability, are characterized against the more commonly used longer-pulse-duration LPP sources. The 23 ps laser produced approximately an order of magnitude weaker SXR flux than the 8 ns laser for a higher power density due to the smaller total energy absorbed by the plasma. The increased repetition rate and an implemented self-referencing scheme still allowed for high-resolution, synchrotron-like NEXAFS measurements of Si3N4 and TiO2 thin films with 2.5 minute acquisition times. It was observed that degradation of the gas jet nozzle led to long-term instability of the source, which can be remediated using alternative nozzle designs. This work demonstrates the feasibility of achieving higher temporal resolution in future time-resolved X-ray absorption measurements using table-top laser-produced plasma sources, Comment: 8 pages, 8 figures

Photoexcitation by ultrashort laser pulses plays a crucial role in controlling reaction pathways, creating nonequilibrium material properties, and offering a microscopic view of complex dynamics at the molecular level. The photo response following a laser pulse is, in general, non-identical between multiple exposures due to spatiotemporal fluctuations in a material or the stochastic nature of dynamical pathways. However, most ultrafast experiments using a stroboscopic pump-probe scheme struggle to distinguish intrinsic sample fluctuations from extrinsic apparatus noise, often missing seemingly random deviations from the averaged shot-to-shot response. Leveraging the stability and high photon-flux of time-resolved X-ray micro-diffraction at a synchrotron, we developed a method to quantitatively characterize the shot-to-shot variation of the photoinduced dynamics in a solid-state electrolyte. By analyzing temporal evolutions of the lattice parameter of a single grain in a powder ensemble, we found that the sample responses after different shots contain random fluctuations that are, however, not independent. Instead, there is a correlation between the nonequilibrium lattice trajectories following adjacent laser shots with a characteristic "correlation length" of approximately 1,500 shots, which represents an energy barrier of 0.38~eV for switching the photoinduced pathway, a value interestingly commensurate with the activation energy of lithium ion diffusion. Not only does our nonequilibrium noise correlation spectroscopy provide a new strategy for studying fluctuations that are central to phase transitions in both condensed matter and molecular systems, it also paves the way for discovering hidden correlations and novel metastable states buried in oft-presumed random, uncorrelated fluctuating dynamics.

Thin-film lithium niobate is a promising photonic platform for on-chip optical sensing because both nonlinear and linear components can be fabricated within one integrated device. To date, waveguided sample interactions for thin-film lithium niobate are not well explored. Compared to other integrated platforms, lithium niobate's high refractive index, birefringence, and angled sidewalls present unique design challenges for evanescent wave sensing. Here, we compare the performance of the quasi-transverse-electric (TE) and the quasi-transverse-magnetic (TM) mode for sensing on a thin-film lithium niobate rib waveguide with a 5 mM dye-doped polymer cladding pumped at 406 nm. We determine that both modes have propagation losses dominated by scatter, and that the absorption due to the sample only accounts for 3% of the measured losses for both modes. The TM mode has better overlap with the sample than the TE mode, but the TM mode also has a stronger propagation loss due to sidewall and sample induced scattering (32.5 $\pm$ 0.3 dB/cm) compared to the TE mode (23.0 $\pm$ 0.2 dB/cm). The TE mode is, therefore, more appropriate for sensing. Our findings have important implications for on-chip lithium niobate-based sensor designs., Comment: 9 pages, 3 figures (main text); 6 pages, 5 figures (supplemental). Submitted to Optics Express

Resonant intermediate states have been proposed to increase the efficiency of entangled two-photon absorption (ETPA). Although resonance-enhanced ETPA (r-ETPA) has been demonstrated in atomic systems using bright squeezed vacuum, it has not been studied in organic molecules. We investigate for the first time r-ETPA in an organic molecular dye, indocyanine green (ICG), when excited by broadband entangled photons in near-IR. Similar to many reported virtual state mediated ETPA (v-ETPA) measurements, no r-ETPA signals are measured, with an experimental upper bound for the cross section placed at $6 \times 10^{-23}$ cm$^2$/molecule. In addition, the classical resonance-enhanced two-photon absorption (r-TPA) cross section of ICG at 800 nm is measured for the first time to be $20(\pm13)$ GM, suggesting that having a resonant intermediate state does not significantly enhance two-photon processes in ICG. The spectrotemporally resolved emission signatures of ICG excited by entangled photons are also presented to support this conclusion.

Superionic conductors, or solid-state ion-conductors surpassing 0.01 S/cm in conductivity, can enable more energy dense batteries, robust artificial ion pumps, and optimized fuel cells. However, tailoring superionic conductors require precise knowledge of ion migration mechanisms that are still not well understood, due to limitations set by available spectroscopic tools. Most spectroscopic techniques do not probe ion hopping at its inherent picosecond timescale, nor the many-body correlations between the migrating ions, lattice vibrational modes, and charge screening clouds--all of which are posited to greatly enhance ionic conduction. Here, we develop an ultrafast technique that measures the time-resolved change in impedance upon light excitation which triggers selective ion-coupled correlations. We also develop a cost-effective, non-time-resolved laser-driven impedance method that is more accessible for lab-scale adoption. We use both techniques to compare the relative changes in impedance of a solid-state Li+ conductor Li0.5La0.5TiO3 (LLTO) before and after UV to THz frequency excitations to elucidate the corresponding ion-many-body-interaction correlations. From our techniques, we determine that electronic screening and phonon-mode interactions dominate the ion migration pathway of LLTO. Although we only present one case study, our technique can extend to O2-, H+, or other charge carrier transport phenomena where ultrafast correlations control transport. Furthermore, the temporal relaxation of the measured impedance can distinguish ion transport effects caused by many-body correlations, optical heating, correlation, and memory behavior.

Efficient on-chip entangled photon pair generation at telecom wavelengths is an integral aspect of emerging quantum optical technologies, particularly for quantum communication and computing. However, moving to shorter wavelengths enables the use of more accessible silicon detector technology and opens up applications in imaging and spectroscopy. Here, we present high brightness ($(1.6 \pm 0.3) \times 10^{9}$ pairs/mW/nm) visible-near-IR photon pair generation in a periodically poled lithium niobate nanophotonic waveguide. The degenerate spectrum of the photon pairs is centered at 811 nm with a bandwidth of 117 nm. The measured on-chip source efficiency of $(2.3\pm 0.5) \times 10^{11}$ pairs/mW is on par with source efficiencies at telecom wavelengths and is also orders of magnitude higher than the efficiencies of other visible sources implemented in bulk crystal or diffused waveguide-based technologies. These results represent the shortest wavelength of photon pairs generated in a nanophotonic waveguide reported to date by nearly an octave., Comment: Main text: 10 pages, 6 figures; Supplementary material: 5 pages, 3 figures. Author initials updated

Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices, however, are usually measured using ultrafast pulsed lasers in a narrow wavelength range. In this work, we prove that steady-state, photomodulated X-ray spectra from a non-time-resolved synchrotron beamline can be used to estimate electron and hole distributions. A set of plasmonic metal core-shell nanoparticles is designed to systematically isolate photothermal, hot electron, and thermalized electron-hole pairs in a TiO2 shell. Steady-state changes in the Ti L2,3 edge are measured with and without continuous-wave illumination of the nanoparticle's localized surface plasmon resonance. Ab initio excited-state X-ray theory developed for transient X-ray measurements is then applied to model the experimental spectra in an attempt to extract the resultant steady-state carrier distributions. The results suggest that, within error, the quasi-equilibrium carrier distribution can be determined even from relatively noisy data with mixed excited-state phenomena., Comment: 41 pages, 18 figures

Small polaron formation is dominant across a range of condensed matter systems. Small polarons are usually studied in terms of ground-state transport and thermal fluctuations, but small polarons can also be created impulsively by photoexcitation. The temporal response of the lattice and local electron correlations can then be separated, such as with transient XUV spectroscopy. To date, photoexcited small polaron formation has only been measured to be adiabatic. The reorganization energy of the polar lattice is large enough that the first electron-optical phonon scattering event creates a small polaron without significant carrier thermalization. Here, we use transient XUV spectroscopy to measure antiadiabatic polaron formation by frustrating the iron-centered octahedra in a rare-earth orthoferrite lattice. The small polaron is measured to take several picoseconds to form over multiple coherent charge hopping events between neighboring Fe3+-Fe2+ sites, a timescale that is more than an order of magnitude longer compared to previous materials. The measured interplay between optical phonons, electron correlations, and on-site lattice deformation give a clear picture of how antiadiabatic small polaron transport would occur in the material. The measurements also confirm the prediction of the Holstein and Hubbard-Holstein model that the electron hopping integral must be larger than the reorganization energy to achieve antiadiabaticity. Moreover, the measurements emphasize the importance of considering dynamical electron correlations, and not just changes in the lattice geometry, for controlling small polarons in transport or photoexcited applications.

Fluorescence lifetime experiments are a standard approach for measuring excited state dynamics and local environment effects. Here, we show that entangled photon pairs produced from a continuous-wave (CW) laser diode can replicate pulsed laser experiments without phase modulation. As a proof of principle, picosecond fluorescence lifetimes of indocyanine green are measured in multiple environments. The use of entangled photons has three unique advantages. First, low power CW laser diodes and entangled photon source design lead to straightforward on-chip integration for a direct path to distributable fluorescence lifetime measurements. Second, the entangled pair wavelength is easily tuned by temperature or electric field, allowing a single source to cover octave bandwidths. Third, femtosecond temporal resolutions can be reached without requiring major advances in source technology or external phase modulation. Entangled photons could therefore provide increased accessibility to time-resolved fluorescence while also opening new scientific avenues in photosensitive and inherently quantum systems.

Ionic conduction in solids that exceeds 0.01 S/cm is predicted to involve collective phonon-ion interactions in the crystal lattice. Here, we use theory and experiment to measure the contribution of possible collective phonon-ion modes to Li+ migration in Li0.5La0.5TiO3. The ab initio calculations reveal that only a few phonon modes, mostly TiO6 rocking modes below 6 Terahertz, provide over 40% of the energy required for the Li+ hop in Li0.5La0.5TiO3. Laser-driving the TiO6 rocking modes decreases the measured impedance ten-fold compared to exciting acoustic and optical phonons at similar energy densities. The decreased impedance persists on thermalization timescales. These findings provide new insights on phonon-coupled ion migration mechanisms, material design rules, and the potential for metastable states for opto-ionic materials.

Electron energy-loss spectroscopy (EELS) can measure similar information to X-ray, UV-Vis, and IR spectroscopies but with atomic resolution and increased scattering cross sections. Recent advances in electron monochromators have expanded EELS capabilities from chemical identification to the realms of synchrotron-level core-loss measurements and to low-loss, 10-100 meV excitations such as phonons, excitons, and valence structure. EELS measurements are easily correlated with electron diffraction and atomic-scale real-space imaging in a transmission electron microscope (TEM) to provide detailed local pictures of quasiparticle and bonding states. This perspective provides an overview of existing high-resolution EELS (HR-EELS) capabilities while also motivating the powerful next step in the field - ultrafast EELS in a TEM. Ultrafast EELS aims to combine atomic level, element specific, and correlated temporal measurements to better understand spatially specific excited state phenomena. Ultrafast EELS measurements also add to the abilities of steady-state HR-EELS by being able to image the electromagnetic field and use electrons to excite photon-forbidden and momentum-specific transitions. We discuss the technical challenges ultrafast HR-EELS currently faces, as well as how integration with in situ and cryo measurements could expand the technique to new systems of interest, especially molecular and biological samples.

On-chip ultraviolet sources are of great interest for building compact and scalable atomic clocks, quantum computers, and spectrometers; however, few material platforms are suitable for integrated ultraviolet light generation. Of these materials, thin-film lithium niobate is the most competitive due to its ability to be quasi-phase-matched, optical confinement, and nonlinear properties. Here, we present efficient (197 $\pm$ 5 %/W/cm$^{2}$) second harmonic generation of UV-A light in a periodically poled lithium niobate nanophotonic waveguide. We achieve on-chip ultraviolet powers of 30 $\mu$W, demonstrating the potential for compact frequency-doubling of common near-IR laser diodes. By using a large cross section waveguide (600 nm film thickness), we achieve insensitivity to fabrication errors, and can attain first-order quasi-phase-matching with relatively long poling periods (>1.5 $\mu$m). The device also demonstrates linear wavelength tunability using temperature. By varying the poling period, we have achieved the shortest reported wavelength (355 nm) generated through frequency doubling in thin-film lithium niobate. Our results open up new avenues to realize ultraviolet on-chip sources and chip-scale photonics.

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core-hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides - $TiO_2, \alpha-Cr_2O_3, \beta-MnO_2, \alpha-Fe_2O_3, Co_3O_4, NiO, CuO, and ZnO$ - are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core-valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Transient X-ray and extreme ultraviolet (XUV) spectroscopies have become invaluable tools for studying photoexcited dynamics due to their sensitivity to carrier occupations and local chemical or structural changes. One of the most studied mate-rials using transient XUV spectroscopy is $\alpha$-Fe$_2$O$_3$ because of its rich photoexcited dynamics, including small polaron formation. The interpretation of carrier and polaron effects in $\alpha$-Fe$_2$O$_3$ is currently done using a semi-empirical method that is not transferrable to most materials. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for arbitrary materials systems. The accuracy of this approach is proven by calculating the XUV absorption spectra for the ground, photoexcited, and polaron states of $\alpha$-Fe$_2$O$_3$. Furthermore, the theoretical approach allows for the projection of the core-valence excitons and different components of the X-ray transition Hamiltonian onto the band structure, providing new insights into old measurements. From this information, a physical intuition about the origins and nature of the transient XUV spectra can be built. A route to extracting electron and hole energies is even shown possible for highly angular momentum split XUV peaks. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Entangled photon pairs are predicted to linearize and increase the efficiency of two-photon absorption, allowing continuous wave laser diodes to drive ultrafast time-resolved spectroscopy and nonlinear processes. Despite a range of theoretical studies and experimental measurements, inconsistencies persist about the value of the entanglement enhanced interaction cross section. A spectrometer is constructed that can temporally and spectrally characterize the entangled photon state before, during, and after any potential two-photon excitation event. For the molecule Rhodamine 6G, which has a virtual state pathway, any entangled two-photon interaction is found to be equal to or lower than classical, single photon scattering events. This result can account for the discrepancies between the wide variety of entangled two-photon absorption cross sections reported from different measurement techniques. The reported instrumentation can unambiguously separate classical and entangled effects and therefore is of importance for the growing field of nonlinear and multiphoton entangled spectroscopy.

Transient XUV spectroscopy is growing in popularity for the measurement of solar fuel and photovoltaic materials as it can separately measure electron and hole energies for multiple elements at once. However, interpretation of transient XUV measurements is complicated by changes in core-valence exciton and angular momentum effects after photoexcitation. Here, we report the photoexcited electron and hole dynamics for ZnTe, a promising material for CO2 reduction, following 400 nm excitation. We apply a newly developed, ab-initio theoretical approach based on density functional theory and the Bethe-Salpeter equation to accurately predict the excited state change in the measured transient XUV spectra. Electrons excited to the conduction band are measured with a thermalization rate of 70 $\pm$ 40 fs. Holes are excited with an average excess energy of ~1 eV and thermalize in 1130 $\pm$ 150 fs. The theoretical approach also allows an estimated assignment of inter- and intra-valley relaxation pathways in k-space using the relative amplitudes of the core-valence excitons.

Transient X-ray absorption techniques can measure ultrafast dynamics of the elemental edges in a material or multiple layer junction, giving them immense potential for deconvoluting concurrent processes. However, the interpretation of the photoexcited changes to an X-ray edge is not as simple as directly probing a transition with optical or infrared wavelengths. The core hole left by the core-level transition distorts the measured absorption and reflection spectra, both hiding and revealing different aspects of a photo-induced process. In this perspective, we describe the implementation and interpretation of transient X-ray experiments. This description includes a guide of how to choose the best wavelength and corresponding X-ray sources when designing an experiment. As an example, we focus on the rising use of extreme ultraviolet (XUV) spectroscopy for understanding performance limiting behaviors in solar energy materials, such as measurements of polaron formation, electron and hole kinetics, and charge transport in each layer of a metal-oxide-semiconductor junction. The ability of measuring photoexcited carriers in each layer of a multilayer junction could prove particularly impactful in the study of molecules, materials, and their combinations that lead to functional devices in photochemistry and photoelectrochemistry.

Silicon nanoparticles have the promise to surpass the theoretical efficiency limit of single-junction silicon photovoltaics by the creation of a "phonon bottleneck", a theorized slowing of the cooling rate of hot optical phonons that in turn reduces the cooling rate of hot carriers in the material. To verify the presence of a phonon bottleneck in silicon nanoparticles requires simultaneous resolution of electronic and structural changes at short timescales. Here, extreme ultraviolet transient absorption spectroscopy is used to observe the excited state electronic and lattice dynamics in polycrystalline silicon nanoparticles following 800 nm photoexcitation, which excites carriers with $0.35 \pm 0.03$ eV excess energy above the ${\Delta}_1$ conduction band minimum. The nanoparticles have nominal 100 nm diameters with crystalline grain sized of about ~16 nm. The extracted carrier-phonon and phonon-phonon relaxation times of the nanoparticles are compared to those for a silicon (100) single crystal thin film at similar carrier densities ($2$ x $10^{19} cm^{-3}$ for the nanoparticles and $6$ x $10^{19} cm^{-3}$ for the thin film). The measured carrier-phonon and phonon-phonon scattering lifetimes for the polycrystalline nanoparticles are $870 \pm 40$ fs and $17.5 \pm 0.3$ ps, respectively, versus $195 \pm 20$ fs and $8.1 \pm 0.2$ ps, respectively, for the silicon thin film. The reduced scattering rates observed in the nanoparticles are consistent with the phonon bottleneck hypothesis.

Entangled photon spectroscopy is a nascent field that has important implications for measurement and imaging across chemical, biology, and materials fields. Entangled photon spectroscopy potentially offers improved spatial and temporal-frequency resolutions, increased cross sections for multiphoton and nonlinear measurements, and new abilities in inducing or measuring quantum correlations. A critical step in enabling entangled photon spectroscopies is the creation of high-flux entangled sources that can use conventional detectors, as well as provide redundancy for the losses in realistic samples. Here, we report a periodically poled, chirped, lithium tantalate platform that generates entangled photon pairs with a $10^{-7}$ efficiency. For a near watt level diode laser, this results in a near $\mu$W-level flux. The single photon per mode limit that is necessary to maintain non-classical photon behavior is still satisfied by distributing this power over up to an octave-spanning bandwidth. The spectral-temporal photon correlations are observed via a Michelson-type interferometer that measures the broadband Hong-Ou-Mandel two-photon interference. A coherence time of 245 fs for a 10 nm bandwidth in the collinear case and a 62 fs for a 125 nm bandwidth in the non-collinear case is measured using a CW pump laser, and, essentially, collecting the full photon cone. We outline in detail the numerical methods used for designing and tailoring the entangled photons source, such as changing center wavelength or bandwidth, with the ultimate aim of increasing the availability of high-flux UV-Vis entangled photon sources in the optical spectroscopy community.

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M$_{2,3}$ edge. It is observed that the core-level absorption lineshape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorption spectrum. Theory predicts, and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($\Delta T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy., Comment: 20 pages, 8 figures

Entangled photons exhibit non-classical light-matter interactions that create new opportunities in materials and molecular science. For example, in entangled two-photon absorption, the intensity-dependence scales linearly as if only one photon was present. The entangled two-photon absorption cross section approaches but does not match the one-photon absorption cross section. The entangled two photon cross section also does not follow classical two photon molecular design motifs. Questions such as these seed the rich but nascent field of entangled light-matter interactions. In this perspective, we use the experimental developments in entangled photon spectroscopy to outline the current status of the field. Now that the fundamental tools are outlined, it is time to start the exploration of how materials, molecules, and devices can control or utilize interactions with entangled photons., Comment: 11 pages, 6 figures. Submitted to Journal of Materials Chemistry C; v2 includes a new section on opportunities in materials science

Transient extreme ultraviolet (XUV) spectroscopy probes core level transitions to unoccupied valence and conduction band states. Uncertainty remains to what degree the core-hole created by the XUV transition modifies the measurement of photoexcited electron and hole energies. Here, the Si {\L_{2,3}} edge is measured after photoexcitation of electrons to the {\Delta}, L, and {\Gamma} valleys of Si(100). The measured changes in the XUV transition probability do not energetically agree with the increasing electron photoexcitation energy. The data therefore experimentally confirm that, for the Si {\L_{2,3}} edge, the time-dependent electron and hole energies are partially obscured by the core-hole perturbation. A model based on many-body approximations and the Bethe-Salpeter equation is successfully used to predict the core-hole{\apos}s modification of the final transition density of states in terms of both electronic and structural dynamics. The resulting fit time constants match the excited state electron thermalization time and the inter-valley electron-phonon, intra-valley electron-phonon, and phonon-phonon scattering times previously measured in silicon. The outlined approach is a more comprehensive framework for interpreting transient XUV absorption spectra in photoexcited semiconductors.

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite ({\alpha}-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 {\pm} 10 fs at 2.2 eV to 350 {\pm} 30 fs at 2.6 eV, whereas the polaron formation probability (85 {\pm} 10%) remains constant. By comparison to hematite ({\alpha}-Fe2O3), an oxide analog, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides.

Metal-oxide-semiconductor junctions are central to most electronic and optoelectronic devices. Here, the element-specificity of broadband extreme ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and recombination kinetics in each layer of a Ni-TiO2-Si junction. After photoexcitation of silicon, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in spectral shifts in the Ni M2,3 XUV edge that are characteristic of holes and the absence of holes initially in TiO2. Meanwhile, the electrons are observed to remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO2, shifting the Ti spectrum to higher oxidation state, followed by electron-hole recombination at the Si-TiO2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps following photoexcitation of the indirect transition to the {\Delta} valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation (BSE) with density functional theory (DFT). The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time.

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical/NIR pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by attosecond transient absorption spectroscopy (ATAS) in the extreme ultraviolet at the germanium M_{4,5}-edge (~30 eV). We decompose the ATAS spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8*10^{20}cm^{-3}. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first order electron and hole decay of ~1 ps suggests a Shockley-Read-Hall recombination mechanism. The simultaneous observation of electrons and holes with ATAS paves the way for investigating few to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions., Comment: Includes Supplementary Information