arXiv subjects

Rafael Quintero-Bermudez

Publications and source records attributed to Rafael Quintero-Bermudez.

5 recordsLinked to original sources

arXiv2026

Probing of Core Excitons in Solid NaF with Polarization-Selective Attosecond Time-Resolved Four-Wave Mixing Spectroscopy

Nonlinear Four-wave mixing processes are a powerful technique to unravel ultrafast dynamics in solid-state systems. Here, we employ attosecond four-wave mixing spectroscopy with one extreme ultraviolet (XUV) pump and two independently delayed, noncollinear near-infrared (NIR) probes to resolve the ultrafast decoherence of both dipole-allowed and dipole-forbidden core excitons at the Na+ L2,3 edge in sodium fluoride (NaF). The decoherence times of the core excitons are observed to be much faster than the 8 fs limit of the instrument response time, which is attributed to strong exciton-phonon coupling. Furthermore, polarization control of the NIR probes (Perpendicular and parallel polarizations) reveals that the bright core excitons exhibit s-like orbital angular momentum, while dark core excitons, reached by two-photon excitation, exhibit p-like angular momentum.

physics.chem-ph↗

arXiv2026

From interface-limited to Auger-dominated carrier dynamics in $π$-SnS

Metastable cubic tin(II) sulfide ($π$-SnS) is an earth-abundant semiconductor whose three-dimensionally bonded chiral lattice may overcome the short minority-carrier lifetime of orthorhombic SnS while maintaining a near-ideal bandgap for tandem photovoltaics. Despite its promise, ultrafast carrier cooling and recombination mechanisms over illumination density remain poorly constrained. We use core-level extreme-ultraviolet attosecond transient absorption spectroscopy at the Sn $4d$ edge to track carrier injection, cooling, and recombination in $π$-SnS with element- and orbital-specific sensitivity. Following femtosecond near-infrared excitation, the Sn $4d\rightarrow$CB onset exhibits conduction-band state filling and a carrier-induced edge shift, enabling extraction of density-dependent kinetics. The transient response follows a biexponential decay with a fast hot-carrier cooling component and a slower recombination component. At low carrier densities, recombination is consistent with interface-limited processes, whereas above $\sim1\times10^{20}$ cm$^{-3}$ both cooling and recombination accelerate, indicating a crossover to carrier-carrier interaction-dominated dynamics. Coherent phonon oscillations with a period of $\sim188$ fs reveal coupling between electronic excitation and lattice motion. These results provide a comprehensive picture of nonequilibrium carrier and phonon dynamics in cubic SnS, reveal a change of mechanisms over a range of carrier densities, and establish the value of using attosecond transient absorption spectroscopy to study ultrafast processes in complex semiconductors that have optoelectronic and energy-conversion applications.

cond-mat.mtrl-sci↗

arXiv2026

Background-free Tracking of Ultrafast Hole and Electron Dynamics with XUV Transient Grating Spectroscopy

Extreme ultraviolet (XUV) transient absorption (TA) and transient reflectivity (TR) spectroscopies enable element-specific insights into attosecond-timescale processes in solids. XUV transient grating spectroscopy (TGS) is an emerging tool that combines the advantages of both absorption and reflectivity while offering intrinsically background-free detection. Here, we implement XUV-TGS by generating a transient grating in germanium solid using two few-cycle near-infrared pulses and probing it with an attosecond XUV pulse, produced via tabletop high-harmonic generation. The spectrally resolved, diffracted XUV pulses directly visualize the separate ultrashort decay times of both photoexcited electrons and holes, without the need for iterative deconvolution. By combining XUV-TA and -TG spectroscopy, we extract the evolution of the complex refractive index, ñ, without the need for Kramers-Kronig reconstruction, as required in XUV-TR, allowing us to extract the roots of the induced optical response. We find reflectivity changes of up to 34% via the real part of ñ, whereas changes in the imaginary part only result in a variation in reflectivity of around 0.5%.

physics.optics↗

arXiv2025

Polarization-Resolved Core Exciton Dynamics in LiF Using Attosecond Transient Absorption Spectroscopy

The ability to control absorption by modifying the polarization of light presents an exciting opportunity to experimentally determine the orbital alignment of absorption features. Here, attosecond extreme ultraviolet (XUV) transient absorption spectroscopy is used to investigate the polarization dependence of core exciton dynamics in LiF thin films at the Li+ K edge. XUV pulses excite electrons from the Li 1s core level into the conduction band, allowing for the formation of a p-orbital-like core exciton, aligned along the XUV light polarization axis. A sub-5 fs near-infrared (NIR) probe pulse then arrives at variable time delays, perturbing the XUV-excited states and allowing the coherence decay of the core exciton to be mapped. The coherence lifetimes are found to be ~2.4 +- 0.4 fs, which is attributed to a phonon-mediated dephasing mechanism as in previous core exciton studies. The differential absorption features are also shown to be sensitive to the relative polarization of the XUV and NIR fields. The parallel NIR probe induces couplings between the initial XUV-excited p-like bright exciton and s-like dark excitons. When crossed pump and probe polarizations are used, the coupling between the bright and dark states is no longer dipole-allowed, and the transient absorption signal associated with the coupling is suppressed by approximately 90%. This interpretation is supported by simulations of a few-level model system, as well as analysis of the calculated band structure. The results indicate that laser polarization can serve as a powerful experimental tool for exploring the orbital alignment of core excitonic states in solid-state materials.

cond-mat.mtrl-sci↗

arXiv2024

Attosecond transient grating spectroscopy with near-infrared grating pulses and an extreme ultraviolet diffracted probe

Transient grating spectroscopy has become a mainstay among metal and semiconductor characterization techniques. Here we extend the technique towards the shortest achievable timescales by using tabletop high-harmonic generation of attosecond extreme ultraviolet (XUV) pulses that diffract from transient gratings generated with 500-1000 nm sub-5 fs near-infrared (NIR) pulses. We demonstrate the power of attosecond transient grating spectroscopy (ATGS) by investigating the ultrafast photoexcited dynamics in an Sb semimetal thin film. ATGS provides an element-specific, background-free signal, unfettered by spectral congestion, in contrast to transient absorption spectroscopy. With the ATGS measurements in Sb polycrystalline thin films, we observe the generation of coherent phonons and investigate the lattice and carrier dynamics. Among the latter processes, we extract carrier thermalization, hot carrier cooling, and electron-hole recombination, which are on the order of 20 fs, 50 fs, and 2 ps, timescales, respectively. Furthermore, simultaneous collection of transient absorption and transient grating data allows us to extract the total complex dielectric constant in the sample dynamics, including the real-valued refractive index, from which we are also able to investigate carrier-phonon interactions and longer-lived phonon dynamics. The outlined experimental technique expands the capabilities of transient grating spectroscopy and attosecond spectroscopies by providing a wealth of information concerning carrier and lattice dynamics with an element-selective technique, at the shortest achievable timescales.

physics.optics↗