Element-Specific Ultrafast Lattice Dynamics in Monolayer WSe.

We study monolayer WSe using ultrafast electron diffraction. We introduce an approach to quantitatively extract atomic-site-specific information, providing an element-specific view of incoherent atomic vibrations following femtosecond excitation. Via differences between W and Se vibrations, we identify stages in the nonthermal evolution of the phonon population.

Direct Observation of Ultrafast Lattice Distortions during Exciton-Polaron Formation in Lead Halide Perovskite Nanocrystals.

The microscopic origin of slow hot-carrier cooling in lead halide perovskites remains debated and has direct implications for applications. Slow hot-carrier cooling of several picoseconds has been attributed to either polaron formation or a hot-phonon bottleneck effect at high excited carrier densities (>10 cm). These effects cannot be unambiguously disentangled with optical experiments alone.

Traversing Double-Well Potential Energy Surfaces: Photoinduced Concurrent Intralayer and Interlayer Structural Transitions in XTe (X = Mo, W).

The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform; , the structure determines the property, a traditional paradigm in materials science. Photoexcitation-driven manipulation of the crystal structure and associated electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications; however, a generalized mechanism for such symmetry engineering is absent. Here, by ultrafast electron diffraction, structure factor calculation, and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in the Td and 1T' phases of XTe (X = Mo, W).

Ultrafast lattice dynamics and electron-phonon coupling in platinum extracted with a global fitting approach for time-resolved polycrystalline diffraction data.

Quantitative knowledge of electron-phonon coupling is important for many applications as well as for the fundamental understanding of nonequilibrium relaxation processes. Time-resolved diffraction provides direct access to this knowledge through its sensitivity to laser-induced lattice dynamics. Here, we present an approach for analyzing time-resolved polycrystalline diffraction data.

Accessing the Anisotropic Nonthermal Phonon Populations in Black Phosphorus.

We combine ultrafast electron diffuse scattering experiments and first-principles calculations of the coupled electron-phonon dynamics to provide a detailed momentum-resolved picture of lattice thermalization in black phosphorus. The measurements reveal the emergence of highly anisotropic nonthermal phonon populations persisting for several picoseconds after exciting the electrons with a light pulse. Ultrafast dynamics simulations based on the time-dependent Boltzmann formalism are supplemented by calculations of the structure factor, defining an approach to reproduce the experimental signatures of nonequilibrium structural dynamics.

Nuclear dynamics of singlet exciton fission in pentacene single crystals.

Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction.

Soft X-ray absorption of thin films detected using substrate luminescence: a performance analysis.

X-ray absorption spectroscopy of thin films is central to a broad range of scientific fields, and is typically detected using indirect techniques. X-ray excited optical luminescence (XEOL) from the sample's substrate is one such detection method, in which the luminescence signal acts as an effective transmission measurement through the film. This detection method has several advantages that make it versatile compared with others, in particular for insulating samples or when a probing depth larger than 10 nm is required.

Anisotropic Nonequilibrium Lattice Dynamics of Black Phosphorus.

Black phosphorus has recently attracted significant attention for its highly anisotropic properties. A variety of ultrafast optical spectroscopies has been applied to probe the carrier response to photoexcitation, but the complementary lattice response has remained unaddressed. Here we employ femtosecond electron diffraction to explore how the structural anisotropy impacts the lattice dynamics after photoexcitation.