Ultrafast Phonon-Diffuse Scattering as a Tool for Observing Chiral Phonons in Monolayer Hexagonal Lattices

At the 2D limit, hexagonal systems such as monolayer transition metal dichalcogenides (TMDs) and graphene exhibit unique coupled spin and momentum-valley physics (valley pseudospin) owing to broken spatial inversion symmetry and strong spin-orbit coupling. Circularly polarized light provides the means for pseudospin-selective excitation of excitons (or electrons and holes) and can yield momentum-valley polarized populations of carriers that are the subject of proposed valleytronic applications. The chirality of these excited carriers have important consequences for the available relaxation/scattering pathways, which must conserve (pseudo)angular momentum as well as energy. One available relaxation channel that satisfies these constraints is coupling to chiral phonons. Here we show that chiral carrier-phonon coupling following valley-polarized photoexcitation is expected to leads to a strongly valley-polarized chiral phonon distribution that is directly measurable using ultrafast phonon-diffuse scattering techniques. Using ab-initio calculations we show how the dynamic phonon occupations and valley anisotropy determined by nonequilibrium observations can provide a new window on the physical processes that drive carrier valley-depolarization in monolayer TMDs

Solving the Jitter Problem in Microwave Compressed Ultrafast Electron Diffraction Instruments: Robust Sub-50 fs Cavity-Laser Phase Stabilization

We demonstrate the compression of electron pulses in a high-brightness ultrafast electron diffraction (UED) instrument using phase-locked microwave signals directly generated from a mode-locked femtosecond oscillator. Additionally, a continuous-wave phase stabilization system that accurately corrects for phase fluctuations arising in the compression cavity from both power amplification and thermal drift induced detuning was designed and implemented. An improvement in the microwave timing stability from 100 fs to 5 fs RMS is measured electronically and the long-term arrival time stability ($>$10 hours) of the electron pulses improves to below our measurement resolution of 50 fs. These results demonstrate sub-relativistic ultrafast electron diffraction with compressed pulses that is no longer limited by laser-microwave synchronization.Comment: Accepted for publication in Structural Dynamic

Ultrafast Electron Diffuse Scattering as a Tool for Studying Phonon Transport: Phonon Hydrodynamics and Second Sound Oscillations

Hydrodynamic phonon transport phenomena, like second sound, have been observed in liquid Helium temperatures more than 50 years ago. More recently second sound has been observed in graphite at over 200\,K using transient thermal grating techniques. In this work we explore the signatures of second sound in ultrafast electron diffuse scattering (UEDS) patterns. We use density functional theory and solve the Boltzmann transport equation to determine time-resolved non-equilibrium phonon populations and subsequently calculate one-phonon structure factors and diffuse scattering patterns to simulate experimental data covering the regimes of ballistic, diffusive, and hydrodynamic phonon transport. For systems like graphite, UEDS is capable of extracting time-dependent phonon occupancies across the entire Brillouin zone and ultimately lead to a more fundamental understanding of the hydrodynamic phonon transport regime.Comment: 7 pages, 4 figure

Tracking ultrafast solid-state dynamics using high harmonic spectroscopy

WWe establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy method for tracking photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal phase transitions in vanadium dioxide. We benchmark the technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon of the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure