With an optical parametric amplification (OPA) system following an 800 nm Ti:Sapphire laser, we generate few-femtosecond, carrier-envelope phase (CEP) stabilized laser pulses throughout the visible and infrared regions in order to subsequently make high-harmonic generated (HHG) attosecond x-ray pulses spanning 20-600 eV. Combinations of such pulses are employed to perform core-level pump-probe experiments with high spatiotemporal resolution and atom-specificity where samples can remain under ultrahigh vacuum and ultracold temperature conditions.
Fig. 1. A moirè lattice consisting of two hexagonal 2D layers twisted relative to one another by an angle θ gives rise to a periodic potential of quantum wells with depths on the order of 100 meV.
We are specifically interested in studying and controlling charge carrier and spin dynamics in novel superlattice structures. We are specifically interested in moirè lattices of twisted van Der Waals 2D materials because they provide a highly-ordered, tunable, and periodic platform for localized excitons, spins, and charged particles (Fig. 1). Such materials are of interest for quantum information, spintronics, and optoelectronics applications, as well as for the study of canonical models of phase transitions, symmetry-breaking, and electron correlations in condensed matter and many-body physics.
We actively collaborate with materials scientists, theorists, and FEL facilities around the world to answer questions like, what is the influence of the moirè potential on the band structure and optoelectronic properties? Can we manipulate the spatiotemporal dynamics of quasiparticles in the superlattice structures in real time? What kinds of couplings exist between the spin, orbit, charge, and lattice degrees of freedom? Can we control how long these couplings persist before decoherence sets in?
Yu, H.; Liu, G. bin; Tang, J.; Xu, X.; Yao, W. Moiré Excitons: From Programmable Quantum Emitter Arrays to Spin-Orbit–Coupled Artificial Lattices. Science Advances 2017, 3 (11), 1–8. https://doi.org/10.1126/sciadv.1701696.
Tran, K.; Moody, G.; Wu, F.; Lu, X.; Choi, J.; Kim, K.; Rai, A.; Sanchez, D. A.; Quan, J.; Singh, A.; et al. Evidence for Moiré Excitons in van Der Waals Heterostructures. Nature 2019, 567 (7746), 71–75. https://doi.org/10.1038/s41586-019-0975-z.
Jin, C.; Regan, E. C.; Yan, A.; Iqbal Bakti Utama, M.; Wang, D.; Zhao, S.; Qin, Y.; Yang, S.; Zheng, Z.; Shi, S.; et al. Observation of Moiré Excitons in WSe2/WS2 Heterostructure Superlattices. Nature 2019, 567 (7746), 76–80. https://doi.org/10.1038/s41586-019-0976-y.
Seyler, K. L.; Rivera, P.; Yu, H.; Wilson, N. P.; Ray, E. L.; Mandrus, D. G.; Yan, J.; Yao, W.; Xu, X. Signatures of Moiré-Trapped Valley Excitons in MoSe 2 /WSe 2 Heterobilayers. Nature 2019, 567 (7746), 66–70. https://doi.org/10.1038/s41586-019-0957-1.
Bloch, I. Ultracold Quantum Gases in Optical Lattices. Nature Physics 2005, 1 (1), 23–30. https://doi.org/10.1038/nphys138.
Wu, F.; Lovorn, T.; Macdonald, A. H. Theory of Optical Absorption by Interlayer Excitons in Transition Metal Dichalcogenide Heterobilayers. 2018, 035306, 1–10. https://doi.org/10.1103/PhysRevB.97.035306.