In our laboratory we perform cutting-edge experiments using state-of-the art laser-driven attosecond sources in the extreme ultraviolet and soft X-ray range. We employ transient absorption/reflection spectroscopy in combination with transient X-ray scattering as methods to study fastest processes in real-time. Further, we perform experiments at facilities such as free-electron lasers to study dynamics using nonlinear X-ray spectroscopies, which will allow accessing dynamics at surfaces and interfaces.
Open research projects include
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 down to 10-11 torr and 4 K.
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.
Solids can be used as a medium to generate harmonics of an intense laser pulse. While gas-phase higher harmonic generation (HHG) is widely used in generating attosecond pulses, recently, solid state higher harmonic generation (sHHG) has gained attention. Compared to HHG, sHHG has richer dynamics due to the periodic structure of the dense medium. Rather than the well-established semi-classical tunnel ionization description that explains HHG (c-d), sHHG process involves tunnel ionization followed by interactions of the charge carriers with the periodic lattice potentials. Therefore, sHHG contains valuable information about the band structure of solids (a-b). Uniquely equipped with a CEP stabilized tunable source and synchronized OPA’s, we are aiming to elucidate interband and intraband dynamics in solids at the strong field regime. We want to develop sHHG as a solid-state spectroscopy tool capable of answering questions about ultrafast dynamics. On this front, we are interested in several classes of materials such as semiconductors, 2D materials and perovskites.
Electrons are valuable probes of the structure of materials, as they yield atomic-level detail of the underlying lattice structure. By generating ultrafast bunches of electrons, pump-probe spectroscopy techniques can be used to generate ”molecular movies” of a materials response to an excitation. These capabilities are like those of X-rays in many ways. The crucial differences include the fact that electrons scatter from the entire atomic potential while X-rays scatter from electrons, and the scattering cross section, which is orders of magnitude higher for electrons. This contrast, in conjunction with the closeness of the techniques, makes them excellent complements. The goal of the HiRES project is to utilize this technique for the study of materials with unique and promising properties. Our instrument has the unique capability of producing high energy electrons (MeV) at a high repetition rate (MHz). In addition, we have the capacity to cool samples to 10K, giving us access to otherwise unobservable phases of materials. This project is in collaboration with Lawrence Berkeley National Laboratory staff scientist Daniele Filippetto.
Another aspect of our research is directed toward the use of X-Ray Free Electron Lasers (FEL) in uncovering quantum-level phenomenon. High tunability, broad spectral bandwidth, and flexible pulse design are all characteristics that contribute to the usefulness of this technology. An FEL achieves optical amplification by accelerating electrons through an undulator, creating high intensity, ultrafast, and coherent X-ray pulses (see illustration). A non-linear second order process, Second Harmonic Generation (SHG), acts as an interface-specific probe for non-centrosymmetric regions of interest.
Use of FEL technology requires a collaborative effort spanning many departments and facilities worldwide (outlined in the beamtime schedule above). Surface properties of solid-state electrolyte (SSE) materials (e.g., Li3xLa2/3-xTiO3 [LLTO]), and liquid carbon interfacial chemistry (amorphous graphite and highly-ordered diamond) are currently being explored with more opportunities to come!
Current FEL beamlines February: Angelique Li K-edge Soft X-ray Second Harmonic Generation (SXR-SHG) at SACLA (Japan) April: Emma Liquid Carbon X-ray Emission Spectroscopy (XES) at PAL (Korea) TBD: Various projects at FERMI (Italy) and FLASH (Germany)