2022 Vol. 2022, No. 6

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Research Article
Tracking Conical Intersections with Nonlinear X-ray Raman Spectroscopy
Deependra Jadoun, Markus Kowalewski
2022, 2022(6) doi: 10.34133/ultrafastscience.0003
Conical intersections are formed when 2 or more electronic states become degenerate and give rise to ultrafast nonadiabatic processes such as radiation-less decay channels and geometric phase effects. The branching of nuclear wave packets near a conical intersection creates a coherent superposition of electronic states, which carries information about the energy difference of the involved states. X-ray Raman techniques have been proposed to observe the coherent superposition of the electronic states and to monitor the evolving electronic state separation. However,these techniques rely on the coherence generated as the wave packet passes through the conical intersection, and the electronic energy gap before the wave packet passes through the conical intersection is not tracked. In this paper, we theoretically demonstrate how a nonlinear Raman detection scheme can be used to gain further insight into the nonadiabatic dynamics in the vicinity of the conical intersection.We employ a combination of a resonant visible/infrared pulse and an off-resonant x-ray Raman probe to map the electronic state separation around the conical intersection. We demonstrate that this technique can achieve high contrast and is able to selectively probe the narrow electronic state separation around the conical intersection.
Transient Superdiffusion of Energetic Carriers in Transition Metal Dichalcogenides Visualized by Ultrafast Pump-Probe Microscopy
Yun-Ke Zhou, Xiao-Ze Li, Qian-Ni Zhou, Ren-Hao Xing, Yan Zhang, Benfeng Bai, Hong-Hua Fang, Hong-Bo Sun
2022, 2022(6) doi: 10.34133/ultrafastscience.0002
Because of the strong Coulomb interaction and quantum confinement effect, 2-dimensional transition metal dichalcogenides possess a stable excitonic population. To realize excitonic device applications, such as excitonic circuits, switches, and transistors, it is of paramount importance for understanding the optical properties of transition metal dichalcogenides. Furthermore, the strong quantum confinement in 2-dimensional space introduces exotic properties, such as enhanced phonon bottlenecking effect, many-body interaction of excitons, and ultrafast nonequilibrium exciton–exciton annihilation. Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices. In this work, we investigated time-resolved exciton propagation in monolayer semiconductors of WSe2, MoWSe2, and MoSe2, with a home-built femtosecond pump-probe microscope. We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm2 s−1 at the initial delay time, followed by a slow linear diffusive regime (20.9 cm2 s−1) in the monolayer WSe2. The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior. We found that in the monolayers MoWSe2 and MoSe2, the energetic carrier-induced exciton expansion is much more effective, with diffusivity up to 668 and 2295 cm2 s−1, respectively. However, the “cold” exciton transport is trap limited in MoWSe2 and MoSe2, leading to negative diffusion behavior at later time. Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems. It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronic devices.
Anti-Correlated Plasma and THz Pulse Generation during Two-Color Laser Filamentation in Air
Zhiqiang Yu, Lu Sun, Nan Zhang, Jianxin Wang, Pengfei Qi, Lanjun Guo, Quan Sun, Weiwei Liu, and Hiroaki Misawa
2022, 2022(6) doi: 10.34133/2022/9853053
The THz generation efficiency and the plasma density generated by a filament in air have been found anti-correlated when pumped by 800 nm+1600 nm two-color laser field. The plasma density near zero delay of two laser pulses has a minimum value, which is opposite to the trend of THz generation efficiency and contradicts common sense. The lower plasma density cannot be explained by the static tunneling model according to the conventional photocurrent model, but it might be attributed to the electron trapping by the excited states of nitrogen molecule. The present work also clarifies the dominant role of the drifting velocity accelerated by the two-color laser field during the THz pulse generation process. The results promote our understanding on the optimization of the THz generation efficiency by the two-color laser filamentation.