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​Prof. Liang Guo published a paper on interaction between femtosecond laser and two-dimensional semiconductor on Nature Physics.

Recently, Prof. Liang Guo (co-first author) publishes a paper on Nature Physics titled with “Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides”. This work reported research findings on interaction between femtosecond laser and two-dimensional semiconductor-monolayer MoS2.This work is conducted during his postdoc research in UC, Berkeley, US.
Monolayer MoS2 and the other related monolayer transition metal dichalcogenides have attracted much attention in recent years. Unlike metallic graphene, these two-dimensional materials are semiconductors, most of which have direct band gap. Therefore, they are potential candidates for making light-emitting diodes, photodetectors and other optoelectronics. Research on interaction between these materials and light could provide insight for exploring novel optoelectronics.

The left figure illustrates the setup of 2DES and the right figure is the characterization result on monolayer MoS2 by 2DES,  in which the cross peaks on the sides of the diagonal line indicates existence of many-body effect between transition A and transition B. 
This work experimentally and theoretically demonstrates the existence of strong exchange interaction between transition A and transition B in monolayer MoS2, two important photo-excited states. The finding unravels the energy conversion landscape during interaction of the related two-dimensional semiconductors and light. In experiments, the authors applied two-dimensional electronic spectroscopy (2DES) to detect transient absorption processes involving transition A and transition B, which reveals many-body effect between the two photo-excited states. 2DES is a femtosecond spectroscopy technique, which enables high resolution simultaneously in time domain and in frequency domain. Besides many-body effect in condensed matters, 2DES can also be utilized to study nanoscale heat transfer, structural biology, and dynamics of chemical reactions. In theoretical calculation, the authors found the strong exchange interaction between the two excited states through solving the first-principles Bethe–Salpeter equations. The experimental results and the theory well agree with each other.

Besides Prof. Liang Guo,the other co-first authors include Meng Wu,Ting Cao,graduates in Department of Physics and Daniele M. Monahan,graduate in Department of Chemistry, UC Berkeley. Corresponding authors are Prof. Graham R. Fleming, Department of Chemistry and Prof. Steven Louie, Department of Physics, UC Berkeley, both are members of United States National Academy of Science. The monolayer semiconductor sample was provided by Prof. Yi-Hsien Lee from National Tsing Hua University, Taiwan.

Link: https: //www.nature.com/articles/s41567-018-0362-y