Study On The Dynamics Of The Quasi-Particles In Monolayer Transition Metal Dichalcogenides

Valley is an additional degree of freedom of electron besides the electron charge and electron spin.Using the valley degree of freedom in information coding,processing and transport,valleytronics is a supplement to the traditional microelectronics and spintronics.It has a great development prospect due to its outstanding characteristics,such as good stability,fast processing speed,high integration,small energy consumption and long transport distance.Recently,monolayer transition-metal dichalcogenides have attracted high scientific interest due to their potential application in valleytronics devices based on spin-orbit interaction and other properties.Meanwhile,monolayer transition-metal dichalcogenides exhibit a unique combination of atomic-scale thickness,direct bandgap,strong spin-orbit coupling and favourable electronic and mechanical properties,which make them interesting for applications in high-end electronics,spintronics and optoelectronics.To fulfill the requirements of these optoelectronic devices such as rapid response rates and high quantum efficiencies,the prerequisite is to understand the ultrafast carrier behaviors existing in monolayer transition-metal dichalcogenides,this has also become a research focus.With the development of experimental techniques,researchers have gradually uncovered the time-scale and physical mechanism of carrier dynamics in monolayer transition-metal dichalcogenides,but there are still some problems not being solved effectively,such as the effective detection of exciton diffusion coefficient,the deep understanding of conversion dynamics between the exciton and trion,etc.Therefore,in this thesis,we use the transient grating spectrum,transient reflection spectrum and time-resolved Kerr rotation spectrum to study the diffusion dynamic of exciton and the conversion dynamics between exciton and trion in monolayer WSe₂.The main research contents and results are as follows:1. Study on the diffusion dynamics of valley exciton in monolayer WSe₂In this work,firstly,we used transient concentration grating spectrum to study the diffusion dynamics of excitons in a large-scale exfoliated WSe₂ monolayer at room temperature.We found that the exciton-exciton annihilation plays a major role in the decay of excitons and distorts the profile of the concentration grating.The exciton diffusion coefficient D_X=0.7 cm²/s is estimated based on Einstein relationship and spin relaxation time.Secondly,we measured the transient spin grating spectra with several grating periods at 10 K,confirming that the exciton diffusion is still weak at low temperature.2. Study on the conversion dynamic between exciton and trion in monolayer WSe₂In this work,we investigated the conversion dynamic between exciton and trion in monolayer WSe₂.In the experiment,we found that the down conversion time from exciton to trion is less than?2 ps,which is consistent with the results of other groups.In addition,we found that the up-conversion from trion to exciton is also ultrafast,with the up-conversion time also less than?2 ps.This fast up-conversion process is mainly caused by the A₁^? phonon in the material,which can be demonstrated by the up-conversion transient reflection spectra and polarized-resolved Raman spectra.