Experimental Advancements Of Two-dimensional Electronic Spectroscopy And Its Application In Two-dimensional Materials

Two-dimensional electronic spectroscopy（2DES）is a kind of advanced spectroscopic technique with femtosecond time-resolved ability.Compared with traditional transient absorption spectroscopy,2DES possesses the ability of two-dimensional resolution on excitation and detection frequency,which is of great benefit in decoupling complex spectral information.In recent years,2DES makes significant contributions to the fields such as coherent coupling and energy transfer process on the complex systems,many-body interaction and exciton dynamics in the semiconductor materials,and so on.This dissertation will give a detailed introduction to the development and application of 2DES.The following is its spectroscopic theory and experimental implementation.On this basis,the works are focused on the advancements in experimental techniques of 2DES.Next,the researches on two-dimensional transition metal chalcogenides（TMDC）by means of 2DES are presented.Achieving the dispersion compensation in wide spectral range,and getting the femtosecond ultrashort pulse is the precondition to obtaining high-time resolution and performing the 2DES experiment.This dissertation has explored various pulse compression methods,including prism pair,grating pair,chirped mirror,pulse shaper,and so on.With the combination of the grating pair and prism pair,a～10 fs ultrashort pulse can be achieved experimentally.The ultrafast pulse obtained by this method of pulse compression has the advantage of very little spectrum chip,which can decrease the artifact on 2DES effectively.On the other hand,the dissertation has performed systematic studies on weak signal detection and scattering suppression schemes of 2DES experiments.The developed signal detection method of multi-pulse accumulation can decrease the lower threshold of excitation energy detection observably.The weak signal detection of monolayer TMDC material is achieved with the help of multi-pulse accumulation.In addition,the experimental method of double-modulation lock-in detection is also complemented successfully.This method can be used to perform a reliable spectral measurement in the condition of strong sample scattering.With the aid of 2DES and broadband transient absorption（BBTA）,the complex many-body interaction in monolayer TMDC material is revealed experimentally.The research results indicate that there exists strong electron-exciton interaction in the WSe₂monolayer.In addition,this many-body interaction process is strongly mediated by the coherent optical phonon.In addition,the 2DES and BBTA results manifest the fact that,on the femtosecond to the picosecond time scale,the coupling strength of exciton and optical phonon can differ considerably for different kinds of excitons in the same single-layer TMDC material.At the same time,the ultrafast experiments indicate that the oscillation signals of coherent optical phonons possess evident intensity node and phase hopping phenomenon.This phenomenon may commonly exist in monolayer TMDC materials.In summary,this dissertation gives a comprehensive presentation of works on experimental advancements of 2DES and TMDC materials studies.The experimental advancements of 2DES improve the reliability of 2DES spectra and broaden the experimental research range of 2DES.The research results of TMDC materials reported in this dissertation show the outstanding ability of 2DES in revealing the complex many-body process and coherent coupling mechanism.