Ultrafast laser-induced spin/electron dynamic in advanced material

I have commissioned the instrumentation of a one-colour time-resolved optical pump-probe experimental system and studied carrier dynamics in a monolayer MoSe2 sample. The system has been first tested and optimized by performing time-resolved measurements at room temperatures in an intrinsic GaAs wafer. Different modulation methods have been extensively investigated in order to increase SNR ratio. Comparing the experiment data and simulation results, double-chopping modulation is found to significantly enhance the signal-to-noise ratio. Transient reflectivity and Kerr rotation have been obtained in the GaAs test sample under various pump fluency, polarisation, and wavelength. The carrier dynamics in the GaAs sample can be reproduced and three distinct time scales have been revealed: τ_1~1.5 ps,τ_2~10 ps,τ_3~100 ps, which are consistent to what have been reported. The same experimental methods are then used to investigate the carrier dynamics of a monolayer MoSe2. From the wavelength dependent transient reflectivity data, the band gap of the MoSe2 sample is confirmed to be lower than 820 nm at room temperature, which is greatly reduced compared with its reported value at zero temperature. The reflectivity is found falling to negative after the initial positive peak at wavelengths longer than 790 nm. We suggest this phenomenon arise from the bound of trion and nonradioactive combination of excitons. All the reflectivity data is fitted by a bi-exponential decay function. fitting weight constants for these two dynamics have a clear dependence on laser wavelength. Transient Kerr rotation is also observed over the range of wavelength from 790 nm to 820 nm. The decay of the Kerr rotation is within the first picosecond and doesn’t show an obvious wavelength dependence, which suggest an ultrafast spin relaxation time in MoSe2.