Ultrafast pump-probe fluence and wavelength dependent relaxation dynamics in graphene
In this dissertation we seek to present a comprehensive fluence and wavelength dependent ultrafast pump-probe transmission study of graphene supported on a soda-lime glass substrate for a range of pump fluences that enable us to observe both decreased and enhanced probe transmission regimes on a femtosecond timescale. Specifically, we study the crossover region from decreased to enhanced probe transmission, which occurs at an intermediate threshold pump fluence. At intermediate threshold pump fluences we observe an order of magnitude decrease in the relaxation time constant of the differential transmission as compared to higher and lower pump fluences. This optical effect can be explained by equal contributions of inter- and intraband transitions with opposite signs to the transient optical conductivity of graphene at intermediate pump fluences. Moreover, the intermediate threshold pump fluence is shown to increase with decreasing probe energy, which is in agreement with the theoretical model. Furthermore, we show that the relaxation time of the electronic temperature increases monotonically over the range of fluences studied. Comparison with the measured relaxation times of the differential transmission implies that pump fluence greatly modifies time-dependent optical properties, while the electron and phonon relaxation processes remain unaffected. In perspective, this work is of importance to graphene- based opto-electronic applications such as light modulators.