Controlling hot carriers in photoexcited graphene

This thesis deals with the dynamics of hot carriers in photoexcited graphene, which is relevant for both fundamental physics and many practical applications of graphene. The main aim is to understand hot-carrier cooling in graphene, and to be able to control its dynamic photoresponse.
Since the typical hot-carrier cooling time is around a picosecond, we use ultrafast terahertz (THz) spectroscopies to follow in time how hot carriers cool in different graphene systems following optical excitation. Using high-quality graphene, we conclude that optical phonon emission is an efficient cooling pathway and the intrinsically limiting factor at room temperature. Interestingly, for electrons initially possessing insufficient energy to emit optical phonons, cooling via optical phonons can still effectively take place as a result of re-thermalization – electron-electron scattering events that redistribute energy among carriers. As a next step, we demonstrate the ability to effectively screen these electron-electron scattering events using liquids with different permittivity, placed directly on graphene. This enables us to tune the hot-carrier lifetimes in graphene. Finally, we report how different cations from electrolyte solutions permeate through graphene defects. We find that the size of the hydrated cations critically governs the permeation kinetics, which in turn affects the Fermi energy, and therefore the photoresponse, of graphene.
Our studies and findings provide a basic understanding and tuning knobs for controlling the dynamic hot-carrier photoresponse in graphene. Our results and discussions are of interest to academia and industry, where graphene shows strong promise as an ideal platform for next-generation optoelectronics and optical communication systems.