Ultrafast terahertz characterisation of surface free carrier dynamics

The invention of ultrafast pulsed laser sources, operating with pulse durations shorter than 10-¹²s, has enabled researchers to fully exploit and observe dynamic systems operating on these timescales. Furthermore, the successful amplification of such short pulses then provided the next leap towards observing such fast processes when under much more extreme nonlinear regimes. The amplification of ultrafast sources was considered such a significant impact that Strickland and Mourou were awarded the Nobel prize in Physics for 2018. One such consequence of the ultrafast laser is the birth of time-resolved pump-probe experimental methodologies, where a weak probe pulse is exploited to observe a sample under investigation, while a strong pump pulse modifies the sample via its nonlinear-optical response at ultrafast timescales. It is then trivial to vary the delay between the pump and probe pulses in order to directly measure the dynamical properties of the sample as it relaxes back to equilibrium. This thesis contains the results obtained in the Emergent Photonics research lab, where I have been developing several embodiments of these pump-probe schemes. Understanding of a system's response to extreme excitation is pivotal to my investigation into the generation and manipulation of Terahertz waves, another field born thanks to the invention of the ultrafast laser. One promising approach to the development of Terahertz technologies is through the nonlinear interaction of ultrashort pulses with matter. Therefore, by developing pump-probe approaches which are able to observe specifically the region where such a nonlinear interaction occurs, I am able to reveal and study the complex processes underpinning the Terahertz generation. This thesis is structured as follows. First, in the introduction I will provide an outline of the physical processes relevant to my work and publications, including a full and critical literature review of the fields impacted by my work. Each chapter will then focus on one of my publications, summarising and describing how it relates to my previous work, and explaining how it was received after publication. I will then finish by outlining how my work could be continued in the future and making any concluding remarks.