Terahertz nonlinear optics & III-V semiconductor nanowires

Most commercially available terahertz spectrometers make use of resonant and non-resonant methods to generate and detect THz radiation. This allows only the low frequency spectrum to be accessed, usually from 0.1 THz to 8 THz, severely limiting the range over which dynamics can be observed. In this thesis, the physics of non-linear optics is exploited and a state-of-the-art terahertz air-plasma generation and detection setup is showcased. THz radiation is generated through non-linear interaction of a two-colour laser field under a dry nitrogen atmosphere producing a plasma, while the detection is accomplished with an Air Biased Coherent Detection (ABCD) technique. The issue of phonon resonances, damage threshold, etalon effects and dispersion of crystals in the THz region are effectively bypassed. The system was specifically designed and built to ensure a robust experimental setup is obtained with several components being made by our physics workshop. Furthermore, significant improvements have been put in place to increase data acquisition speed and maximising signal-to-noise ratio for better results. III-V semiconductor nanowires are playing an increasingly important role in the field of condensed matter physics. From nanoscale components to highly efficient photovoltaic devices, there is a strong urge to have a solid fundamental understanding of their optoelectronic properties. The air plasma system was used to demonstrate two novel ultrafast THz modulators based on GaAs semiconductor nanowires operating at very high THz bandwidth (up to 40 THz). A significant improvement in modulation depth was achieved, up to 45%, together with picosecond modulation speed. Additionally, to further widen the study of III-V nanowires, this thesis presents a study showing a direct relation between the nanowire length and the frequency of surface plasmon resonance in the terahertz regime. Theoretical calculations have been done to further strengthen this study with the simulations revealing higher electron mobilities than the previously reported values. Finally, two smaller chapters were dedicated to the the study of the temperature-dependence of the refractive index of c-cut sapphire in the terahertz regime and the optoelectronic properties of hexagonal boron nitride (hBN) at equilibrium state. These provide fundamental insights into the potential usage of sapphire substrates as a replacement to quartz substrates as they are more robust and can support higher temperatures during growth and with the hBN laying the foundation for further avenues of research.