Terahertz scattering-type scanning near-field optical microscopy for measurement of plasmonic effects

Recent advances in the development of scattering-type scanning near-field optical microscopy (s-SNOM) using terahertz (THz) frequency quantum cascade lasers (QCLs) have opened up new opportunities for THz measurements on the micro- and nano-scale. In this thesis the further development of THz-s-SNOM using QCLs is reported, as well as its application to the investigation of plasmonic effects in a range of systems.
It is shown how the frequency tuning of a QCL can be measured with high resolution using the self-mixing effect, but also exploited for coherent measurement of the scattered field in s-SNOM. The THz-s-SNOM is characterised with an optimisation of the spatial resolution achieving a value of 29 nm corresponding to λ/3000. Using THz-s-SNOM, a novel coherent stepped-frequency system is then reported, in which a generalised phase-stepped algorithm is employed to measure magnitude and phase data with as little as 4 sampling points per imaging pixel. This approach is used to successfully image the out-of-plane electric field supported by a THz micro-resonator.
A theoretical and experimental investigation of the excitation of surface plasmon polaritons (SSPs) in topological insulators (TI) is then presented. Experimental measurements are presented for unpatterned and patterned thin-film Bi2Se3 samples. A Bi2Se3 thin-film sample incorporating a metallic top gate is also investigated and it is demonstrated that the s-SNOM approach can successfully probe the TI surface beneath the top-gate. By expressing the resulting imaging data in the complex plane, it is seen that SPPs can be successfully launched and measured on the TI surface using this technique.
Furthermore, an improved metamaterial waveguide structure supporting spoof surface plasmon polaritons (SSPP) has been theoretically and experimentally investigated, wherein the out-of-plane electric field associated with SSPPs has been imaged using THz-s-SNOM showing a waveguide-to-waveguide SSPP energy transfer.