Spectroscopic Studies of Thin Film Materials in the Terahertz Frequency Range

Abstract

This thesis reports the results of spectroscopic techniques including terahertz time-domain spectroscopy (THz-TDS) that were used to characterise and develop the understanding of a variety of thin film materials, including few-layer graphene and spintronic metallic heterostructures. The complex transmission of few-layer graphene at terahertz frequencies was measured using THz-TDS, and was used to determine figures of merit including frequency-dependent conductivities, DC conductivities, and mean scattering times. DC conductivities ranging from 0.2-2.8 mS, and scattering times ranging from 3-130 fs were extracted by fitting to the Drude model of conductivity, and were found to be correlated to the number of layers of each sample, demonstrating the practical uses of THz-TDS for characterising these materials. Supporting measurements using Raman and ultraviolet-visible (UV-VIS) spectroscopy were compared and contrasted against that of THz-TDS. These methods suggest that characterisation of graphene requires a range of different spectroscopic techniques to be used in parallel to fully describe its properties. A suite of thin metallic film heterostructures including Co20Fe60B20/Pt and Ni80Fe20/Pt were investigated, which were used as alternative sources of terahertz emission. These heterostructures consisted of nanometre-thick layers of ferromagnetic and non-magnetic metals, and are thought to utilise the spin of the electron and the inverse spin-Hall effect (ISHE) to generate broadband terahertz radiation, which was measured at frequencies of up to 15 THz. The magnetic field dependence of terahertz emission of these heterostructures was studied to determine the role that the magnetic structure had on their emission process. It was found that the terahertz emission closely followed the magnetisation properties of the samples found via vibrating sample magnetometry (VSM) measurements, displaying a uniaxial magnetic anisotropy depending on the direction of the externally applied magnetic field. The effects of the heterostructure design on the emission were also investigated, as well as the consequences of other external conditions such as temperatures as low as 5 K and laser pump fluences from 0.05-50 mJ/cm^2. These findings show promise towards further development of this new generation of low-cost terahertz emitters with tunable emission properties.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jessica Boland (Supervisor) & Darren Graham (Supervisor)