Electric field, Magnetic field and Magnetization: THz time-domain spectroscopy studies

Abstract

Terahertz radiation is electromagnetic waves with frequencies from 0.1-10 THz. THz radiation can pass through cardboard, paper, plastics, ceramics and many other materials. Hence, it can be used for non-destructive imaging. Another important application of THz radiation is spectroscopy. Many organic molecules absorb light at THz frequencies and these absorption lines can be used for the identification of the molecules. This spectroscopic technique is called terahertz time domain spectroscopy (THz-TDS). It is a valuable tool for studying the properties of the material. In THz-TDS we measure the amplitude and phase of the THz pulse in the time domain using coherent detection techniques. Usually, in THz-TDS technique we measure the THz electric field using electro-optic detention technique. However, in thesis, the main goal is to focus on the magnetic aspect of THz generation and detection using THz-TDS. This thesis is divided into three research problems, in which THz-TDS plays the key role. In the first part, the THz-TDS setup is used for characterising the metamaterial elements. Metamaterials are artificially structured materials that are used to control and manipulate light. A split-ring resonator is one of the most common metamaterial elements. Usually these split-ring resonators are studied in far-field but near-field interactions are important to understand the properties of metamaterials. In the past electric near-field of SRRs are already studied. For the first time, we have directly measured the magnetic near-field of SRRs. The second research problem investigated in the thesis is generation of THz radiation from ferromagnetic cobalt thin film. When femtosecond laser pulses are incident on ferromagnetic metals prepared on a glass substrate, THz pulses are emitted via ultrafast demagnetization. It is often forgotten that nonmagnetic metals are also capable of emitting THz light and that such a contribution to the emitted THz field cannot a priori be excluded for magnetic metals. Our work clearly establishes a strong correlation between the magnetization and the emission of THz light following the excitation of cobalt with a femtosecond laser pulse. It does this by highlighting the role of the orientation of the magnetization in the terahertz emission from ferromagnetic thin films. We find that as we increase the cobalt film thickness, the polarisation direction of the emitted THz pulse changes, correlating with the transition from a predominantly in-plane to a predominantly out-plane magnetisation, as measured with magnetic force microscopy. When femtosecond laser light is incident on a semiconductor thin film, emission of THz radiation is observed. The emission can be enhanced if semiconductor materials are deposited on metal surfaces. In the last research problem of this thesis, in chapter 5, THz spectroscopy was used for studying the BiVO4/Au inter- faces. BiVO4 is a semiconductor material which is widely used for water splitting. When a BiVO4/Au interface is illuminated with ultrashort laser pulses, due to reflection from various interfaces, a standing wave pattern is observed. As a result, a difference in carrier concentration builds up which gives rise to THz emitting dipole. In short, this thesis discusses the possibilities of using terahertz time domain spectroscopy for studying the generation of THz radiation and using it for imaging and material characterization.