Open Access

A Faraday rotation spectroscopy (FRS) technique is presented for measurements on the micrometer scale. Spectral acquisition speeds of about two orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. The experimental method is based on charge-coupled-device detection, avoiding speed-limiting components, such as polarization modulators with lock-in amplifiers. At the same time, FRS spectra are obtained with a sensitivity of 20 µrad () over a broad spectral range (525–800 nm), which is on par with state-of-the-art polarization-modulation techniques. The new measurement and analysis technique also automatically cancels unwanted Faraday rotation backgrounds. Using the setup, Faraday rotation spectroscopy of excitons is performed in a hexagonal boron nitride-encapsulated atomically thin semiconductor WS2 under magnetic fields of up to 1.4 T at room temperature and liquid helium temperature. An exciton g-factor of −4.4 ± 0.3 is determined at room temperature, and −4.2 ± 0.2 at liquid helium temperature. In addition, FRS and hysteresis loop measurements are performed on a 20 nm thick film of an amorphous magnetic Tb20Fe80 alloy.

Electromagnetic radiation in the terahertz (THz) frequency range from 0.1 to 30 THz can be highly useful for spectroscopy and imaging experiments in fundamental scientific research as well as for industrial applications. However, as THz regime bridges the gap between electronic and optical frequencies, emitter systems are still expensive and limited in power and bandwidth. A novel approach to overcome these challenges is given by the so-called spintronic terahertz emitters, which are based on ferromagnetic (FM) and non-magnetic metal (NM) layers with thicknesses of a few nanometers. Excitation of a FM/NM bilayer with a femtosecond optical laser pump pulse leads to the formation of an ultrafast spin current Js from the FM toward the NM layer, which is caused by the excitation of spin-polarized electrons of the FM layer above the Fermi level. In the NM layer, Js is converted into a transverse charge current pulse Jc due to the inverse spin Hall effect, which leads to the emission of electromagnetic radiation in the THz frequency regime. The emission properties of the emitters can be optimized by utilizing different materials, layer thicknesses, or more complex multilayer structures. The present work shows studies of spintronic THz emitter systems that are based on different magnetic thin films combined with Pt and W layers. The experimental studies can be divided into two parts. The main goal of the first part was to investigate how the magnetic properties of different FM and in particular also ferrimagnetic (FI) materials are reflected in the THz emission properties of a spintronic emitter system. Therefore, thin bilayers consisting of FM CoFe, or FI TbFe or GdFe alloy thin films with varying Fe content (0 ≤ x ≤ 1), combined with Pt layers have been prepared. The laser-excited spintronic THz emission has been investigated in dependence on the applied magnetic field, the temperature, and the pump fluence of the excitation laser. The results have been explained with regard to detailed characterizations of the structural, magnetic, electrical, and optical properties of the samples. The second goal of this work was set on the development of more functional multilayer emitter systems that allow for the control of the THz emission amplitude between a high- and a low-amplitude state and also might open the way for higher THz emission amplitudes. Based on the results of the previously investigated FI Pt/GdFe bilayer emitter system, a new concept of a THz emitter that can be switched by a temperature change from a high- to a low-amplitude state has been developed. Additionally, the use of a spin-valve system as a spintronic emitter system that allows for the switching of the emission amplitude by small applied magnetic fields in the range of a few millitesla has been demonstrated.