Development of techniques for magneto-spectroscopy at terahertz frequencies

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Abstract

In this thesis, I present my research into the development and use of a magneto-spectrometer operating at terahertz frequencies. Molecular samples with unpaired electrons were studied using electron paramagnetic resonance (EPR) spectroscopy and the properties of semiconducting samples were measured by cyclotron resonance.A terahertz time-domain spectrometer was constructed, fine-tuned and benchmarked. The use of a large area photoconductive antenna (PCA), with a bias voltage modulated at 1 MHz, allowed for a signal-to-noise ratio of 13800:1 to be achieved, which was equal or superior to comparable systems in use. The thermal behaviour of the PCA was studied and modelled to find that a 28% increase in emission occurred when the PCA was cooled to 100 K and a 22% increase in the maximum usable bias voltage was possible when the emitter substrate was cooled in a liquid nitrogen cryostat. The spectrometer was used to study a mixed type I/type II GaAs quantum well heterostructure as a test sample, in the process extending the existing research performed on this controllable terahertz filter.Together with a Gunn diode, bolometer and a Schottky diode, a pulsed magnet was developed into a 135 GHz, continuous-wave EPR spectrometer. EPR was detected in DPPH allowing for the spectrometer magnetic field accuracy to be determined and in ruby, which provided the crystal orientation of the sample and the base temperature of the spectrometer. Following this, three novel mixed-metal fluoride-centred triangles were studied on a multiple frequency EPR spectrometer allowing for the exchange interactions to be calculated along with the g-factors for two energy states. Two dimetallic cobalt complexes were studied to find that small changes to the molecules resulted in significant changes in the measured EPR spectra and so these molecules could be a useful part of the study of exchange coupled systems. The terahertz time-domain spectrometer was then combined with the pulsed magnet and an asynchronous optical sampling (ASOPS) system. Use of ASOPS allowed the number of waveforms measurable during a magnetic field pulse to be increased by at least a factor of 20 compared to contemporary alternatives. The spectrometer was used study cyclotron resonance in a two-dimensional electron gas (2DEG) formed at a GaAs/AlGaAs interface. Cyclotron resonance was successfully measured in the 2DEG at frequencies up to 1.6 THz, finding an electron effective mass of 0.071 m e , a sheet electron density of 3.3 × 10 11 cm -2 and a mobility of 1.5 × 10 5 cm 2 V -1 s -1 . Furthermore, using this system, the detection of EPR in a ruby sample was achieved, providing the first demonstration of ASOPS being used to study EPR at terahertz frequencies.

Keyword(s)

Cyclotron resonance; EPR; Pulsed magnets; Terahertz; Time domain

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