Application of THz pulses in semiconductor relaxation and biomedical imaging studies

This thesis is primarily concerned with the experimental study of media using THz frequency radiation. It covers a cross-section of research from the field of THz science and highlights the growing interest in this region of the electromagnetic spectrum. The dynamics of holes in valence quantum well samples are investigated in Chapters 2 to 5, providing insight into carrier behaviour essential for the development of semiconductor THz technology. Traditional spectroscopic techniques are used in Chapter 2 to study hole intersubband transitions. The spectra obtained show good agreement with the non-parabolic subband structure predicted by theory. In Chapter 3 THz pulses are used to probe the intersubband transition using saturation-absorption techniques. The results indicate complex relaxation behaviour and a significant thermal contribution at high intensity. The relaxation is examined further in Chapters 4 and 5 using time-resolved pump-probe techniques. A wide variation in the relaxation lifetime is observed for different well widths and FELIX intensities. This behaviour is explained effectively by a thermalised population picture, which predicts LO-phonon emission from thermally excited carriers in high energy states. Such a model is described in detail in Chapter 5 where the impact of the carrier and lattice temperatures on the relaxation are also explored. At low carrier temperatures strong variation is observed in the acoustic decay rates for the different valence samples, due in part to the large variation in the hole effective mass. In Chapter 6 the carrier dynamics results are examined from a wider perspective by considering them directly in relation to THz emitter research. Current studies using quantum well systems are reviewed and, in particular, the impact of carrier heating on the design and operation of efficient devices is discussed. Following on from this, recent advances in the generation and detection of THz radiation have opened the way for new imaging technology using THz waves. In Chapter 7 a Terahertz Pulse Imaging system constructed at the Toshiba Cambridge Research Laboratory is used to highlight the potential of THz frequency radiation in Biomedical research. The results of preliminary studies of animal tissue are presented showing the ability to distinguish between different tissue types and to produce high-quality three-dimensional tomographic images. Images are also presented of model human skin tissue, demonstrating the ability to penetrate and spectrally analyse moist dermal tissue, and to identify small variations in tissue composition for multi-layered samples. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN038755 / BLDSC - British Library Document Supply CentreGBUnited Kingdo