Terahertz (THz), the frequency region that bridges the gap between the radio and optical frequencies in the electromagnetic spectrum, is of both scientific and technological importance. Its broad applications, however, have been limited by the availability of bright sources and sensitive detectors. The invention of modelocked femtosecond lasers has made possible the generation and detection of electromagnetic transients on the picosecond to sub-picosecond time scale, which correspond to broadband coherent radiation up to a few or 10’s THz. This time-domain technique has had a significant impact on spectroscopy, imaging and sensing.
While detectors and emitters have been the main interest in the development of THz technology, other components such as modulators, reflectors and filters are also of importance. In Chapter 2 we apply THz transmission spectroscopy to characterize polymeric materials and nano-composites. We demonstrate the design and fabrication of THz mirrors based on one-dimensional photonic crystals consisting thermoplastic polymers and BaTiO3 nanoparticles. Stop bands have been achieved in the THz region, and the results agree well with the prediction of the transfer matrix method.
The main focus of this thesis is to study charge transport and carrier dynamics in low dimensional materials using THz time-domain spectroscopy. In contrast to conventional transport measurements, where electrical contacts are required, the THz method based on freely propagating electromagnetic transients can probe the electronic transport in a contactless fashion. In Chapter 3 we investigate highly oriented pyrolytic graphite (HOPG) using the THz emission spectroscopy. The direction, spectrum, polarization and dynamics of the emitted THz radiation from graphite upon ultrafast photoexcitation reveal the emission mechanism as well as the out-of-plane transport properties of graphite.
In Chapter 4 metal phthalocyanines have been investigated by the optical pump/THz probe technique. Disc-like phthalocyanine molecules self-assemble into columns and form quasi-1D conduction channels for efficient charge transport. With appropriate side groups the material can have both crystalline and liquid crystalline phases. The photoconductivity spectrum, its dependence on the pump-probe delay time and sample temperature suggest the mechanism of as well as the influence of crystallinity on the photoconductivity in metal phthalocyanines.