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This thesis demonstrates two application-oriented terahertz (0.1-10 THz) systems developed with ErAs:In(Al)GaAs photoconductors, and introduces new models to describe their performance as terahertz detectors. ErAs:In(Al)GaAs photoconductors, and photoconductors in general, have been widely used as terahertz emitters and detectors in homodyne terahertz spectrometers for more than a decade. However, some fundamental aspects of their operation as terahertz detectors had not been fully modelled yet. The first part of this thesis proposes new models for those fundamental aspects. Such models are not only valid for ErAs:In(Al)GaAs photoconductors, but also for any short carrier-lifetime photoconductive mixer acting as a CW terahertz detector. The focus is on two aspects: the influence of the driving laser on the noise performance, and the relationship between the incoming terahertz power and the current output by the mixer. The existing models for noise in short carrier-lifetime photoconductors neglect the influence of laser noise assuming that thermal noise is the dominant one. However, experimental evidence shows the contrary, at least for the optical power levels normally used in photoconductors. Thus, the model proposed in this thesis incorporates the influence of laser noise and that of generation-recombination noise into the picture. Its predictions were tested experimentally. Since no comprehensive models to quantitatively describe the relationship between incoming terahertz power and output current existed for CW photoconductive detectors, the one proposed here for that purpose is the first one of its kind. Its predictions were confirmed by experimental measurements. In addition to these models applicable to CW photoconductive terahertz detectors, the first part of this thesis also introduces a model to perform a detailed estimation of the spectral structure of the terahertz pulses emitted by a pulsed photoconductor when pumped by a MLL. Such models are required to examine some of the applications presented in the second part of this thesis. The second part of this thesis consists of two different applications developed using ErAs:In(Al)GaAs photoconductors working as CW terahertz detectors. The first one is a novel detection technique for terahertz pulses emitted by pulsed photoconductors driven by MLLs. The novel detection technique, coined FreSOD, allows to detect each of the modes composing a terahertz pulse with a resolution that exceeds by several orders of magnitude the resolution achieved with any time-domain optoelectronic detection technique. Such spectral resolution is not only relevant for terahertz measurements, but also for the characterization of the noise in the MLL driving the pulsed photoconductor. The second one is a fully optoelectronic spectrum analyzer for the sub-terahertz range operating without the need of an external electronic spectrum analyzer. The demonstrated resolution is on the 1-Hz level, with an operating range that goes from 0.05 to 0.09 THz. The range can easily be extended to more than 0.3 THz with relatively minor modifications. |
