Terahertz spectroscopy of semiconductor nanostructures
Kabir, Nafees Aminul
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Electromagnetic waves centered at a frequency of 1 terahertz (THz) lie between the realms of photonics and electronics. Only recently have techniques been established to generate and detect electromagnetic waves in this regime, which is of fundamental importance since it corresponds to the characteristic energy range for many physical, chemical and biological processes. Since semiconductors may play a critical role in the development of such technologies, in this thesis the THz-transmission properties of high-mobility GaAs and InAs two-dimensional-electron-gas (2DEG) systems are explored using terahertz time domain spectroscopy (THz TDS). The momentum-relaxation time of electrons in these systems is determined from an analysis of these measurements, using a modified Drude model, it is found to be systematically lower than that inferred from the results of (quasi-) DC magneto-conductance measurements. This discrepancy is particularly pronounced at high frequencies (> ∼0.7 THz) and low temperatures (< ∼20 K), and suggests that small-angle scattering leads to weaker heating of 2DEGs at lower temperatures than would be expected from the DC relaxation time. In addition to these studies, THz TDS is also performed on large-area arrays of quantum dots, realized by electron-beam lithography and wet etching. The interest here is in exploring the possibility of plasmonic detection in the sub-THz range. It is believed that these results may be very useful in the future for the successful design of a frequency-resolved THz detector.