Abstract. The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1 THz. Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.
The electron capture time in superlattice structures consisting of periodically spaced layers of self-assembled ErAs nanoislands and In0.53Ga0.47As is investigated on photoconductive switches as a function of the superlattice period using photocurrent autocorrelation and pulsed laser excitation at 1.55μm. The capture time can be tuned from picoseconds all the way down to 0.2ps by changing the periodicity. Two different Be doping schemes are explored to reduce the dark current. The resulting characteristics indicate that ErAs:InGaAs may serve as a high performance photoconductive material at this wavelength for pulsed terahertz emission and detection.
We demonstrate that branching of the electron flow in semiconductor nanostructures can strongly affect macroscopic transport quantities and can significantly change their dependence on external parameters compared to the ideal ballistic case, even when the system size is much smaller than the mean free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs two-dimensional electron gas, we observe a splitting of the commensurability peaks in the magnetoresistance curve. We show that a model which includes a random disorder potential of the two-dimensional electron gas can account for the random splitting of the peaks that result from the collimation of the electron beam. The shape of the splitting depends on the particular realization of the disorder potential. At the same time, magnetic focusing peaks are largely unaffected by the disorder potential
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