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We use ultrafast terahertz spectroscopy to observe terahertz-frequency electron oscillations in a modulation-doped InGaAs/AlGaAs parabolic quantum well with subband spacing 0.01 eV. Our study examined how the extrinsic electron density in the well influences terahertz emission efficiency and we found no strong dependence. This indicates that terahertz emission in this structure arises from quantum beating of the photogenerated electrons. Terahertz emission from the cold extrinsic electrons due to ultrafast field screening plays at most a secondary role. [ABSTRACT FROM AUTHOR]

We report the molecular beam epitaxy growth of Al0.3Ga0.7As/GaAs multijunction solar cells with epitaxial, semimetallic ErAs nanoparticles at the interface of the tunnel junction. The states provided by these nanoparticles reduce the bias required to pass current through the tunnel junction by three orders of magnitude, and therefore drastically reduce the voltage losses in the tunnel junction. We have measured open-circuit voltages which are 97% of the sum of the constituent cells, which result in nearly double the efficiency of our multijunction cell with a conventional tunnel junction. [ABSTRACT FROM AUTHOR]