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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 demonstrate subpicosecond photocarrier lifetimes at 1.55 μm in GaSb/ErSb nanoparticle superlattices grown by molecular beam epitaxy. Pump–probe measurements were made with a 1.55 μm mode-locked laser in transmission geometry to determine the photocarrier lifetime. The lifetime is found to be dependent on the size of the ErSb particles, amount of ErSb, and the distance between layers of particles. Through manipulation of these three parameters the photocarrier lifetime can be tuned down to less than 300 fs, the temporal limit of our experiment. [ABSTRACT FROM AUTHOR]