THz Charge Oscillations in a Modulation Doped Parabolic Quantum Well

We used ultrafast terahertz (THz) spectroscopy to observe THz-frequency electron oscillations in a modulation-doped InGaAs/AlGaAs parabolic quantum well. THz emission and absorption measurements yielded an electron subband spacing of 0.01eV, in agreement with sample design. Our study examined how the extrinsic electron density in the well influences THz emission efficiency, and we found no strong dependence. This indicates that THz emission in this structure arises from quantum beating of the photogenerated electrons. In contrast to a previously published report [1], we find that THz emission from the cold extrinsic electrons due to ultra-fast field screening plays at most a secondary role, even when the density of extrinsic electrons [$\sim $10$^{11}$ cm$^{-2}$] exceeds the density of photogenerated charge. This work was funded by the National Science Foundation under the NSF-RUI Program (DMR-0606181). \newline [1] R. Bratschitsch, et. al., APL 76, 3501 (2000).