Terahertz emission by quantum beating in a modulation doped parabolic quantum well

We use ultrafast terahertz spectroscopy to observe terahertz-frequency electron oscillations in a modulation-doped InGaAs∕AlGaAs parabolic quantum well with subband spacing 0.01eV. 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.