Abstract
We report on few-cycle terahertz (THz) emission from modulation-doped parabolic quantum wells. The quantum wells are optically excited by near-infrared femtosecond laser pulses. The observed THz emission corresponds to the intersubband plasmon of the parabolic quantum well. The emission frequency is independent of the number of optically generated carriers. We identify the excitation mechanism of the intersubband plasmon and hence THz emission to be ultrafast field screening. This mechanism allows for an optically driven THz emission from a completely symmetric nanostructure, in contrast to quantum beats which require a broken symmetry for their excitation.
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