Coherent terahertz emission from optically pumped intersubband plasmons inparabolic quantum wells

We report on few-cycle terahertz (THz) emission from modulation-doped parab olic quantum wells. The quantum wells are optically excited by near-infrare d femtosecond laser pulses. The observed THz emission corresponds to the in tersubband 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 drive n THz emission from a completely symmetric nanostructure, in contrast to qu antum beats which require a broken symmetry for their excitation. (C) 2000 American Institute of Physics. [S0003-6951(00)04624-6].