Abstract
We demonstrate that we can generate oscillatory terahertz radiation in single quantum wells after the coherent optical excitation of both the light hole and the heavy hole excitons, with femtosecond pulses from a modelocked Ti:sapphire laser. The sample consists of 15 periods of 17.5 nm GaAs wells separated by 15 nm AlGaAs barriers. By applying a voltage to a Schottky contact on top we can control the electric field in the sample. Figure 1 depicts the experimental setup. Figure 2 shows the generated THz waveforms detected with a 50-μm. photoconducting dipole antenna, as a function of the electric field in the structure at 10 K. The first cycle of radiation results from the excitons already being in a polarized state upon creation. This leads to a time-dependent polarization P that grows with the integrated pulse energy and hence radiates an electrical transient according to E ∝ ∂2P / ∂t2. 1 The later oscillatory radiation originates from the time evalution of the superposition of the lh and hh exciton states giving rise to a time dependent dipole moment that radiates.2 The THz radiation can easily be tuned from 1.4 to 2.6 THz and we find an excellent agreement between the THz oscillation frequency and the hh-lh splitting.
© 1993 Optical Society of America
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