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HL: Halbleiterphysik

HL 29: Ultrakurzzeitdynamik I

HL 29.11: Vortrag

Donnerstag, 25. März 1999, 13:00–13:15, H2

Few-cycle THz spectroscopy of semiconductor quantum structures — •Rudolf Bratschitsch¹, Roland Kersting¹, Edwin Thaller¹, James Heyman², Gottfried Strasser¹, and Karl Unterrainer¹ — ¹Institut für Festkörperelektronik, TU-Wien, Floragasse 7, A-1040 Wien — ²Macalester College, St. Paul, MN 55105, USA

In our experiments we generate few-cycle THz pulses by ultrafast laser excitation of coherent plasma oscillations in n-doped GaAs. This THz radiation can drive electronic intersubband transitions in modulation doped quantum well structures. By using a cross-correlation technique we are able to detect both amplitude and phase of the transmitted radiation. We show results for both resonant and non-resonant excitation of the quantum well intersubband transition which serves as a model two level system. While resonant excitation shows the signature of the free induction decay a completely different observation is made in the off-resonant case: at early times the coherent polarization oscillates with the frequency of the exciting THz-pulse but after the end of the driving pulse the electrons inside the quantum well perform a phase jump and begin to oscillate with the eigenfrequency of the intersubband transition. Experiments are under way to selectively populate the upper state of the two level system by interband pumping and to probe the population by a delayed THz pulse.