Paul C. M. Planken

THz pulses from the creation of polarized electron‐hole pairs in biased quantum wells

We report generation of coherent terahertz electromagnetic transients from GaAs/Al0.3Ga0.7As quantum wells in perpendicular fields. Although, at low temperature, the quantum well barriers suppress the transport current perpendicular to the layers by at least two orders of magnitude compared to bulk, we observe terahertz signals that are comparable in strength to those generated from bulk GaAs surfaces. This directly proves that the field‐induced polarization of photoexcited electron‐hole pairs is an important mechanism for the generation of terahertz radiation at semiconductor surfaces.

Generation of terahertz electromagnetic pulses from quantum-well structures

Terahertz generation from semiconductor quantum-well structures pumped by a femtosecond optical pulse is studied. We propose a three-level model for the electrons and holes in the quantum wells. We then solve the coupled optical Bloch equations directly using a Runge-Kutta method and calculate the terahertz radiation field. We study optical rectification and quantum beats caused by charge oscillations in 1) a coupled quantum well in which quantum beats occur between two electron states of the coupled system and 2) a single-quantum-well structure in which quantum beats occur between light-hole and heavy-hole excitons. Our theoretical results agree very well with the experimentally measured terahertz data. >

Repetitive excitation of charge oscillations in semiconductor heterostructures

We create forced excitonic charge oscillations in semiconductor heterostructures by excitation with periodic optical pulse sequences. The far‐infrared radiation that accompanies the charge oscillations shows temporal interference patterns for pulse sequences with different optical phase profiles, providing evidence for phase induced population transfers between exciton levels in a semiconductor.

Optical generation of terahertz pulses in quantum wells

We report on the generation of terahertz pulses from the optical excitation of excitons in GaAs/Al0.3Ga0.7As quantum wells. In general there are two contributions to the THz signals for nonzero field. The first arises from the creation of excitons in states in which they are already polarized. This leads to a time-dependent polarization that grows with the integrated pulse energy and radiates a short electrical transient. The second one arises from the coherent excitation of both light hole and heavy hole excitons. The time evolution of the coherent superposition of these two exciton states give rise to charge oscillations that radiate a terahertz signal consisting of several oscillations of the electric field. By applying a field to the sample we can tune the oscillation frequency from 1.4 to 2.6 THz.