Terahertz excitation dynamics in semiconductor heterostructures

Summary form only given. The advent of terahertz time-domain spectroscopy (THz-TDS) gave research on intersubband dynamics a new dimension of time resolution. THz-TDS allows the measurement of a transient polarization with subcycle time resolution thus providing full information on the intersubband response to ultrafast and nonadiabatic excitation pulses. We have investigated the dynamics of intersubband excitations in modulation n-doped AlGaAs/GaAs heterostructures. Their parabolically shaped conduction band leads to equally spaced intersubband transitions, which have typical resonance frequencies between 1.5 THz and 3.0 THz. The quantum wells are modulation n-doped at about 10/sup 12/ cm/sup -2/ and have electron mobilities of about 80,000 cm/sup 2//Vs, which correspond to dephasing times of about 2 ps. The electron density in the quantum wells can be controlled electronically by Schottky grating contacts fabricated on top of the structures. In our time-resolved experiments we excite the intersubband transitions with few-cycle THz pulses. Switching the electron density within the devices gives a modulation signal, which directly shows the polarization of the electronic quantum transitions. The polarization build-up and the free induction decay are observed on nonadiabatic time-scales. The results are compared to solutions of the optical Bloch equations. In order to follow the nonadiabatic dynamics our model uses an ansatz, which goes beyond the slowly-varying envelope approximation and unveils the population dynamics at low excitation densities. As a first application for signal processing we present a THz phase modulator device.