Ultrafast carrier capture and THz resonances in InGaAs quantum posts

Semiconductor quantum posts (QPs) - nanowire-like InGaAs heterostructures in a GaAs matrix - resemble many properties of regular self-assembled quantum dots (QDs), to which they are closely related. Due to their increased size as compared to QDs, QPs have proven to be suitable for very low threshold interband lasers. However, their well controllable height makes them attractive for precise tuning of the interband energy spacing that in QDs can only be achieved via post-growth annealing. Specifically, the 1s - 2p transition energy is expected to drop below LO-phonon energies at post heights of more than 30 nm, making them attractive as frequency-agile structures at terahertz frequencies. In the work presented here we explore the capture dynamics of QP structures after photoexcitation into the GaAs matrix. While the combined electron-hole dynamics are studied using time-resolved photoluminescence spectroscopy, optical pump - THz probe experiments were performed in order to solely study the electron dynamics. The results of the THz experiment show that after ultrafast excitation, electrons relax within a few picoseconds into the quantum posts, which act as efficient traps. The saturation of the quantum post states, probed by photoluminescence, was reached at approximately ten times the quantum post density in the samples. Also studied was the presence of possible electronic resonances after direct photoexcitation into QPs where a broad absorption around 1.5 THz was observed.