Carrier relaxation in quantum wires: consequences for quantum wire laser performance

Abstract

Quantum wire lasers are expected to require very low threshold currents owing to the nature of the 1D density of states which develops a sharp peak at the band edge and ensures superior laser characteristics. However, carrier relaxation processes in quasi-1D structures may be much slower than in bulk material owing to reduction in the momentum space. For very long relaxation times, these equilibrium processes are expected to limit the maximum modulation frequency of the quantum wire lasers. We perform a Monte Carlo simulation of electron relaxation in quantum wires with the inclusion of the electron-bulklike polar optical and acoustic phonon, electron-electron and electron-hole interactions as well as Thomas-Fermi screening. We find that for a carrier density of 1018 cm-3 the electron relaxation time ranges from 120 ps for the 100 AA*100 AA wire to 30 ps for the 200 AA*200 AA wire. Since the threshold current in a quantum wire laser increases with the wire cross section, within the limits of our relaxation model, this indicates possible existence of a trade-off between speed and efficiency in a quantum wire laser. We also analyse the effects of carrier relaxation on gain compression in quantum wire lasers by solving the Boltzmann equation using a novel Monte Carlo technique. A spectral hole forms in the carrier distribution at high injected currents with the resulting decrease in the slope of the light-current characteristic. The effect of a non-fermi-Dirac distribution of electrons is found to result in a suppression of the peak gain as compared with the peak gain calculated using the equilibrium distribution.