Ultrafast dynamics in quantum cascade lasers: Electronic transport and coherent light -matter interaction.

Abstract

To date, quantum cascade lasers are the only semiconductor laser sources that can cover a broad range of wavelength from mid-infrared down to terahertz frequency. The operation physics of quantum cascade lasers is qualitatively different from other conventional lasers; the lasing action is based on the unipolar characteristics, i.e. intersubband transition, giving rise to an atomic-like gain transition, and the superlattice transport, relaxation channel for hot electrons, which can not be found any analogues in any conventional laser systems. In this thesis, the nature of electronic transport and the femtosecond pulse propagation in quantum cascade lasers have been addressed by using ultrafast optical techniques. In particular, the electronic transport in quantum cascade lasers was extensively investigated by using femtosecond time-resolved pump-probe techniques. Sub-picosecond resonant tunneling injection from the quantum cascade laser injector ground state into the upper lasing state was found to be incoherent due to strong dephasing in the active subband. The few-picosecond gain recovery due to transport through superlattice was observed and interpreted in terms of dielectric relaxation within the superlattice miniband. We also observed the strong coupling of the electronic transport to the intra-cavity photon density, which we term photon-driven transport. By using an ultrafast upconversion method, the dispersion of the active waveguide was characterized by measuring the wavelength-dependent propagation delay. Contributions from material dispersion, waveguide dispersion, and small-signal gain dispersion were separated, and compared to the experimentally measured pulse broadening as for a self-consistency check. Pulse re-shaping and possible contribution of coherent effects on pulse propagation and gain dynamics were investigated by comparing weak versus strong pulse injection for various bias conditions. In order to support the experimentally measured data, several quantum transport models were used: density-matrix formalism for the resonant tunneling process, Monte-Carlo simulation for the superlattice relaxation, and Maxwell-Bloch equation for the pulse propagation. All theoretical models strongly support the experimental results.