Ultrafast time-resolved pump-probe measurements are used to study low energy excitations and dynamics of electronic
transport in various semiconductor nanostructures. In quantum cascade lasers, we observe ultrafast gain recovery
dynamics due to electronic transport in the structures. In particular, the nature of electronic transport had been addressed
by using ultrafast optical 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. We also observed the strong coupling of the electronic transport to the intra-cavity photon density, which we
term "photon-driven transport". Note that this invited paper reviews the details of our recent observations (H. Choi et al.,
Phys. Rev. Lett., 100, 167401, 2008 and H. Choi, et al., Appl. Phys. Lett. 92, 122114 (2008)).