Effects of two-photon absorption on carrier dynamics in quantum-dot optical amplifiers

We investigate ultrafast gain-dynamics in a quantum-dot (QD) semiconductor optical amplifier where two-photon absorption (TPA) occurs in the bulk region of the waveguide during propagation of a sub-picosecond optical pulse, and gain bleaching occurs in the QD active region. Our calculation reveals that TPA provides optical pumping which is quasi-synchronous in time and space with QD carrier depletion, leading to enhancement of carrier capture into the QDs on an ultrafast timescale. The TPA-induced optical pumping, which is qualitatively different from electrical pumping, changes qualitatively carrier recovery dynamics, reducing pattern effects when a train of optical pulses is injected at the ultra-high speed.