Above- and below-band femtosecond nonlinearities in active AlGaAs waveguides

Femtosecond optical pulses were used to study the gain and refractive index dynamics in a bulk AlGaAs diode laser. Experiments were performed at wavelengths above and below the band gap of the diode-laser active region. The above-band gain dynamics exhibit an ultrafast transient that has not been seen before in AlGaAs devices. We attribute this dynamic to spectral-hole burning and a delay in carrier heating. The below-band refractive index measurements demonstrate that carrier heating by free-carrier absorption is important and that there is a delay in the onset of this heating of approximately 120 fs.