We report on the observation of photoluminescence from the second and third electronic subbands in undoped GaAs-AlAs superlattices under application of an electric field perpendicular to the layers. The occupation of the higher subbands is achieved by sequential resonant tunneling of electrons. We determine the relative occupation of the second subband at resonance. Together with the electrically measured transport time between adjacent wells this novel spectroscopic effect provides a new method to determine the intersubband relaxation time.

• Received 21 November 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.2426