Narrowing of the intersubband absorption spectrum by localization of continuum resonances in a strong electric field

Abstract
Direct observation of strong localization of continuum resonances in the presence of an electric field is reported. Our structure consists of doped GaInAs quantum wells cladded by AlInAs/GaInAs quarter wave electron stacks and by AlInAs barriers. In a strong electric field of the appropriate polarity the first continuum resonance is strongly confined above the wells by the high reflectivity λ/4 stacks and by the field-induced triangular potential barrier, while the second continuum resonance becomes localized in the latter. This manifests itself in a strong narrowing of the intersubband absorption spectrum as a function of the electric field as a greater fraction of the oscillator strength is concentrated into a transition from the ground state of the well to a quasibound state in the continuum.