Unipolar semiconductor laser

Abstract

This application discloses, to the best of our knowledge, the first unipolar laser. An exemplary embodiment of the laser was implemented in the GaInAs/AlInAs system and emits radiation of about 4.2 .mu.m wavelength. Embodiments in other material systems are possible, and the lasers can be readily designed to emit at a predetermined wavelength in a wide spectral region. We have designated the laser the 'quantum cascade' (QC) laser. The QC laser comprises a multilayer semiconductor structure that comprises a multiplicity of essentially identical undoped 'active' regions, a given active region being separated from an adjoining one by a doped 'energy relaxation' region. In a currently preferred embodiment each active region comprises three coupled quantum wells designed to facilitate attainment of population inversion. In the currently preferred embodiment the energy relaxation regions are digitally graded gap regions. However, other energy relaxation regions are possible. Disclosed are also embodiments that rely primarily on 'vertical' transitions in a given quantum well. Such lasers preferably comprise superlattice Bragg reflectors. The unipolar plasma in a unipolar laser can be manipulated by means of an electric 'control' field, facilitating, for instance, beam steering or external control of the modal gain of the laser. Means for accomplishing this are discussed.