Polarization versatility of surface emitting ring cavity quantum cascade lasers

Recently, our group presented the surface emitting ring cavity quantum cascade laser (QCL). This kind of light source shows lower thresholds, higher slope-efficiencies and narrower emission beams as compared to equivalent Fabry-Perot devices. Typically, such lasers show a doughnut-shaped, rotational symmetric far field with azimuthal polarization. When the excited modes in the cavity are off-resonance to the DFB grating, the light is emitted under a broader angle. This leads to a widening of this `doughnut´ and the polarization of the far field switches to radial polarization. Additionally, we present ring-QCLs with linear polarized far fields. For this task, devices with PI-shifts in the grating period were used. The opposing fields in the ring resonator interfere constructively in the centre of the far field. We observed a central lobed far field, where pure linear polarization in this central lobe was measured. To extend this polarization property to the entire far field, a wire grid polarizer was fabricated onto the substrate with the wires perpendicular to the favoured polarization direction. The devices were optimized for substrate emission.