Room Temperature, Low Threshold Distributed Feedback Quantum Cascade Lasers at 7.7 μm

In this paper, we present low threshold DFB QCLs (λ ≡ 7.7 μm) with top grating by innovating the waveguide structure and the method of grating fabrication. Highly doped InP cladding layer was taken as the host layer for the grating, and was anisotropically etched along [O^-₁₁] direction with the lateral etching being negligible. This method releases the limitation on the grating depth. By careful design and control a deep first-order grating, with a thin highly doped InP layer beneath it, was formed in the plasmon enhanced cladding layers. The deep grating ensures strong feedback effect and the thin highly doped InP layer separates the high loss metals in the groove of the grating from the active region, suppressing the increase of the waveguide loss. It allows the fabrication of DFB QCLs with low threshold current density.