Electrically pumped photonic crystal distributed feedback quantum cascade lasers

In parallel with the effort to improve the efficiency of Quantum cascade lasers (QCL) for high power continuous wave (CW) operations, the peak power in pulsed mode operation can be easily scaled up with larger emitting volumes, i.e., processing QCLs into broad area lasers. However, as the emitter width increases, multi-mode operation happens due to poorer lateral mode distinguishability. By putting a two dimensional photonic crystal distributed feedback (PCDFB) layer evanescently coupled to the main optical mode, both longitudinal and lateral beam coherence can be greatly enhanced, which makes single mode operation possible for broad area devices. For PCDFB laser performance, the linewidth enhancement factor (LEF) plays an important role in controlling the optical coherence. Being intersubband devices, QCLs have an intrinsically small LEF, thus serving as better candidates over interband lasers for PCDFB applications. We demonstrate electrically pumped, room temperature, single mode operation of photonic crystal distributed feedback quantum cascade lasers emitting at λ. ∼ 4.75 μm. Ridge waveguides of 50 μm and 100 μm width were fabricated with both PCDFB and Fabry-Perot feedback mechanisms. The Fabry-Perot device has a broad emitting spectrum and a broad far-field character. The PCDFB devices have primarily a single spectral mode and a diffraction limited far field characteristic with a full angular width at half-maximum of 4.8 degrees and 2.4 degrees for the 50 μm and 100 μm ridge widths, respectively.