Single mode quantum cascade lasers

Quantum cascade lasers (QCLs) are based on intersubband transitions. Selection rules for transitions in quantum wells allow gain only for TM polarized electromagnetic waves. Therefore conventional vertical cavity surface emitting laser design is not suitable for QCLs. A metal-stripe surface grating structure allows high surface emission efficiency for the TM-polarized light. The emitted power via the surface is in the range of a few Watts and exceeds the emitted power from the facets. A double-lobed surface emission far field pattern is obtained for the lasing mode from DFB gratings without phase shift. The respective grating coupling coefficient is determined to be K=29 cm. This technique can also be used to fabricate sophisticated waveguide structures, like Bragg mirrors or photonic crystal QCLs operating in the mid-infrared to the THz wavelength regime. We show QCL structures with etched Bragg mirrors operating around 11μm. Compared to other fabrication techniques, like FIB-cut DBRs, this ICP-RIE technique requires less effort and gives more freedom in design and processing of these structures.