Low-Loss Surface-Mode Waveguides for Terahertz Si–SiGe Quantum Cascade Lasers

A new design of low-loss terahertz waveguide for Si-SiGe quantum-cascade lasers (QCLs) is presented. Periodic surface gratings are used to define waveguides without the requirement of cleaved or etched end facets. As Si cleaves along the (111) planes and not in the vertical direction for standard Si (100) substrates, this significantly aids the fabrication of waveguides. Losses down to 2 cm
^-1
with modal overlap of 0.4 can be achieved for shallow gratings with etched depths of only 0.56 mum for an active material layer thickness of 8 mum. Such low loss and high modal overlap is key to any Si-SiGe QCL being realized