Cut-off-based dual-taper reflectors in on-chip hollow waveguides

We describe back-to-back (dual) tapers embedded within hollow Bragg waveguides clad by omnidirectional Si/SiO₂-based mirrors, and fabricated using a thin film buckling approach. The back-reflection of light subject to mode cutoff in the narrowed tunnel section results in a short-pass transmission characteristic. Thus, the dual taper can act as a waveguide filter with the upper pass-band edge determined by the lithographically controlled height of the tunnel section. We also report preliminary results on the use of these dual tapers as in-plane reflectors (for operation in the cutoff regime), with potential to enable a novel class of open-access hollow-waveguide microcavities.