Femtosecond Technology Research Association scientists in Tsukuba, Japan, have constructed coupled-cavity waveguides by forming cylindrical airholes in ridge waveguides using electron-beam lithography and reactive ion-beam etching. The group had previously suggested that coupled-cavity waveguides could enable the efficient optical delay of ultrashort photonic pulses.

The researchers generated periodically spaced defects in the silicon on silicon-dioxide ridge waveguides by increasing the distance between the airholes. With the proper lattice constant, they produced quasiflat impurity bands at approximately 1.55 µm. A fabricated delay line with a length of 20 µm produced a 600-fs delay for a 110-fs-wide optical pulse.

They described the work in the Sept. 9 issue of Applied Physics Letters.