Guided modes of a width-reduced photonic-crystal slab line-defect waveguide with asymmetric cladding

The authors fabricated GaAs-based width-reduced photonic-crystal (PC) slab line-defect waveguides with asymmetric cladding, characterized optically by broadband transmission spectral results. These waveguides were vertically sandwiched between air and oxide claddings. Transmission peaks originating from the guided mode located within the bandgap were observed, thereby showing the presence of a bandgap effect for the transverse-electric (TE)-like guided mode even with the coexistence of a fundamental transverse-magnetic (TM)-like mode. Propagation loss spectra were derived from transmission spectra for both TE and TM polarization inputs. Comparing the obtained spectra to band diagrams, the authors detected a mini-stopband effect in the transmission spectra arising from a folding of the fundamental TM-like mode at the Brillouin zone boundary. The coupling coefficient was then estimated to be ∼0.01 [a^-1] (with a being a lattice constant). Dips in spectral intensity were also observed and were attributed to TE-TM mode conversion based on a comparison to the band diagram.