Beam recirculation and mode-order conversion via compact Mach-Zehnder-Fano interferometers.

In this Letter, photonic crystal (PC) waveguide-based interferometer design is studied; spectral as well as temporal analyses have been conducted. Intentional structural modifications inside the interferometer trigger Fano resonances, allowing for extraordinary optical effects, such as enhanced beam recirculation and mode-order conversion. The proposed Mach-Zehnder-Fano interferometer is compatible with conventional silicon-on-insulator (SOI) technology and consists of two arms: the lower arm, with no point defects, creates continuum states, whereas the upper arm, including a Fano defect, creates discrete states. The PC waveguide channel with intentional point defects, which possess effective discreteness, creates the required phase retardation of the propagating beam, enabling Fano resonance excitations. Such a type of PC waveguide-based interferometer allows for the realization of compact mode-order converters with a broad 3 dB-bandwidth of 65 nm (at the wavelength range of λ = 1517 - 1582 nm) and efficient optical switching as well as sensing capabilities, operating at optical telecommunication bands, favoring inherent beam recirculation characteristics.