Controlling light propagation and mode coupling in optical waveguides using one dimensional phased antenna arrays

Phased antenna arrays patterned on optical waveguides can strongly affect mode coupling and propagation in the waveguides. The unidirectional effective wavevector provided by the phased antenna array can facilitate phase matching between different waveguide modes; the effective wavevector can also breaks the symmetry of optical power flow in waveguides. These effects can be used to realize novel integrated photonic circuits. We used full-wave simulations to demonstrate broadband, small-footprint optical waveguide mode converters, polarization converters, and optical power diodes in the telecom and mid-infrared spectral ranges. The designed optical devices have small footprints (i.e., length of antenna arrays ranges from a few to ten times of the free-space wavelength). The efficiency of mode conversion (defined as the percentage of optical power coupled from one waveguide mode to the other) ranges from 10% to 60%, and the purity of converted modes approaches unity. The on-off ratio of optical power diodes (defined as the ratio between optical power transmission in two opposite directions) can be larger than 20dB over a broad wavelength range. We developed a coupled mode theory to model the asymmetric coupling between different waveguide modes and between waveguide modes and surface waves propagating along antenna arrays. The results calculated using the coupled mode theory can successfully explain mode evolution in our waveguide structures observed in simulations. We believe the platform of optical waveguides integrated with phased antenna arrays offers great opportunity to study novel physics originating from interaction between guided waves and designer structures. The platform also has great implication for integrated photonics device applications.