Flexible transmission control of mode conversion in a TiO2 cladding silicon subwavelength waveguide by engineering inter-waveguide mode coupling

Abstract

Mode conversions can be effectively accomplished in coupled mode silicon photonic waveguide systems when specific phase-matching conditions are fulfilled. However, conventional device using such principles is optimized to a certain coupling length, which can only achieve a fixed energy transmission. Moreover, the conventional method to tune the refractive index is insufficient for effective transmission manipulation. In this paper, we systematically analyze the coupling strategy in the silicon-based dual-waveguide coupled-mode system, and propose an effective transmission tuning method by engineering the inter-waveguide mode coupling. We introduce titanium dioxide as cladding material to a subwavelength waveguide-based directional coupler to change the refractive index variable, which is both CMOS compatible and has negative thermo-optical coefficient. By implementing different temperature on waveguide, the dispersion curves of stripe waveguide and subwavelength waveguide can be tailored independently, thus the coupling coefficient can be changed without changing the geometry of waveguide. We demonstrate prototype of TE0-TE1 multi-mode variable optical attenuator. The output can be tuning 0.05%–96.4% of input energy when the temperature rises 0-400 K. We extend the device to TE2 and TE3 with output intensities of 0.04-91.4% and 0.04-88.8% over the temperature rise range of 317 K and 214 K, respectively. Cascading mode converters for different mode-order, we can arbitrarily mix the mode percentage in output waveguide to generate different interference patterns. We demonstrate a reconfigurable optical tweezer using this setup, which is capable of manipulating nanoscale particles. We envision that such methodology can be implemented in flexible multimode optical networks and labs on chip.