Amorphous silicon-lithium niobate thin film strip-loaded waveguides

The heterogeneous integration of an amorphous silicon (a-Si) film with a lithium niobate (LN) thin film combines both the mature micro-processing technology of Si and the excellent optical properties of LN. An a-Si thin film was deposited on an LN thin film, and strip-loaded waveguides were designed, fabricated, and characterized. A full-vectorial finite difference method was used to explore the single-mode conditions and appropriate dimensions for the strip-loaded waveguides. The waveguide mode size could be as small as 0.36 μm2. By adjusting the thickness and width of the a-Si loading strip, the distribution of light power could be mainly confined in the LN layer. The maximal light power that could be confined in LN was 91%, which was obtained at an a-Si thickness of 65 nm. A set of waveguides with widths of 2‒7 μm were prepared by inductively coupled plasma (ICP) etching of the a-Si thin film. Following annealing at 300°C in air for 1 hour, light transmission was observed in the waveguide. The 2-μm-wide waveguide showed propagation losses of 20 dB/cm for the quasi-TM (q-TM) mode and 42 dB/cm for the quasi-TE (q-TE) mode at 1550 nm. The root-mean-square (RMS) surface roughness of the a-Si thin film before and after annealing was 1.04 and 0.35 nm, respectively. High-resolution transmission electron microscopy (HRTEM) was performed to investigate the interface morphologies. A well-defined interface was clearly observed, and the structure of the a-Si thin film was proved to be amorphous.