Fig. 1. (a) Measured complex refractive indices of AlN. (b) Calculated effective indices dependent on the waveguide width. Scanning electron microscope (SEM) images of the AlN waveguide (c) without top cladding and (d) with SiO2 top cladding.

Fig. 2. Simulation, fabrication, and measurement results of directional couplers at 1550 nm wavelength and TE mode. (a) Directional coupler simulation showing 50:50 output ratio. (b) SEM image of fabricated directional couplers. (c) Simulation and (d) experiment results of the through port (1→3) output ratio map obtained by changing the coupling waveguide length and gap. The black dashed lines indicate a coupling condition of 50:50 ratio. (e) Simulation and (f) experiment results of the output ratios at through (1→3) and coupling (1→4) ports depend on coupling gap at a fixed coupling waveguide length of 8 μm.

Fig. 3. (a) Calculated ΔDt dependent on the coupling gap at 1550 nm wavelength. (b) Calculated PBS transfer distance (DPBS) with different n, thus different coupling gap. (c) Coupling simulation in the case of n=4 condition when TE or TM modes are launched in the upper waveguide. Waveguide width, 700 nm. (d) SEM images of fabricated directional couplers for PBS. Up, top view; bottom left, cross section of coupling part; bottom right, top view zoom in of coupling part. (e) Measured output ratio for TE/TM input with different coupling waveguide lengths at 1550 nm wavelength and 600 nm gap. (f) Measured extinction ratios and their Gaussian fitting for TE (blue) and TM (red) inputs from the PBS with a gap of 600 nm and a coupling waveguide length of 614 μm.

Fig. 4. (a) SPDC setup for single photon pairs generation. (b) Lensed fibers to chip coupling setup scheme of two inputs and two outputs. (c) Twofold coincidence counting measurement setup. (d) HOM interference result from the AlN waveguide beam splitter. The integration time of the photon count for the y axis is 10 s.