Mechanisms that transfer light between layers of multi-chip photonic assemblies

A multi-chip photonic assembly includes first and second photonic integrated circuits having first and second waveguides vertically stacked such that first vertical dimensions of the first and second waveguides occupy different horizontal planes in the stack. At least one of the first and second waveguides has a region that has a second vertical dimension that is larger than the first vertical dimension and either horizontally overlaps the other waveguide and/or vertically contacts the other waveguide. Light moving through one of the waveguides from the first vertical dimension to the other vertical dimension changes modes vertically so that the light moves from one waveguide to the other.