  We experimentally demonstrate broadband waveguide crossing arrays showing
ultra low loss down to $0.04\,$dB/crossing ($0.9\%$), matching theory, and
crosstalk suppression over $35\,$dB, in a CMOS-compatible geometry. The
principle of operation is the tailored excitation of a low-loss spatial Bloch
wave formed by matching the periodicity of the crossing array to the difference
in propagation constants of the 1$^\text{st}$- and 3$^\text{rd}$-order TE-like
modes of a multimode silicon waveguide. Radiative scattering at the crossing
points acts like a periodic imaginary-permittivity perturbation that couples
two supermodes, which results in imaginary (radiative) propagation-constant
splitting and gives rise to a low-loss, unidirectional breathing Bloch wave.
This type of crossing array provides a robust implementation of a key component
enabling dense photonic integration.
