  Optical isolation is a long pursued object with fundamental difficulty in
integrated photonics. As a step towards this goal, we demonstrate the design,
fabrication, and characterization of on-chip wavelength-scale optical diodes
that are made from the heterojunction between two different silicon
two-dimensional square-lattice photonic crystal slabs with directional bandgap
mismatch and different mode transitions. The measured transmission spectra show
considerable unidirectional transmission behavior, in good agreement with
numerical simulations. The experimental realization of on-chip optical diodes
using all-dielectric, passive, and linear silicon photonic crystal structures
may help to construct on-chip optical logical devices without nonlinearity or
magnetism, and would open up a road towards photonic computers.
