Ultracompact Photonic Circuits without Cladding Layers

Cladding layers of waveguides prevent interchannel interference yet are unfavorable for the integration of photonic circuits. Here, we report the realization of ultracompact waveguide arrays, bends, and circuits with essentially zero interchannel separation. This supercompactness is achieved via arrays of waveguides with shifted spatial dispersions, where each waveguide functions as both the transmission channel and an effective “cladding layer” of its neighboring waveguides, and has been experimentally realized in low-loss all-dielectric photonic crystals. We show that the zero-spacing transmission array possesses the remarkable features of negligible crosstalk, high-efficiency sharp bending, and ultracompact photonic routing where the light can traverse the entire physical space. This finding opens a new avenue for extreme space utilization efficiency in waveguide physics and integrated photonics.