  Dielectric optical waveguides constitute the main building blocks of photonic
integrated circuits (PICs). Channels with high refractive index contrast can
provide very compact PIC components whereas structures with lower index exhibit
less propagation loss. A hybrid concept that can combine the best of high- and
low-index materials is highly required. Here, we devise a new approach to
realize compact and low-loss hybrid optical waveguides based on the interaction
of weak optical modes. This is a rather universal approach that can be applied
to a wide range of optical materials. To prove the principle, the hybrid
waveguide structure is formed by combining a low-index polymer and a thin layer
of silicon nitride. For this material combination, a minimum bending radius of
90 micron (for a bending loss of 0.005 dB/90o ) and an upper limit for the
optical loss of 0.7 dB/cm are achieved. The viability of this platform is
demonstrated through a series of high-performance novel PIC components. This
hybrid waveguide platform enabled by a powerful and simple design concept holds
great promise for high-density and low-loss PICs.
