  Waveguide design is crucial in developing efficient light delivery systems,
requiring meticulous material selection, precise manufacturing, and rigorous
performance optimization, including dispersion engineering. Here, we introduce
universal diffractive waveguide designs that can match the performance of any
conventional dielectric waveguide and achieve various functionalities.
Optimized using deep learning, our diffractive waveguide designs can be
cascaded to each other to form any desired length and are comprised of
transmissive diffractive surfaces that permit the propagation of desired guided
modes with low loss and high mode purity. In addition to guiding the targeted
modes along the propagation direction through cascaded diffractive units, we
also developed various waveguide components and introduced bent diffractive
waveguides, rotating the direction of mode propagation, as well as programmable
mode filtering and mode splitting diffractive waveguide designs, showcasing the
versatility of this platform. This diffractive waveguide framework was
experimentally validated in the terahertz (THz) spectrum using 3D-printed
diffractive layers to selectively pass certain spatial modes while rejecting
others. Diffractive waveguides can be scaled to operate at different
wavelengths, including visible and infrared parts of the spectrum, without the
need for material dispersion engineering, providing an alternative to
traditional waveguide components. This advancement will have potential
applications in telecommunications, imaging, sensing and spectroscopy, among
others.
