In photonics, a meta-waveguide is a physical structures that guides electromagnetic waves with engineered functional subwavelength structures. Meta-waveguides are the result of combining the fields of metamaterials and metasurfaces into integrated optics. The design of the subwavelength architecture allows exotic waveguiding phenomena to be explored. Meta-waveguides can be classified by waveguide platforms or by design methods. If classified by underlying waveguide platform, engineered subwavelength structures can be classified in combination with dielectric waveguides, optical fibers, or plasmonic waveguides. If classified by design methods, meta-waveguides can be classified as either using design primarily by physical intuition, or by computer algorithm based inverse design methods. Meta-waveguides can provide new degrees of design freedom to the available structural library for optical waveguides in integrated photonics. Advantages can include enhancing the performance of conventional waveguide based integrated optical devices and creating novel device functionalities. Applications of meta-waveguides include beam/polarization splitting, integrated waveguide mode converters, versatile waveguide couplers, lab-on-fiber sensing, nano-optic endoscope imaging, on-chip wavefront shaping, structured-light generations, and optical neural networks. The meta-structures can also be further integrated with van der Waals materials to add more functionalities and reconfigurability.
Nonlinear optical applications Meta-waveguides can be used to enhance or control nonlinear optical interactions by combining subwavelength field engineering with the optical confinement provided by a waveguide. In nonlinear frequency-conversion processes, efficient interaction commonly requires conservation of optical momentum through phase matching. Patterned meta-structures can provide additional momentum, alter modal symmetry, or control the spatial overlap between interacting modes, enabling nonlinear conversion and manipulation within integrated waveguides. One implementation used a gradient metasurface composed of dielectric optical antennas on a thin-film lithium niobate waveguide. The metasurface supplied additional optical momentum and enabled second-harmonic generation over multiple coherent lengths without conventional birefringent or periodically poled phase matching. Grating metasurfaces patterned on lithium-niobate slab waveguides have also been proposed for simultaneous second-harmonic generation and control of the generated wavefront, including focusing and Airy-beam generation. Meta-waveguides have also been investigated for intensity-dependent optical processing. A 2024 experiment integrated arrays of titanium–gold split-ring resonators with a silicon waveguide. The resonant structure produced a slow-light response that enhanced two-photon absorption, with a reported effective absorption coefficient of 424 cm/GW, approximately 1,200 times the value used for an unmodified silicon waveguide. The measured nonlinear transmission was evaluated as an all-optical activation function in a handwritten-character recognition model, which achieved an inference accuracy of 98.36%.
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