Self-Adaptive Waveguide Boundary for Inter-Mode Four-Wave Mixing

We propose a universal approach to provide wide-band multimode four-wave mixing independently of the intrinsic dispersion of the involved waveguide modes. Concepts from quantum mechanics and subwavelength engineering are adopted to design an effective lateral confinement photonic well, i.e., with a graded potential along the waveguide cross section providing flexible control over the modes' confinement. The self-adaptive nature of the waveguide boundary allows different spatial modes with equi-spaced frequencies and shared propagation wavevector, thus automatically fulfilling both energy conservation and wavevector phase matching conditions. Capitalizing on this concept, we show phase-matching among modes separated by 400 nm (bridging from telecom wavelengths to almost 2 mu m), with less than 5% deviation in a remarkably large bandwidth exceeding 300 nm. Furthermore, we also show the flexibility of the proposed approach that can be seamlessly adapted to different technology platforms. This strategy opens a new design space for versatile on-chip nonlinear applications in which the manipulation of energy spacing and phase matching is pivotal, e.g., all-optical signal processing with four-wave mixing, mid-infrared supercontinuum or frequency comb light generation, or Brillouin scattering with selectable phonon energy.