Multiple mode couplings in waveguide array for broadband near-zero dispersion and supercontinuum generation

Photonic chips can enhance the light-matter interaction by confining light within a submicron-scale waveguide cross-section and have advanced many nonlinear optical processes, such as supercontinuum and Kerr frequency comb generations. These nonlinear frequency conversion processes rely on the phase-matching of an optical system, which can be achieved by engineering the dispersion of the photonic waveguides. A typical dispersion engineering method, however, is bounded by the material dispersions, limiting the spectral bandwidth and efficiency of many nonlinear processes. Moreover, it causes certain fabrication challenges; for example, a thick silicon nitride film is required for anomalous dispersion, which is easy to crack due to high film-stress and incompatible with the complementary metal-oxide-semiconductor (CMOS) foundry. Thus, to develop fully CMOS compatible on-chip photonic devices, further advanced dispersion engineering is required. This work presents a coupled waveguide array scheme that introduces multiple mode couplings at different wavelengths and achieves a near-zero dispersion profile at near-infrared. A broadband near-zero dispersion profile, spanning over the spectral range of 1350-1900 nm within ±140 ps/nm/km, is obtained on a 300 nm-thick silicon nitride film, which is compatible with the CMOS process but shows high normal dispersion unless otherwise engineered. Using this engineered dispersion profile, an octave-spanning supercontinuum is numerically generated by pumping near the spectral regions of the mode couplings. The concept of utilizing multiple mode couplings to achieve broadband near-zero dispersion can be applied to other spectral regimes and material platforms as well, by engineering the structural parameters to shift the positions of the zero-dispersion-wavelengths. This CMOS compatible waveguide array design would allow mass production in supercontinuum generating chips, which can then be used in molecular sensing and spectroscopic applications.