  Dispersion engineering is among the most important steps towards a promising
optical frequency comb. We propose a new and general approach to trim frequency
combs using a self-adaptive boundary of the optical mode at different
wavelengths in a sub-wavelength structured waveguide. The feasibility of
ultra-wide bandwidth dispersion engineering comes from the fact that light at
different wavelengths automatically self-adapts to slightly different effective
spatial spans determined by the effective indices of the mode. Using this
self-adaptive variation on the confinement, we open up the window of
low-anomalous dispersion in a large wavelength range, and theoretically
demonstrate frequency combs with improved bandwidths with respect to the
state-of-art in several different waveguide configurations considered, for a
matter of illustration, in the silicon photonic platform. This strategy opens
up a new design space for trimming the spectrum of frequency combs using
high-index-contrast platforms and provides benefit to various versatile
nonlinear applications in which the manipulation of energy spacing and phase
matching are pivotal.
