  We computationally study a subwavelength dielectric grating structure, show
that slab waveguide modes can be used to obtain broadband high reflectivity,
and analyze how slab waveguide modes influence reflection. A structure showing
interference between Fabry-Perot modes, slab waveguide modes, and waveguide
array modes is designed with ultra-broadband high reflectivity. Owing to the
coupling of guided modes, the region with reflectivity R > 0.99 has an
ultra-high bandwidth ( {\Delta}f/f > 30%). The incident-angle region with R >
0.99 extends over a range greater than 40{\deg}. Moreover, an asymmetric
waveguide structure is studied using a semiconductor substrate.
