Enhanced anti-crossing in resonant reflection via structured waveguide gratings

Gratings with structured periods allow for a controlled ratio between the strength of optical coupling in different diffraction orders. A structured waveguide grating may produce relatively weak coupling in the first diffraction order leading to sharp mode excitation resonances, while the second order coupling can be strong, resulting in a large anti-crossing that can exceed the spectral width of the mode excitation resonances by an order of magnitude or more. This dramatic revelation of strong coupling turns structured waveguide gratings into a platform for studying strongly coupled systems. Here, we describe the concept of structured period gratings, illustrate it with numerical simulations of resonant reflection, and provide experimental verification using a GaN/sapphire system operating in the mid-IR spectral range.