Single-material coupling-tolerant semi-planar microresonator using Littrow diffraction

Resonators carved in a single material such as disc, rings, spheres, do not require genuine mirrors since they exploit shape resonances, but they then suffer from poor coupling to standard free-space collimated beams. Exploiting the idea of Littrow resonators combining total internal reflection and grating diffraction, we propose trapezoidal prism-type geometries that include a strong grating on one face, and total internal reflection on the other face. Resonant modes are localized within a length of the same order as the prism thickness, but are well coupled to free-space beams. The resonator core can be seen as a broad waveguide with multimode coupling of “Littrow modes”, a configuration recently pointed out for its capability to produce a “collective” slow-light regime. A more “planar” version, with a saw-tooth input-coupling grating for vertically collimated beam is also proposed.