  Using Fano-type guided resonances (GRs) in photonic crystal (PhC) slab
structures, we numerically and experimentally demonstrate optical reflectivity
enhancement of high-Q SiNx membrane-type resonators used in
membrane-in-the-middle optomechanical (OM) systems. Normal-incidence
transmission and mechanical ringdown measurements of 50-nm-thick PhC membranes
demonstrate GRs near 1064 nm, leading to a ~ 4\times increase in reflectivity
while preserving high mechanical Q factors of up to ~ 5 \times 10^6. The
results would allow improvement of membrane-in-the-middle OM systems by virtue
of increased OM coupling, presenting a path towards ground state cooling of
such a membrane and observations of related quantum effects.
