An exemplary refractive-index sensor includes a photonic crystal microcavity structure, a light source, and a detector. The photonic crystal microcavity structure includes a photonic crystal layer having first holes and a second hole defined therein. The first holes are arranged in a regular pattern of staggered parallel rows. The second hole is at an approximate center of the regular pattern, instead of a first hole. A diameter of the second hole is different from that of the first holes. The first holes at each of opposite ends of the row having the second hole are omitted, thereby defining an input waveguide and an output waveguide. The light source is adjacent to the input waveguide. The detector is adjacent to the output waveguide.