In a general aspect, vapor cells are disclosed that include a dielectric body having a first surface and a second surface. The dielectric body includes a plurality of cavities extending from the first surface to the second surface and ordered periodically to define a photonic crystal structure in the dielectric body. Each cavity has a first opening defined by the first surface and a second opening defined by the second surface. The photonic crystal structure has a photonic band gap. The vapor cells additionally include a first optical window covering the first openings and having a surface bonded to the first surface of the dielectric body to form a seal around each of the first openings. A second optical window covers the second openings and has a surface bonded to the second surface of the dielectric body to form a seal around each of the second openings.