A focal plane array (FPA) imaging sensor includes an array of multiple tuning cells disposed on a monolithic substrate, in which each tuning cell includes: (a) a tunable etalon disposed in the monolithic substrate, and (b) a THz detector assembly comprised of at least two antennae disposed in the monolithic substrate. The THz detector assembly is disposed below the tunable etalon. Each tunable etalon includes a Fabry-Pérot interferometer comprised of upper and lower mirrors, spaced by a gap between the mirrors, the gap forming an etalon of the interferometer. The FPA imaging sensor may also be formed as a hybrid, in which portions of the tuning cells are fabricated separately from the THz detector assembly and aligned to each other. The lower mirrors may also be fabricated monolithically with the substrate and the upper mirrors may be formed separately and then aligned to the lower mirrors.