Efficient coupling of long‐wavelength infrared (LWIR) radiation to a two‐dimensional (2‐D) array of GaAs/AlGaAs multiple quantum well detectors is achieved by illumination through chemically etched diffraction gratings. Gratings were fabricated on the back surface of the GaAs substrate as well as selectively on the top contact of the detector mesas. Both top and bottom illumination schemes were employed. In all cases, high coupling efficiency (>90%) of the gratings was observed as measured by comparing the responsivity to that of an identical detector illuminated through an angle‐polished facet. The results demonstrate the feasibility of high‐sensitivity GaAs LWIR imagers.