Ultra-narrowband perfect absorbers and emitters are proposed and realized by engineering multiple-beam interference in Gires-Tournois etalon with the presence of low metallic loss. The absorption mechanism and spectral characteristics of the Gires-Tournois resonators are numerically and experimentally investigated for three configurations: dielectric cavity on metal, metal-dielectric-metal resonator, and distributed Bragg reflector (DBR)-dielectric-metal resonator. Narrowband thermal emitters based on the metal-dielectric-metal cavity and (DBR)-dielectric-metal cavity are experimentally demonstrated with an emissivity of 0.8 and 0.82, and a quality factor of 21 and 85, respectively. A DBR-dielectric-metal resonator-based absorber is directly loaded onto a LiTaO 3 film for the first time to constitute an on-chip ultra-narrowband pyroelectric detector with an excellent quality factor of 151 at the absorption band of methane.