Abstract: A photodetector comprises a substrate, and supported by the substrate, a configuration to act as optical resonator and to absorb incident radiation of a band, including infrared. The configuration comprises: a resonant frontside structure facing the incident radiation; a backside structure and arranged between the frontside structure and the substrate; and a layer of an active material made from a semiconducting material, and configured to convert at least part of the incident radiation of the band into charge carriers. The frontside structure or the backside structure is made from electrically conducting material and is in contact with the active material. The configuration is configured to selectively absorb the incident radiation of the band. The frontside structure or the backside structure that is in contact with the active material is contacted by electrical contacts for sensing the charge carriers in the active material. The active material comprises amorphous or polycrystalline material.