Photodetector incorporating: • at least one active area for detecting optical radiation and • one structure, positioned on the said active area, designed for optically coupling of infrared radiation, characterized by the fact that the said structure combines the properties of coupling gratings and Bragg gratings, and has at least one defect which, from an incident wave, makes it possible to obtain a plasmon wave which is both coupled and localized in the active area, with the said active area having dimensions that are small in comparison to those of the said structure; • with the said active area being of small dimensions in relation to those of the aforesaid structure, and with the said active area being elongated in form; • with the said structure being composed of mutually-parallel grooves parallel to the length of the said active area; • with the structure being produced in a layer of at least one dielectric material transparent in the spectral range of the infrared radiation, with the said layer incorporating one essentially-flat lower face in contact with the active area, and one upper face incorporating the grooves of the structure, with the said upper face being covered with a metal film, with the optical thickness of the said layer separating the lower face from the upper face along an axis (Ox) perpendicular to the direction of the grooves, and parallel to the plane of the lower face varying with the superimposition of at least one first so-called coupling grating, of which the pitch is at a first spatial frequency, and a second so-called localization grating, of which the pitch is at a second spatial frequency that is twice the first spatial frequency and incorporates a first central defect. • at least one active area for detecting optical radiation and • one structure, positioned on the said active area, designed for optically coupling of infrared radiation, characterized by the fact that the said structure combines the properties of coupling gratings and Bragg gratings, and has at least one defect which, from an incident wave, makes it possible to obtain a plasmon wave which is both coupled and localized in the active area, with the said active area having dimensions that are small in comparison to those of the said structure; • with the said active area being of small dimensions in relation to those of the aforesaid structure, and with the said active area being elongated in form; • with the said structure being composed of mutually-parallel grooves parallel to the length of the said active area; • with the structure being produced in a layer of at least one dielectric material transparent in the spectral range of the infrared radiation, with the said layer incorporating one essentially-flat lower face in contact with the active area, and one upper face incorporating the grooves of the structure, with the said upper face being covered with a metal film, with the optical thickness of the said layer separating the lower face from the upper face along an axis (Ox) perpendicular to the direction of the grooves, and parallel to the plane of the lower face varying with the superimposition of at least one first so-called coupling grating, of which the pitch is at a first spatial frequency, and a second so-called localization grating, of which the pitch is at a second spatial frequency that is twice the first spatial frequency and incorporates a first central defect.