An ultrasensitive optical detector with high resolution in time, using a waveguide, and a processes for manufacturing this detector. The detector is configured to detect at least one photon and includes a dielectric substrate and at least one detection element on the substrate, configured to generate an electrical signal starting from energy of the photon received, and a guide element to guide the photon, the energy of which is then absorbed by the detection element at an absorption zone which is less than 100 nm thick. The detection element is substantially straight on the substrate and is short, and the guide element includes a single mode light waveguide with strong confinement, placed on the detection element. The detector is particularly applicable to detection and localization of operating defects in a semiconducting circuit.