A high time-resolution ultrasensitive optical detector, using a planar waveguide leakage mode, and methods for making the detector. The detector includes a stacking with a dielectric substrate, a detection element, first and second dielectric layers, and a dielectric superstrate configured to send photon(s) into the light guide formed by the first layer. The thicknesses of the layers is chosen to enable a resonant coupling between the photon(s) and a leakage mode of the guide, the stacking having an absorption resonance linked to the leakage mode for a given polarization of the photon(s).