A fuel injector for use with an internal combustion engine. The IEDM includes a bellmouth coupled to a forward end of the IEDM and an exhaust pipe coupled to an aft end of the IEDM. When light to be detected is made incident and photoelectrons occur in the light absorption layer 1, the photoelectrons are accelerated to the plane of emission by the electric field formed in the light absorption layer 1, and emitted to the outside of the transmission type photocathode.