Optical means for enhancing the sensitivity of a tri-alkali photocathode

With conventional transmissive photocathodes, an appreciable amount of the incident light passes through the photocathode layer without contributing to the electron emission. It is possible, at certain predetermined wavelengths, to reduce these light losses and thereby to increase the quantum efficiency of the cathode by making it part of a reflection interference filter. In practice, this involves the deposition of the photocathode on a dielectric which is, in turn, deposited on an opaque mirror. The thin-film optics of the above arrangement has been considered theoretically, and the possible increase in light absorption, and hence in electron emission, could be predicted. Evaluation of actual tri-alkali (S-20) photocathodes constructed along the above lines agree with, and at times even exceed, the theoretical enhancement factor of about three to five, thus establishing the practicability of the method considered.