Analysis of interference photocathodes

The efficiency of photodetection and guidelines for its improvement can be determined by an analysis of the photodetection process. In this paper, the effects of interference within a reflecting-translucent photocathode are studied. Equations to describe the efficiency of generating photoelectrons by optical absorption of narrow-band light are developed, analyzed, and solved numerically using the IBM 7094 computer. The collection efficiency, the quantum yield, and the percent absorption have been evaluated as functions of 1) the thickness of the emitter, 2) the absorption coefficient, 3) the incident wavelength, 4) the electron diffusion depth, and 5) the refractive index of the photodetection medium. Graphs have been drawn which relate the variance of these three functions with the incident wavelength at constant emitter thickness and with the emitter thickness at constant wavelength, respectively. Minima and maxima, corresponding to destructive and constructive interference, occur alternately at emitter optical thicknesses of λ/4, λ/2, 3λ/4, and λ, respectively. It is found that only when the absorption coefficient is low (≤ .0013/Å) will interference phenomena enhance the quantum yield above that obtained due to absorption for reasonable thickness.