Resolution characteristic of graded band-gap AlGaAs/GaAs transmission-mode photocathodes

Abstract

The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs transmission-mode photocathodes is numerically solved from the two-dimensional continuity equations. According to the MTF model, we calculate the theoretical MTF of graded band-gap and uniform band-gap transmission-mode photocathodes, and analyze the effects of Al composition, wavelength of incident photon, and thickness values of AlGaAs and GaAs layer on the resolution. The calculated results show that compared with the uniform band-gap photocathode, the graded band-gap structure can increase the resolution of photocathode evidently. If the spatial frequency f ranges from 100 to 500 lp·mm-1, the increase of resolution is more pronounced. Let f=200 lp·mm-1, the resolution of graded band-gap photocathode generally increases 150%-260%. The resolution improvement of graded band-gap photocathode is attributed to the built-in electric field. While too high built-in electric field will influence the spectral response of long-wavelength photons due to higher Al composition in the AlGaAs/GaAs photocathodes.