fotokathoder

Fotokathoder, commonly called photocathodes, are electron-emitting surfaces used in vacuum devices and photodetectors. They convert light into a current of electrons via the photoelectric effect: photons with energy above the material's work function liberate electrons from the surface into vacuum. The performance is described by the quantum efficiency, the fraction of incident photons that yield emitted electrons, which varies with wavelength, surface condition, and material choice.

Materials span metal, semiconductor, and compound categories. Metal photocathodes, including cesium, potassium, and Ag-O-Cs alloys, offer robustness and broad spectral response but often lower efficiency in certain bands. Semiconductor and compound photocathodes, such as GaAs-based structures activated with cesium and oxygen, can achieve higher quantum efficiency in the visible and near-infrared, yet require strict ultra-high vacuum and careful surface preparation to maintain performance. Some approaches employ negative electron affinity surfaces to lower the emission barrier.

Applications include photomultiplier tubes, image intensifiers, and other light-detection systems that amplify weak signals. They are also used as electron sources in accelerators and vacuum electronics. Key challenges include maintaining vacuum integrity, surface contamination, and long-term degradation of quantum efficiency under illumination and aging.

Overall, fotokathoder remain essential for converting light into a controllable electron signal, with material choices balancing spectral coverage, efficiency, and stability.