Photocathodes

Photocathodes are electron-emitting surfaces that release electrons when illuminated, based on the photoelectric effect. They are typically operated in high vacuum and serve as the electron source in devices such as photomultiplier tubes, image sensors, night-vision apparatus, X-ray detectors, and accelerator injectors.

Principle: When photons with sufficient energy are absorbed, electrons near the surface gain enough energy to overcome the surface barrier and escape into the vacuum. In metals the barrier is the work function; in semiconductors it is the bandgap and surface electron affinity. Semiconductor photocathodes can be engineered to have negative electron affinity, via surface activation, which lowers the escape barrier and increases quantum efficiency.

Types: Metal photocathodes (for example Cs-doped Ag-O-Cs or similar cesium-activated layers) have long history in vacuum tubes. Alkali-antimonide and related compound photocathodes (such as K2CsSb or Cs3Sb) offer higher QE in the visible range. Semiconductor photocathodes such as GaAs and GaAsP are commonly activated with cesium and oxygen to create a negative electron affinity surface, yielding high QE at near-visible to near-infrared wavelengths. There are transmission-mode photocathodes, where light enters from the opposite side to the emission layer, and reflection-mode photocathodes, where illumination and emission occur from the same side.

Performance: Quantum efficiency (QE) measures emitted electrons per incident photon and depends on material, wavelength, and surface treatment. Metal and alkali-antimonides typically range from 10^-4 to a few percent, while NEA GaAs-based photocathodes can achieve higher QE in appropriate spectral regions. Longevity is limited by vacuum quality and exposure to contaminants; aging or contamination reduces QE and may require re-activation with cesium deposition.

Applications: Photocathodes enable fast, sensitive light-to-electron conversion in photomultiplier tubes, linac photoinjectors, streak cameras, X-ray detectors, and imaging sensors.