fotokatodi

Fotokatodi, or photocathodes, are electron-emitting surfaces that release electrons when illuminated by light. They form the electron source in many vacuum-electron devices, including photomultiplier tubes, image intensifiers, and electron guns for accelerators and microscopes. The emission relies on the photoelectric effect: when photons with energy above the material’s work function are absorbed, electrons gain enough energy to overcome the surface barrier and are emitted into vacuum. The efficiency of conversion from photons to emitted electrons is described by the quantum efficiency (QE), which depends on material, surface treatment, and wavelength.

Photocathodes are made from a range of materials chosen for spectral response and lifetime. Common room-temperature materials include alkali-antimonide compounds such as Cs3Sb and Cs2Te, which respond in the visible region. For infrared sensitivity, GaAs and its variants (often with negative electron affinity activation) are used. Metal photocathodes, such as Cs- or Ag-based coatings, are more durable but typically have lower QE. The surface is prepared and activated in ultra-high vacuum, sometimes with cesium and oxygen to tune the work function and achieve NEA activation for high QE.

Performance is a balance between spectral range, QE, dark current, and lifetime. QE can range from a fraction of a percent to several tens of percent, depending on wavelength and device design. Lifetimes are limited by contamination, vacuum quality, and current loading.

Applications include light detection and imaging in photomultiplier tubes, night-vision devices, scientific instrumentation (time-resolved electron spectroscopy, electron microscopes), and accelerator-based electron sources. The development of fotokatodi has evolved with materials science to optimize response, speed, and durability.