Editing Photocathode (section)

==Construction==
Photocathodes operate in a vacuum, so their design parallels [[vacuum tube]] technology. Since
most cathodes are sensitive to air the construction of photocathodes typically occurs after the enclosure has been evacuated. In operation the photocathode requires an [[electric field]] with a nearby positive anode to assure electron emission. [[Molecular beam epitaxy]] is broadly applied in today's manufacturing of photocathode. By using a substrate with matched lattice parameters, crystalline photocathodes can be made and electron beams can come out from the same position in lattice's [[Brillouin zone]] to get high [[Beam emittance|brightness]] electron beams.

Photocathodes divide into two broad groups; transmission and reflective. A transmission type is typically a coating upon a glass window in which the light strikes one surface and electrons exit from the opposite surface. A reflective type is typically formed on an opaque metal electrode base, where the light enters and the electrons exit from the same side. A variation is the double reflection type, where the metal base is mirror-like, causing light that passed through the photocathode without causing emission to be bounced back for a second try. This mimics the [[Tapetum lucidum|retina]] on many mammals.

The effectiveness of a photocathode is commonly expressed as quantum efficiency, that being the ratio of emitted electrons vs. impinging quanta (of light). The efficiency varies with construction as well, as it can be improved with a stronger electric field.