Photocathode

A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron beams. Electron beams generated with photocathodes are commonly used for free electron lasers and for ultrafast electron diffraction. Photocathodes are also commonly used as the negatively charged electrode in a light detection device such as a photomultiplier, phototube and image intensifier.

Quantum efficiency is a unitless number that measures the sensitivity of the photocathode to light. It is the ratio of the number of electrons emitted to the number of incident photons. This property depends on the wavelength of light being used to illuminate the photocathode. For many applications, QE is the most important property as the photocathodes are used solely for converting photons into an electrical signal.

Quantum efficiency may be calculated from photocurrent (${\displaystyle I}$), laser power (${\displaystyle P_{\text{laser}}}$), and either the photon energy (${\displaystyle E_{\text{photon}}}$) or laser wavelength (${\displaystyle \lambda _{\text{laser}}}$) using the following equation.

$${\displaystyle {\text{QE}}={\frac {N_{\text{electron}}}{N_{\text{photon}}}}={\frac {I\cdot E_{\text{photon}}}{P_{\text{laser}}\cdot e}}\approx {\frac {{\overset {[{\text{amps}}]}{I}}\cdot 1240}{{\underset {[{\text{watts}}]}{P_{\text{laser}}}}\cdot {\underset {[{\text{nm}}]}{\lambda _{\text{laser}}}}}}}$$

For some applications, the initial momentum distribution of emitted electrons is important and the mean transverse energy (MTE) and thermal emittance are popular metrics for this. The MTE is the variance of the transverse momentum in a direction along the photocathode's surface and is most commonly reported in units of milli-electron volts.

$${\displaystyle {\text{MTE}}={\frac {\langle p_{\perp }^{2}\rangle }{2m_{e}}}}$$

In high brightness photoinjectors, the MTE helps to determine the initial emittance of the beam which is the area in phase space occupied by the electrons. The emittance (${\displaystyle \varepsilon }$) can be calculated from MTE and the laser spot size on the photocathode (${\displaystyle \sigma _{x}}$) using the following equation.

$${\displaystyle \varepsilon =\sigma _{x}{\sqrt {\frac {\text{MTE}}{m_{e}c^{2}}}}}$$