Enhancement of photomultiplier sensitivity with anti-reflective layers

Photomultipliers are fast, sensitive and low noise light detectors which operate across the ultraviolet–optical–near infra red region of the electromagnetic spectrum. Sensitivity is determined by the composition of the photocathode layer in which incident photons excite photoelectrons and the thickness of this layer. Incident light is partially reflected from and partially transmitted through the photocathode layer, which is typically ∼20 nm thick, and this energy is unavailable for photoelectron excitation, limiting sensitivity. Typical reflectance and transmittance values at 500 nm are 21% and 33%, respectively, for KCsSb; 27% and 24% for RbCsSb and 36% and 35% for Na2SbK : Cs. These substantial losses can be reduced by the addition of one or more transparent impedance matching layers between the photomultiplier tube (PMT) window and the photocathode, resulting in enhanced sensitivity without effecting the PMT geometry, photocathode deposition process or altering the acceptance angle of the photomultiplier. The impedance matching serves to reduce reflectance losses, increasing cathode absorptance. By using published measurements of the dispersive properties of bialkali and trialkali (S20) photocathode compositions, accurate modelling of the electromagnetic field distribution within the photocathode layer is possible. This model facilitates the prediction of the enhancement of sensitivity obtainable with an anti-reflective layer of zirconium dioxide (zirconia). The authors present the enhancement factors possible with standard bialkali and trialkali photocathode compositions using the material zirconia for impedance matching. Enhancement factors up to 27% and 38% are predicted for KCsSb and RbCsSb photocathodes, respectively, while an enhancement factor of up to 44% is predicted for the S20 photocathode.