Designs for waveguide and structured photocathodes with high quantum efficiency

Abstract

Conventional S20 multialkali photocathodes have a wide wavelength coverage from < 200 to > 850 nm, but their high transparency and the surface work function result in low quantum efficiencies at longer wavelengths. Theoretical modelling of the photon and excited electron interactions that define the cathode performance provides a realistic prediction of the measured response. The theory emphasizes that the basic light absorption is strongly sensitive to the cathode thickness, wavelength, polarization and incident angle. Parameters can be selected which predict that even at long wavelengths (e.g. 900 nm), absorption may be increased from ∼1% to ∼100%. Cathode topographies can be designed to exploit these responses and offer increased absorption at the longer wavelengths. Alternative designs, which include waveguiding of light within the cathode window, or in structured surfaces, can similarly lead to almost total absorption of the incident light by increasing the number of interactions. These concepts of optimal incidence and waveguiding have been both theoretically modelled and demonstrated in newly fabricated cathode designs. The methods have variously reached quantum efficiencies in excess of 50% at wavelengths in the range from 200 to > 750 nm under different operational conditions. The improvement factors relative to normal incidence on planar cathodes increase for longer wavelengths, and examples of 20–50 times by ∼900 nm were noted. Whilst the absolute S20 efficiency values at long wavelengths are still small, the improvements offer a usable sensitivity even beyond 1 µm, as demonstrated by spectroscopy data up to at least 1140 nm.

Enhancement of photomultiplier sensitivity with anti-reflective layers

“…There are two standard bialkali photocathode compositions: KCsSb and RbCsSb, and the dispersive properties of these two bialkali compositions have been well documented [8,9] and the data obtained from measurements by Motta and Schonert [8] are used here. The constants for the trialkali cathode (S20) have been documented by Hallensleben et al [2,3,10]. Selection of the thickness of the zirconia layer used for impedance matching is of great importance as can be seen from the enhancement factor surfaces plotted in figure 3.…”

“…In these applications semiconductor photodiode devices have still not displaced the venerable PMT and considerable advancement in photodiode technology will be required before this occurs. However, 3 Author to whom any correspondence should be addressed.…”

“…These techniques rely on controlling the angle that the incident light impinges upon the PMT window using additional optical elements, resulting in higher absorptance values than are achievable with normal or near normally incident light [1,2]. Other methods propose the use of multiply reflecting structures, such as pyramids or cones, fabricated on the inside of the PMT window to reduce reflectance and enhance absorptance, or waveguiding within the tube window to enhance the overall absorption efficiency [3][4][5]. Such techniques can offer significant increases in sensitivity, but so far they have not been commercialized.…”

Unexploited Information from Luminescence Spectra

“…Fig.2 sketches one such arrangement and improvements on a standard S20 (trialkali) photocathode. An even greater performance enhancement accrues by any coupling that causes the light to waveguide across the cathode surface within the window (or support layer) [4]. Here there are multiple interactions at steep angles and so, even for the longest wavelength red light, absorption approaches 100%.…”

Common mistakes in luminescence analysis

“…Hence a spectral scan from short to long wavelengths can differ from scans made in the reverse direction. In recent years various ways of coupling the light to the PM tube have offered enhanced red performance [10][11][12][13]. Some of these same enhancements can be achieved with S20 cathode photon imaging tubes as are used in spectrometer systems.…”