INSTRUMENTATION
Advisory Panel Jonathan W . Amy Glenn L. Booman Robert L. Bowman
Jack W . Frazer G. Phillip Hicks Donald R. Johnson
Howard V. Malmstadt Marvin Margoshes William F. Ulrich
Total Internal Reflection Enhancement of Photodetector Performance Photocathode quantum efficiencies may be increased by optical modification of existing photomultipliers.
Total internal reflection cathodes offer the greatest promise for enhancing
the photodetector performance of modern absorption and emission instruments TOMAS HIRSCHFELD Block Engineering, Inc., 19 Blackstone St., Cambridge, Mass. 0 2 1 3 9
RECENT
DEVELOPMENTS in electrooptical technology, involving breakthroughs in light sources, optical components, electronics, and data handling, have gradually created a situation where photodetector quantum efficiencies place a ceiling on the performance of electrooptical instruments. I n analytical chemistry this is particularly obvious for fields such as Raman spectroscopy, luminescence spectroscopy, or the now emerging techniques of remote optical analysis, originated by space and environmental technology. However, the limitation can also be felt in more established fields, such as emission and absorption spectroscopy.
Among photon detectors today, the photomultiplier tube dominates the field, its near perfect amplification further optimized by high gain dynode stages, cooling, and photon counting within a narrow pulse energy window. But, before this stage is reached, a photoemissive cathode must be used to transform as many photons as possible into emitted photoelectrons. And here the bottleneck lies, for even the best commercial photocathodes are easily the least efficient component of most optical systems. The reason for this can be seen by observing the two types of photocathodes in use today (Figure 1). The transmission photocathode (superior from an electron optical point of view) must be simultaneously thick enough to absorb most of the incident light, and thin enough so that the generated photoelectrons can traverse it while re-
Figure 1 .
taining enough energy to overcome the work function barrier at the vacuum interface. Similarly, in the reflection photocathode, those photons that are absorbed too deeply within the material will generate photoelectrons that can no longer get to its surface and be emitted. For optimum efficiency a photocathode material must have the highest possible absorption coefficient for light and the lowest possible energy absorption coefficient for photoelectrons. The material must also have a low work function to extend its spectral coverage to longer wavelengths. Further demands concern the chemical and electrical properties of the cathode, as well as its suitability for production in a more or less controllable fashion. It is, therefore, not surprising that research on improving photocathodes became centered on improving photo-
Photocathode structures
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970 · 87 A
Instrumentation
Wavelength, A Figure 2. Envelope of commercial photocathode quantum efficiencies
cathode materials (1), leading to several decades of incremental improvements through very hard work. The final results of this effort were the extremely sophisticated, expensive, and temperamental semiconductor photocathodes of today, whose peak quantum efficiences of around 30% help us to live with their eccentricities and individualities. However, these efficiencies are available only in a small fraction of the spectrum, below about 5000 Â, and decay rapidly towards the red, becoming exceedingly small in the infrared before failing altogether near 1.2 microns (Figure 2). This is owing to the seemingly universal tendency of photocathode materials to become more transparent at longer wavelengths, where, on the other hand, lower photoelectron energies mean reduced electron range within the cathode layer. Even these performances require using several different photocathodes, each of which is optimal only over a small fraction of the total range.
Figure 3.
Interference photocathodes
The situation clearly called for a now approach, and this was suggested in a 1957 paper by Deutscher (β), where he proposed improving the absorption of light in a photocathode by the tech niques of physical optics, for a given material and layer thickness. The method he chose for this pur pose consisted of coating the photocathode on top of a dielectric spacer layer deposited on a mirror (Figure 3), the hole forming an optical cavity whose reflectivity could be made arbi trarily small by interference at prese lected wavelengths. These are fixed for a given tube by the thickness of the spacer layer. This approach pro vided gains in quantum efficiency that increased toward longer wavelengths as the photocathode became more trans parent, up to a maximum factor of three to five. Unfortunately, this gain was available only over a range of 300 to 500 Â on either side of the design wavelength, outside of which the quantum efficiency was actually lower than for a normal photocathode. Along these same lines, Love and Sizelove (3, Jf) studied photocathode behavior through a mathematical model in an effort to analyze the behavior of reflection, transmission, and intereference photocathodes. Their equations allow prediction of the relative performances of these photocathodes, as well as optimum photocathode and spacer layer thicknesses for them, with a fairly good accuracy. To the extent that the layer thicknesses had not been arrived at empirically, the results developed by this group allowed some further gains in quantum efficiency to be achieved. But it was a third physical optics approach, introduced by Rambo in 1964 (
