INSTRUMENTATION J o n a t h a n W. A m y Glenn L. B o o m a n R o b e r t L. B o w m a n
Jack W. F r a z e r G. Phillip H i c k s Donald R. J o h n s o n
H o w a r d V. M a l m s t a d t Marvin Margoshes W i l l i a m F. U l r i c h
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. 02139 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. In analytical chemistry this is particularly obvious for fields such as Raman spectroscopy, luminescence spectroscopy, or the now emerging techniques of remote optical analysis, originated b y space and environmental technology. However, the limitation can also be felt in more established fields, such as emission and absorption spectroscopy. ECEST
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-
taining enough energy to overcome the work function barrier a t 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 t o its surface and be emitted. For optimum efficiency a pliotocathode 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 lorn work function to extend its spectral coverage t o 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-
Figure 1. Photocathode structures ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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cathode materials ( I ) , lesding 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 quant u m 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 A, 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 Tvithin the cathode layer. Even these performances require using several different photocathodes, each of ir-hich is optimal only over a small fraction of the total range. Dielectric
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The situation clearly called for a new approach, and thia was suggested in a 195T paper by Deutscher (W),where he proposed improving the absorption of light in a photocathode b y the techniques of physical optics, for given material and layer thickness. The method he chose for this purpose consisted of coating the photocathode on top of a dielectric spacer layer deposited on a mirror (Figure 31, the hole forming an optical cavity whose reflectivity could be made arbitrarily small b y interference a t preselected wavelengths. These are fixed for a given tube b y the thickness of the spacer layer. This approach provided gains in quantum efficiency that increased toward longer wavelengths as the photocathode became more transparent, up to a maximum factor of three t o five. Unfortunately, this gain was available only over a range of 300 t o 500 A 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. 4) 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 la! er thicknesses for them, with a fairly good accuracy. T o the extent that the layer thicknesses had not been arrived a t empirically, the results developed b y this group allowed some further gains in quantum eficiency to be achieved. But it m s a third physical optics approach, introdured b y Rambo in 1964 ( 5 ), that achieved the most significant results so far, and became the most widely used technique for physical optical enhancement of photocathode performance. As shown in Figure 4, it was based on illuminating a transmission photocathode a t an incidence angle beyond the critical one for total reflecEntrance
tion for the window-vacuum interface. Any fraction of the photom that was not absorbed in their first passage through the photocathode was then bounced back and forth between both sides of t h e window until eventually absorption was complete. Independently rediscovered by Gunter and coworkers a t NASA Ames, the teclinique aroused considerable interest, and research efforts on it were started a t a number of institutions, notably the Electron Tube Division of ITT, the Electronic Components and Devices Group of RCA, Optics Technology, and Block Engineering. The technique in its initial form was applied in a more or less empirical fashion to photocathode layers of the S-11 (6),S-20 ( 7 ) , extended red S-PO (8))and S-1 types (9). The typical behavior of the quantum efficiency enhancement as a function of angle and of wavelength can be seen in Figures 5 and 6. The largest enhancement was usually found in the vicinity of the critical angle, and grew from a factor < 2 near the peak response wavelength t o as
20 40 60 80 Internal Reflection Angle of Incidence, Degrees Figure 5. Angular dependence of TIR enhancement of photocathode quantum efficiency (S-11 photocathode) (6)
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Figure 4. Total internal reflection (TIR) enhancement of photocathode quantum efficiency (5)
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Figure 6. TIR enhancement of photocathode quantum efficiency as function of wavelength (S-20 photocathode) (16)
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Various geometries used in TIR-enhanced photocathodes (16, 18, 19)
much as 7 to 9 near the long wavelength threshold of the material. The improvement is quite polarization sensit i r e (10). At about this time, another technique for enhancing photocathode quantum efficiences was developed, based on using a strong external electric field to reduce the photocathode vacuum potential barrier and increase the fraction of photoelectrons crossing it (11). While the enhancement is modest, it is multiplicative with the one obtained by T I R , allowing even larger efficiency gains (12. 13). An interesting variant of the technique wa; ite inadvertent application by Samson and Cairns ( I d ) , mho increased the quantum efficiency of a faruv metallic reflectire cathode by using multiple reflections a t grazing incidence inside a concave polygonal cathode. As the refractive index of metals is below unit>- in this spectral region, grazing incidence external reflection becomes identical I\-ith TIR for a high index material of unity index. As expected, their experimental results resemble those cited above. The phenomenon was later studied in detail by Pepper (15).
I n absolute terms, very high sensitivities ( 4 0 0 r l / Z m ) ( 7 ) and quantum
efficiencies (-40%) ( 1 6 ) have been realized through the application of TIR to photocathodes. However, the geometry of these systems was quite inconvenient, as their angular aperture \vas limited to an JIG cone and the entrance aperture vias a small fraction of the area of the hypotenuse face of the entrance prism. Improved apertures were first described b y Hirschfeld ( 1 7 ) , who used high index materials for the entrance prism For a sapphire prism of index 1.76, an f/1.2 cone could be used B u t even the optimum width of this entrance prism was of the order of the window thickness and too small for many applications. Optically contacting a glass disk t o the window increased its effective thickness, and thus that of the aperture, but only by reducing the nuniber of bounces across the tube diameter, and, therefore, lessening the enhancement. Other optical layouts giving higher apertures b y reducing the number of reflections to tjvo were proposed by Oke and Schild ( 1 8 ) (Figure 7a), Hirschfeld (19) (Figure 7b), and Gunter et al. ( 1 6 ) (Figure 7c). T h e largest apertures so far, however, resulted from a system ( 1 9 ) (Figure Sd) which combined the large aperture of a cone condenser-deflector with a thickened window, while maintaining a large
number of bounces by double-passing the beam along the windox radius. The aperture of such a system, in terms of the original window size, is shown in Figure 8. The system of Figure 7c mill remage a line on itself, allowing simultaneous TIR-enhanced measurement of a whole spectrum in an image intensifier ( 2 0 ) . With auxiliary optics (1.5, 21) even a two-dimensional image can be doublepassed in this way for viewing a t high quantum efficiencies with an intensifier. Still other geometries (15) have been developed to allow adding TIR enhancement to existing systems without displacing the detector. V i t h the primary goal of high quantum efficiencies within reach, efforts were made to analyze the enhancement numericallg , in order to optimize it. The advantages expected from this were manyfold : (1) Reducing the number of bounces required for a given enhancement improves the geometry, particularly for imaging applications. ( 2 ) Higher enhancements still are needed in the red and infrared region for most photocathodes. (3) For a large enough enhancement, a simultaneously thin and iiot yery opaque cathode would be adequate. This removes both the optical and the electronic transmission behamor constraints on the photocathode material, and nidens the range of compounds that may be used, allowing one to hope that a material may be found (18) permitting operation beyond the 1 %micron long u arelength limit reached more than a generation ago .inother hope is to find somewhat more tract-
Internal Reflection Angle of Incidence, Degrees Figure 8. Relative aperture of cone entrance for TIR-enhanced photomultiplier for 4 reflections with f/5.5 beam as function of incidence angle (19)
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Figure 9. Absorption by very thin film of S-20 material in single total internal reflection at 60" incidence at 5000 A as as function of thickness (21)
able materials than those in use today. (4) Surface, low-level defect state, and phonon-aided photoemitters have never been practical because of low absorptions into these states, a problein that large enhancements might circumvent. ( 5 ) h photocathode one or a few atomic layers thick often shows large reductions in n-ork function (22i, again enhancing IR performance Since initial calculations by Sizelove and Love (4. 23) were in error, as shown b y Seachman ( Z k ) , the first analysis published was one by Ramberg (%), n h o showed the possibility of a significant TIR enhancement in a single bounce. lIeanwhile, the theoretical under-
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Figure 11% Experimental values of quantum efficiencies for single reflection ultrathin TIR-enhanced photocathode
standing of the total reflection phenomenon had been enormously ad\-anced b y its use as a sampling technique for abeorption spectroscopy through the efforts of Harrick (26). By analyzing the electromagnetic field in the vicinity of a totally reflecting surface, it was possible to predict to a good approximation the behavior of a thin absorbing film in contact with it. He furthermore showed that in some circumstances the effective thickness of such a film for absorptioii was much higher than its real one. These approximate formulae were then adapted by Hirschfeld (19) for the case of photocathodes. and used to generate optimization criteria, and predict the performances obtainable by using them. Thus for perpendicularly polarized light, a low index window it-ould allow a 15-fold enhancement in a single bounce near the critical angle and a n Sfold one for a cone having an ,f,'l aperture. For parallel polarization, the 012timum wiiidoiv index of 1 9 allowed 3fold enhancements a t the optimum angle (about l o 3 beyond the critical) and 1.8-fold over even a 150' ( ! ) field of view, all for a single bounce. The development of an exact theory for absorption by thin films at a totally reflecting boundary b y Hansen (27) allowed a n exact acalysis of TIR cathodes (20, 2 8 ) , proving the validity of these earlier conclusions, and yielding a new and very surprising fact. The absorption b y a very thin abeorbing film in the total reflection region, after steadily decreasing with thickness as one xould expect, suddenly changes be-
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