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Improved photon efficient cathodes with applications in biological luminescence (IMPECABLE)


ETL has designed and built photomultiplier tubes to demonstrate the potential of using a waveguide structure to improve the quantum efficiency over a broad spectral range. These photomultipliers used a photocathode deposited onto a thin sheet of glass that acted as a waveguide for the incoming light. These photomultiplier tubes were of novel design, as the necessarily thin waveguides could not be used to support a vacuum within the device against atmospheric pressure on the outside. The use of a waveguide to achieve many interactions with a photomultiplier tube photocathode has led to significant improvements in quantum efficiency, especially towards the red end of the spectrum where current photocathodes typically have low sensitivity. The increase in QE of waveguide photocathodes compared to standard mode illumination is x2 at 400nm, x3 at 550 nm, x4 at 650nm and x9 at 800nm Relevance of this result: As time progresses there is an increasing demand for low background photomultipliers with greater sensitivity towards the green and red regions of the spectrum (especially for biological applications). The use of multiple interactions of light with the photocathode means that as much of the available radiations as possible will be absorbed and may be converted into photoelectrons. It is also becoming more common for the light to be detected to be transported to the photomultiplier tube by a small fiber-optic. These sources are very suitable to be optically coupled into a narrow waveguide. Current Status: The large gains that have been demonstrated within this project make this an attractive possibility to incorporate into the design of the next generation of photomultiplier tubes. Further exploitation of this technology will depend on the future development of the photomultiplier tube market.
The design and realisation of optical devices to efficiently couple light into photomultiplier tubes has been completed. Polymer optical components have been fabricated that significantly increase the effective sensitivity of photomultiplier tubes over a broad range of wavelengths (400-850nm). This was achieved by causing incoming light to undergo multiple interactions with the photocathode by being reflected within the photomultiplier tube window. A variety of PMMA (Perspex) demonstrators were trialled incorporating polymer optical fibres (POF), simple prisms and cylindrically symmetric designs. It was found that the use of POF was impractical and the modest gains achieved were easily bettered by the simpler, easier to manufacture reflecting prisms. The designs were finally refined to a cylindrical symmetry, to make optimum use of the photocathode area and facilitate the easy mounting of the devices photomultiplier tubes. The prototype devices increase the quantum efficiency of a standard blue-green sensitive bialkali photocathode photomultiplier tube over its entire working range, with an improvement in sensitivity at 550nm of up to a factor of two (i.e. 100% better than tube without device). For an S20 photocathode improvement factors of up to seven have been observed at 800nm. Relevance of this result: Any general increase in the quantum efficiency of our products will make them more attractive in a highly competitive world market. These devices are initially aimed at the rapidly growing market for biological luminescence where a key wavelength monitored is 550nm. At this wavelength green-enhanced bialkali photocathodes are currently used, but they give rise to a much higher dark count than a standard blue-green sensitive bialkali photocathode. It is expected that use of the new optical coupling devices will extend the viable working range for the blue-green sensitive bialkali photocathode to provide good sensitivity, low-background photomultiplier tubes for use at 550nm. Current Status: Prototypes have been made and tested and will shortly be distributed to selected customers for appraisal. A thorough market research report has been also been completed to assess the impact of the improved performances that are now achievable.
Theoretical modelling of the performance of the S20 photocathodes has been made with input from the measured dielectric constants. For the planar photocathodes there is a close match in terms of the predicted a priori absolute quantum efficiency and the standard tube data. The evaluations extend to include the angular dependence of the optical absorption, polarisation effects, the effect of ridges, cones and other structures on the cathode substrate, and the results of using waveguide geometries to enhance cathode performance. The developments of the theory have been matched with experimental data in many cases and there are good agreements. The fundamental aspects of the work have been published, or are in print. The same theoretical approach also predicted the observed reflectivity data seen for “black silicon”. Commercially sensitive predictions have been retained but used in modelling of new tube designs and retrofits. The theory has included both quantum mechanical and simplistic alternatives. The former approach has been necessary to evaluate the way in which electrons interact with the cathode-vacuum surface and their probability of escape as a function of incident wavelength. This latter feature is a major consideration for long wavelength improvements in sensitivity.
Effective Medium Theory (EMT) developed by Rytov was successfully applied to obtain the effective refractive index of sub-wavelength, periodic, one-dimensional structures. Here, an algorithm to solve such bi-dimensional structures is proposed. This provides an approximate method for solving the reflectance and transmittance of two-dimensional structures that are periodic, providing that the radiation wavelength is much smaller than the structure’s periods. The method starts from the EMT for 1D structures and is applied to solve 2D periodic sub-wavelength structures by considering the latter problem as a superposition of two 1D problems. Such a method is highly useful, considering the long computational time required by the more accurate Rigorous Coupled-Wave Analysis (RCWA) now in use. The theory of the optical properties for normal incidence of inhomogeneous media with two-dimensional periodicity has been developed, based on effective index methods. A software code has been developed to simulate optical properties of sub-wavelength structures. The theory avoids the approximation made by Ritov, and the calculations are exact. A software code has been written in Visual Basic. This programming language permits a friendly interface with the user, where the parameters can be easily modified. The software allows inclusion of absorbing structures, having an arbitrary number of layers. In this way, arbitrary profiles can be simulated. Also, the code includes the possibility of different angles of incidence, as well as treating separately both polarisations.
A range of structures have been produced on the inside of the windows of ETL’s standard photomultiplier tubes in order to investigate their effect on the quantum efficiency the devices. The structures have been produced in two distinctive ways: - Sol-Gel: The sol-gel process has been used to create precise square-based pyramids in silica glass. The arrays of pyramids could then be incorporated into photomultiplier tubes. Different pyramid sizes and cone angles were tested and compared with the photocathode spectral response from a flat window. - Laser-Ablation: A selection of windows of different glass types were laser ablated by LZH to produce microstructures (approx. 50micron) on their surfaces (see LZH report for details). These were incorporated into standard photomultiplier tubes and the quantum efficiencies compared between the flat and structured parts of the windows. The ejection of photoelectrons from structured surfaces has been modelled in 3D using the Simion7 electrostatics program. Pyramids of different sizes and cone-angles were investigated. It was evident that features of the order of 1mm in size facilitated the escape of photoelectrons from the photocathode surface, while keeping the effects on the timing characteristics with acceptable limits. Both sol-gel and laser-ablated structured cathodes give large increases in cathode quantum efficiency, i.e. up to x1.5 at 400nm, x1.8 at 550nm, x2.2 at 650nm, and x2.5 at 800nm). However, laser-ablation is not commercially viable with the current level of technology. Relevance of this result: ETL currently produces photomultiplier tubes with “prismatic” windows that have square–based pyramids, pressed into the window. These give enhancement of up to x1.6 at 550nm (for an S20 photocathode). The pressed structures do not have sharply defined features and it has been seen that enhancement is not uniform. Extra enhancement can be achieved with sol-gel structures due to the ability to for very precise repeatable structures and sharp features. This technique is relatively simple to implement and works well for all photocathode types. Greater enhancement is observed, due to the extra interaction with the photocathode, at longer wavelengths where absorption of the light by the photocathode is weak. Any general increase in the quantum efficiency of our products will make them more attractive in a highly competitive world market. A structured window may be used with little impact on the rest of a photomultiplier tube, making easy to incorporate into existing designs. Current Status: Laser ablated windows were successful in achieving gains in photomultiplier tube performance, but the technique is not commercially viable to be used in production. Sol-gel windows were also very successful and ETL are interested in ways to viably incorporate the silica windows into our standard products. ETL are also interested in further possible sol-gel applications in cooperation with Novarra.
The sensitivity of the photomultipliers with improved cathodes was investigated for detection of the biological luminescence. New results on tissue luminescence including improvements of the original detector system were obtained. The advance in sensitivity was considerable (ie>10,000 times). Medical luminescence exploitation of the PM tubes was presented with new examples of biologically relevant signals and spectral response, particularly for the detection of cancer. Three cationic zinc phthalocyanines (ZnPcs), tetrakis-(3-methylpyridyloxy), tetrakis-(3-hexyl-pyridyloxy), and tetrakis-(3-dodecylpyridyloxy) phthalocyanine zinc (ZnPcMe, ZnPcXe and ZnPcDo) have been studied as advanced fluorescent contrast agents for pigmented melanoma. UV-Vis spectroscopic properties of the monomers were investigated. Their photophysical behaviour as a substantial part of dye-induced fluorescence was evaluated. The selective accumulation and labelling capacity towards B16F0 pigmented melanoma tumor were determined. Melanin containing cells were isolated and incubated with ZnPcs at several time intervals (1, 2.5 and 6hours) following the kinetics of cellular uptake. A highest accumulation was found for ZnPcXe. A comparable uptake was detected for the extremely lipophilic ZnPcDo and hydrophilic ZnPcMe. Fluorescence diagnostic potential of ZnPcs towards pigmented melanoma by using an argon-dye laser detection set-up was demonstrated.
Nature of result: Know-how of Laser based processes for manufacturing large areas with fine detailed surface structures (5-100µm) on glass or silica substrates has been gathered. Many aspects of geometrical shaping the material, and process development for reducing surface roughness have been successfully carried out. Polishing with surfaces of optical quality has been achieved. Relevance: The new surface structures offer one starting point for enhancement of photon efficiency of photomultipliers. The small dimensions of the generated geometry are crucial. The generation of structures for large cathode areas necessitated development of rapid and large-scale laser technology to achieve good economics. The compatibility with subsequent fabrication processes, such as cathode coating with active layers, as well as optical coatings, is directly linked to the condition of the machined surface. Therefore the reduction of surface roughness is an important aspect for the fabrication of micro-optical components. In contrast with the developed laser based process, mechanical methods will be limited to very simple geometries, if applicable at all. The newly developed technology is applicable to other technologies such as polishing of micro-optics, generation of functional structures for micro-biological analysis etc. The machining of non-glass microstructures is valuable for making masters in solgel technology. This symbiosis opens a new and wide range topological processing. Potential: Currently, further areas with need for high quality microstructures are being evaluated for possible technology transfer. They include micro-optics, technical optics, and electro-optics as well as chemical and biological analysis. Current status: Extension of a pre-existing patent to other dimensions e.g. micro-optics.