The principles of the VUV detectors are those used for the visible and infrared detections – surface photoemission and electron-hole pair creation in semiconducting materials. But most of the time, VUV detectors should operate in an evacuated environment, especially when EUV or Hard X-Ray radiations have to be detected.
A photodiode is a p-n junction or p-i-n junction. It converts light into current on voltage using the inner photoelectric effect. The use of silicon photodiodes at VUV wavelengths has traditionally been limited by the strong absorption of VUV photons in the outer SiO2 passivation layer that covers the p-n junction of these devices.
Thinning the SiO2 passivation region to a thickness of about 5 to 10 nm significantly improves its sensitivity. Common VUV photocathode materials are CsTe, CsI and KBr. These materials combine high VUV quantum efficiency with a solar blind response (insensitive to visible light).They are robust and inexpensive.
A PMT is built in a vacuum tube equipped an input window in front of a photocathode, focusing electrodes, multiple dynode stages and an anode. The dynodes act as electron multipliers, enabling the detection of low incident flux. Useful PMT window materials are Fused Silica, MgF2, LiF and Sapphire in the 105-200 nm range.
For spectral ranges below 105 nm to few nm, a fluorescent coating can be deposited on the PMT window to down- convert the VUV light to longer wavelengths. Coatings such as Sodium Salicylate is commonly used to convert the VUV radiation to visible. Its efficiency is relatively constant for VUV light from 30 to 200 nm with a pic emission around 430 nm. In such condition a sealed classical PMT working behind a MgF2 window, Sodium Salicylate coated, can operate in an atmospheric pressure housing. Lumogen, Terphenyl, and Coronene are other alternatives to the Sodium Salicylate.
Some “PMTs” may be operated directly evacuated without any window/photocathode, in this case, the first dynode is directly sensitive to VUV radiations. Beryllium oxide is mainly used but other materials such as CsI, CuI, KCl and MgF2 can be evaporated on the dynode for an optimization of the detector response. The typical response of such detectors is 30 to 140 nm. If the EMT principle is quite simple, such detectors have to be stored and handled with care because of the deliquescence of their material.
A MCP-PMT consists in a 2D array of capillaries working as individual electron multipliers and an anode. MCPs are directly sensitive to EUV and X-Ray radiation (150 nm to 1Kev) and do not require a photocathode such as those working in the UV/Visible range. With their VUV photon detection capability and a rise time of hundred picoseconds, such detectors are the best choice for a EUV spectral investigation and nanosecond scale domains such as fluorescence life time (TCSPC technics). They are an affordable cost compared to most of the CCD detectors.
MCP assembled with a phosphor screen can equip a 2D array detector. In this case, it combines the single-photon counting sensitivity of a PMT with the high resolution imaging capability of a CCD detector.
VUV CCD detectors are classical scientific grade cameras. They can be used for imaging or spectroscopy purposes. Only the Back-Illuminated (BI) without window is compatible with the full VUV range because of the easiest penetration of the UV photons in their thinner silicon structure. They are fully vacuum compatible (detector placed inside the vacuum chamber) or with an external attachment (internal evacuation only). EMCCD or sCMOS detectors start to be employed in VUV experiments. Front-illuminated open electrode detector may be sensitive down to 120 nm when they are Lumogen coated.
In conclusion, back-illuminated full frame CCDs are the best choice for VUV spectroscopy.
The theory of CCD is explained in the OSD section of this book. We ask you to refer to this chapter for more detailed explanation.
A spectrograph is an instrument that disperses an incoming light into a spectrum while a monochromator selects a narrow band of radiation from the wide radiation spectrum. A monograph configuration allows the instrument to be operated in either monochromatic mode or spectrographic mode.
The linear dispersion of a grating varies with wavelength. The shorter the wavelength is, the higher the dispersion will be. In consequence, the dispersion of a spectrograph is most of the time, less than 1 nm/mm for a full coverage of about 25 nm with a decent efficiency in EUV. Therefore, the coverage of 100 nm requires more than one grating.
The reflectance of all materials drops dramatically below about 100 nm in normal incidence. As the reflectance increases with incidence angle, VUV instruments are better EUV optimized when their optics work with a grazing configuration.
In this VUV optical layout, a large deviation angle is used. Fig. 17 shows the reflectance of aluminum with Fluoride Magnesium coating (AlMgF2), from which we can see the improvement of the efficiency below 100 nm at 160° deviation. Such a large angle between the beam and the surface is referred to as “grazing angle.”