Detection of ultraviolet (UV) light with high single-photon efficiency is required in many fields, from dark matter experiments to astronomical surveys, and even air pollution monitoring. Traditional detection techniques are based on the use of photo-multiplier tubes, which have typical single-photon efficiency of 20-25% for wavelengths λ < 300 nm, and are susceptible to thermal noise. Arrays of aligned carbon nanotubes (CNTs) could bring exciting new possibilities to this field, because of two key features: (i) their intrinsic anisotropy, which makes them ‘hollow’ in the direction of the tube axes; and (ii) their relatively high work function (4.3 eV) which makes them insensitive to thermal noise and blind to visible light, while maintaining sensitivity in the deep UV range (λ < 290 nm).
NanoUV is a concept of a new solid-state UV photo-sensor, based on arrays of aligned CNTs coupled with a silicon photodiode. The CNT array will be placed in a vacuum and in a strong uniform electric field, parallel to the tube axis. The open ends of the tubes will be pointing to the photodiode, which will be place a few centimetres away. The CNT array would serve as photo-emitting component: the incident UV radiation would be absorbed in the tubes, and extract electrons from the Carbon lattice through the photoelectric effect. These electrons will have an energy of a few eV, which is below the anelastic cross-section threshold, therefore they will traverse the whole CNT length without being re-absorbed, and exit the tubes from their open end. Once outside the CNT array, they would be accelerated by the external electric field, reaching an energy of a few keV before hitting the silicon photodiode, where they would produce a visible current.
NanoUV could represent a breakthrough in UV-light detection, as it would allow for detectors practically unaffected by thermal noise and dark currents, insensitive to visible light backgrounds, and with the potential to achieve high efficiency in single-photon counting, by avoiding photoelectron re-absorption in the photocathode, which is the leading source of inefficiency in current UV light detection devices.
High-efficiency UV light detection would have immediate applications in dark matter searches, such as state-of-the-art large volume xenonbased detectors, and could also affect the design of next-generation space telescopes, for which high-precision UV scans of the universe are now central physics objectives. They could also play a role in developing more precise air-pollution monitoring devices, which today represents a crucial healthcare issue for the citizens of the European Union.