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Posted: June 5, 2007
Photo-detector with a very narrow bandwidth
(Nanowerk News) For a fast detection of hazardous biological and chemical agents in air, photo-detectors analyzing scattered light in the ultraviolet (UV) spectral range are often employed. Every material exhibits a kind of optical fingerprint, which for example can be retrieved by laser excitation. For this purpose, photo-detectors with an extremely narrow bandwidth are necessary, which are only sensitive to particular wavelengths.
Scientists of the Paul Drude Institute for Solid State Electronics (PDI) together with colleagues from India and Spain have recently presented such a detector for UV radiation in the journal Applied Physics Letters ("Very narrow-band ultraviolet photodetection based on strained M-plane GaN films"). This detector exhibits a bandwidth of only six nanometers (nm), being therefore five times narrower than the one of comparable photo-detectors. In addition, the photo-detector is sensitive to polarized light, helping to reduce the scattered background radiation.
The international research team led by Holger Grahn (PDI) developed the detector using a photo-sensitive layer of non-polar gallium nitride (GaN) on a substrate of lithium aluminate (LiAlO2). The active GaN layer is 0.4 micrometers thick, about ten times smaller than the diameter of a typical dust particle. The GaN layer was produced at the PDI (Oliver Brandt), the photo-detector was processed at the Universidad Politécnica de Madrid in Madrid, Spain (Carlos Rivera, Jose Luis Pau und Elias Muñoz), and the measurements were performed at the Tata Institute of Fundamental Research in Mumbai, India (Sandip Ghosh).
The new detector is suitable for real-time identification of biological and chemical agents in air, which are excited by a laser beam to produce fluorescence in the UV spectral range. The detector can then be used to extract the optical fingerprint from the scattered light. In order to simultaneously detect as many substances as possible, a larger number of photo-detectors with very narrow bandwidths are necessary, each of them being sensitive to a particular wavelength. The device presented by the international research team is as small as a pin head and detects only UV radiation of 360 nm.