Laser diodes with world's shortest wavelength for bioterror detection

Here are two observations: First, when it comes to terrorist attacks — and, without being flippant, also to one’s personal health — detection and prevention are better than response and mitigation. Hence the enormous efforts and amounts of money governments put into encouraging the development of various detection and sensing systems. Second, at times small differences matter; in evidence: The difference between 343nm and 337nm is so small, it is relevant only on the molecular level. The molecular level, however, is exceedingly important for some applications. These two observations were evoked by a piece of encouraging — and potentially very important — news: Researchers at Sheffield and Bristol universities will next month launch a project to develop laser diodes with the world’s shortest wavelength. If successful, the technology could be used in a cheap, portable system to be used in continuously monitoring biowarfare pathogens. It could also be used to increase the storage capacity of DVDs. The scientists are fabricating the first 337nm laser diode based on gallium nitride/aluminum gallium nitride (GaN/AlGaN) to replace the existing nitrogen gas-based laser for use in the detection of biological particles. The researchers told the Engineer that the shortest laser diode so far reported measures 343nm — which is problematic: Most biological molecules show strong absorption in the ultraviolet spectral region ranging from 280nm to 340nm. “The project would mainly focus on developing a light source that works in the ultraviolet spectral range, which is needed to excite the biological molecules,” said Professor Martin Kuball, an expert in thermography technologies at Bristol. Dr. Tao Wang, a specialist in III-nitride materials and devices at Sheffield, added: “In order to detect any biological molecules we have to use a very short wavelength, such as the 337nm. The challenge to achieve the shorter wavelength is crystal quality.”

Semiconductor materials are typically grown using a process known as metal-organic chemical vapor deposition, whereby the materials are grown through the surface reaction of metalorganics containing the required chemical elements on a substrate. It is normally a homoepitaxial process, which means the semiconductor grown on the substrate is the same as the substrate material. “Homoepitaxy means that, for example, in the ordinary semiconductor the gallium arsenide is grown on the gallium arsenide substrate. For the shorter wavelength we have to use GaN, which we would normally grow on the GaN substrate. Unfortunately this is not