A commercial THz imaging system using Lumped Element Kinetic Inductance Detectors

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Detector arrays are used in many familiar technologies for forming images of the world we live in. The most common detector array known as the charged coupled device (CCD) array forms the basis of many digital and phone cameras. Such detector arrays are sensitive to light in the optical region of the electromagnetic spectrum (the colours of the rainbow - red through to blue). However there is a wealth of information contained in the regions of the electromagnetic spectrum outside of the range of the human eye. For example you may have seen infrared images taken from police or rescue helicopters using special cameras searching at night. Infrared light is no different to optical light and in most cases is generated by the same mechanism - heat. If one were to heat a piece of metal to a few hundred degrees centigrade we would notice it glowing red. Heat it further and it would glow yellow then white. If we let it cool again we would see the white glow fade to yellow which would fade to red and then to no glowing at all. In fact this is not the case. The metal is now glowing in a region of the electromagnetic spectrum known as the infrared. The only reason we do not observe this is because our eyes are insensitive to this light. We can however sense infrared light and it is what we more commonly refer to as heat. If one moved their hand near a warm piece of metal (such as a household radiator) without touching it we would feel that it was hot from the infrared radiation warming our skin. If the metal were to cool back down to room temperature it would still be glowing in the infrared but now much less intensely. The infrared region of the spectrum lies just beyond the red region of the visible spectrum but as we move further past the infrared from the visible spectrum we enter what is known as the THz region of the electromagnetic spectrum. The THz region of the spectrum is of great interest to research and industry alike. For example many materials that are opaque to visible light are transparent to THz light. In this example if one had an array of THz detectors one could image objects beneath a surface that would otherwise be invisible. You may have experienced such systems in some airports where they are used for security purposes to detect concealed objects on passengers. Beyond security, the imaging of THz light has many applications ranging from quality control (imaging the invisible circuitry of an encased computer chip for example) to looking at the THz light emitted from biological samples used to deduce their chemical composition. However to date developing detectors that can sense THz light has proven complex and expensive hence THz imaging arrays are not commonplace in the world of research or industry.The proposed research will develop a new type of detector called the Lumped Element Kinetic Inductance Detector (LEKID). The LEKID is sensitive not only to optical, THz and infrared light but also ultra-violet light and X-rays. The LEKID is also very simple to fabricate into large imaging arrays making it a viable option for the commercial and industrial applications. The one drawback of the LEKID is that it must be cooled to very low temperatures. Known as cryogenic temperatures the temperature the LEKID operates at is of order -273 degrees centigrade and is close to the lowest temperature physically possible referred to as absolute zero. However, recent development in cryogenic technology has made achieving these low temperatures relatively simple. Detectors operating at these low temperatures have significant advantage over their room temperature rivals, being of order 10,000 times more sensitive and generally much faster. This property allows for the first time THz imaging at video frame rates. The idea of a THz video has excited many research scientists as they would now have the ability to watch how a system emitting THz light evolves in real time which has never before been possible.