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Fresnel lens uses concentric circles to bring terahertz waves to a focus
Professor David Cumming, Dr Timothy Drysdale, Dr Ata Khalid, Dr Anna Sheridan and Dr Edward Walsb
Terahertz waves, invisible to the naked eye, are being used to provide 'superhuman vision'. 'With terahertz imaging it is possible to see the body beneath the clothes not just the bones that you see with x-rays,' says Tim Drysdale of the University of Glasgow.
Terahertz waves lie between the highest radio frequencies and the far infrared representing the last unexplored frontier of the radio and light wave spectrum. The breakthrough came when it was discovered that sending ultra-fast pulses of visible light through specially engineered crystals could generate terahertz waves. The terahertz waves then generate 3D images of objects. 'It is the pattern of reflection and absorption of terahertz waves that build the image,' says Tim. 'The depth of structures can be calculated by the tiny time delay between the wave being emitted and reflected back.'
Apart from providing structural information, terahertz waves can identify materials. David Cumming, who leads the Microsystems group, explains, 'Different molecules absorb and reflect terahertz waves in a recognisable way, they have a kind of 'terahertz fingerprint'.' This makes the technique particularly useful for pharmaceutical applications where rapidly and precisely identifying the makeup of drugs is essential.
The Glasgow group develop devices that allow the terahertz waves to be deftly manipulated, often in collaboration with developers of terahertz imaging and spectroscopy systems. 'New developments, such as continuous wave imaging, are expected to lead to significant cost reductions,' says David. 'So we are continuing to develop components that improve the performance of terahertz systems or add new functionality.'
Filters are used to improve the imaging speed by blocking out unwanted intermodulation products that arise when a continuous stream, rather than pulse of terahertz waves, is used. Intermodulation products are additional frequencies that are created when two frequencies combine. The rock guitar power chord is based on this effect two specific notes are played to create further frequencies or notes and distortion is added. 'We didn't want some of those extra signals,' says Tim. 'So we had to filter them out.'
Fresnel lenses have been developed to 'focus' the terahertz waves. 'The lens has a frequency dependent focus,' explains Edward Walsby. 'Each frequency focuses at different lengths.' In a broad band pulsed terahertz system one that contains a broad range of frequencies the waves focus at different lengths enabling one pulse to create a stack of images throughout an object.
Rings of metal much smaller than the diameter of a hair are being developed to enhance the sensitivity of 'terahertz fingerprinting'. 'When terahertz waves are shone on these rings, they resonate like a bell hit with a hammer, making a much larger sound than just the hammer alone produces,' says Anna Sheridan. 'That lets you detect the response of much smaller numbers of cells.'
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