Today, I will not tell you something new about the computer industry, which runs at gigahertz speeds. Let's look instead to the laser industry which is exploring terahertz frequencies.
They lie, unexploited, between microwaves at long wavelengths and infra-red at short. They are neglected because no one has developed a convenient source of terahertz radiation. Not yet, anyway. But a laser unveiled by Alessandro Tredicucci of the Scuola Normale Superiore in Pisa, Italy, at the recent International Conference on the Physics of Semiconductors, in Edinburgh, lights the way to the future.
Research into terahertz sources has been driven hard by demand from industry. Terahertz frequencies have great potential in medical imaging because they are strongly absorbed by large biological molecules and by water, and so promise to reveal tissues in astonishing detail. Research carried out by a company called TeraView in Cambridge, England, for example, has indicated that terahertz imaging might distinguish cancerous skin cells from healthy cells.
Dr Tredicucci used quantum cascade lasers to produce terahertz radiation.
Quantum cascade lasers? What are they?
Quantum cascade lasers were invented a few years ago at Bell Labs, in America, and work rather differently from conventional lasers.
A quantum cascade laser also works by pumping up electrons. But instead of dropping back in a single step, their fall is controlled so that they give up their energy a bit at a time. The electron 'cascades' down as it moves through the laser, which is built from thin layers of different semiconductors. The amount of energy given up at each step, and hence the wavelength of the radiation given off, is governed by the thickness of the layer not the properties of the material. A cascade laser is therefore tuneable. Dr Tredicucci has tuned one to terahertz frequencies.
Anyway, it doesn't look like they will be put at work for practical medical imaging anytime soon. These lasers are only operational around 30 degrees Kelvin.
Source: The Economist, August 8, 2002
5:45:48 PM
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