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Laser chip emits terahertz waves

17 Jun 2002

A UK-Italian collaboration has developed a semiconductor laser that emits an intense beam of radiation at 4.4 THz.

Portable imaging devices that can detect early stages of skin cancer, explosives and hidden metal objects are now a step closer to reality thanks to the development of a gallium arsenide laser that emits terahertz radiation (Nature 417 132).

A research team from the National Institute for the Physics of Matter (INFM) in Italy and the University of Cambridge, UK have made a prototype laser that is just a few millimetres in size and emits 2 mW of singlemode radiation at a frequency of 4.4 THz (wavelength of 67  µm)

Terahertz waves, which lie between the infrared and microwave parts of the electromagnetic spectrum, have great potential for medical imaging, security screening and process control as they have the ability to highlight specific biological tissues and penetrate clothing or plastic.

However, until now, development of the technology has been hampered by the lack of a convenient source of radiation. To date, terahertz sources have relied on large gas lasers or germanium lasers that only operate at liquid helium temperatures and often require the presence of a strong magnetic field.

The UK-Italian laser, which is based on a so-called quantum cascade structure, gives a promising alternative. "Definitely the biggest advantages are its compactness and portability," explained Rüdeger Köhler, a researcher from INFM in Pisa. "The actual laser is just 1.5 mm long and 0.2 mm wide. It can therefore be easily implemented into small portable devices or laboratory setups."

At the moment the laser operates in pulsed mode and at a temperature of 50 K, however, mid-infrared lasers based on similar quantum cascade principles now operate at room temperature and are commercially available.

"It took about 7 years from the initial demonstration of a mid-infrared quantum-cascade laser, which initially exhibited similar performance to our prototype, to the first device operating in continuous wave at room temperature," said Köhler.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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