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Generating terahertz radiation from thin air

Laser-generated microplasma in air can be used as a source of broadband terahertz radiation, researchers at the University of Rochester's Institute of Optics have shown.

PhD student Fabrizio Buccheri demonstrated that an approach for generating terahertz waves using intense laser pulses in air can be done with much lower power lasers than previously.

The application of terahertz radiation can be divided into two categories: imaging and spectroscopy. For imaging applications, such as replacing X-rays, a narrow range of terahertz frequencies is needed. This can be generated using specific terahertz devices, such as diodes or lasers. However, for spectroscopy applications, such as security scanning, the terahertz radiation needs to be as 'broadband' as possible, which requires a plasma.

Until now, broadband sources of terahertz radiation have usually used an elongated plasma generated by combining two laser beams of different frequencies. This 'two-colour' approach requires powerful, expensive lasers. The 'one-colour' approach uses one laser frequency to generate the plasma.

An advantage of this 'one-colour' approach is the terahertz waves propagate in a different direction to the laser beam. This makes it easier for potentially coupling the terahertz waves to a wave guide on a microchip, for example.

Through experiments into the polarisation of light Buccheri was able to exploit the physics to use lower laser energies than previously thought possible to generate broadband terahertz waves in air. He replaced elongated plasmas, with lengths ranging from a few millimetres to several centimetres, with a microplasma, about the width of a human hair. He thinks that even lower operation powers could be achieved by fine tuning the type of laser used and changing to a different gas.

Tom Austin-Morgan

Source:  www.newelectronics.co.uk

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