Researchers at the University of Rochester's Institute of Optics have shown that a laser-generated microplasma in air can be used as a source of broadband terahertz radiation. In a paper published this week in Optica, Fabrizio Buccheri and Xi-Cheng Zhang demonstrate that an approach for generating terahertz waves using intense laser pulses in air -- first pioneered in 1993 -- can be done with much lower power lasers, a major challenge until now. Ph.D. student and lead author Buccheri explains that they exploited the underlying² physics to reduce the necessary laser power for plasma¹ generation. He adds that it could potentially be improved for applications in the monitoring of explosives or drugs.

 

Buccheri explains that applications for terahertz radiation, a form of electromagnetic radiation named after its frequency, can be divided into two categories: imaging and spectroscopy. Imaging using terahertz waves is similar to imaging using X-rays, but unlike X-rays it is not a form of ionizing radiation. Imaging with terahertz can, for example, allow us to look under layers of painting. For imaging applications, a narrow range of terahertz frequencies is needed. He adds that this can be generated using specific terahertz devices, such as diodes or lasers. However, for spectroscopy applications, "such as analyzing³ food for poisons or baggage for drugs or explosives it is useful for the terahertz radiation to be as 'broadband' as possible," according to Buccheri. That is, it contains waves of many different frequencies within the terahertz range. For this, a plasma is needed.

 

Buccheri explains that spectroscopy works by looking at which frequencies are absorbed by certain materials. Different materials have different spectra⁴ -- they have peaks and troughs at different frequencies. But depending on the spectral⁵ resolution, these features might look very similar for the different materials.

 

"Spectroscopy is like taking a picture," said Buccheri. "If the camera has a low resolution, the resulting image might be blurry⁶ and the object difficult to identify."

 

For common applications, however, higher spectral resolution is not feasible as it is more costly⁷ and requires more sophisticated equipment. In these cases, more points of comparison are needed, just like in fingerprint⁸ analysis. The more points of comparison that are available, the more precise the analysis, and this is what a broadband source can provide, says Buccheri.

 

"If you were only using a source of radiation with a range around 1 terahertz you might not be able to tell two different materials apart at low spectral resolution, as you might only have one feature in the spectrum⁹ to compare," added Buccheri. "If instead you compare their spectra over a range of tens of terahertz, the 'fingerprints¹⁰' of the two materials will differ and the materials will be more clearly identifiable, even at lower spectral resolutions."

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