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. Fabrizio Buccheri and Xi-Cheng Zhang recently demonstrated 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.

A microplasma is created by focusing intense laser pulses in air. Besides visible light, the microplasma emits electromagnetic pulses at terahertz frequencies that can be used to detect complex molecules, such as explosives and drugs. (Photo: J. Adam Fenster / University of Rochester)

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. This can be generated using specific terahertz devices, such as diodes or lasers. However, for spectroscopy applications, “such as analyzing 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 spectra – they have peaks and troughs at different frequencies. But depending on the spectral 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 blurry and the object difficult to identify.”

For common applications, however, higher spectral resolution is not feasible as it is more costly and requires more sophisticated equipment. In these cases, more points of comparison are needed, just like in fingerprint 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 spectrum to compare,” added Buccheri. “If instead you compare their spectra over a range of tens of terahertz, the ‘fingerprints’ of the two materials will differ and the materials will be more clearly identifiable, even at lower spectral resolutions.”

Until now, approaches to use a plasma as a broadband source of terahertz have commonly used an elongated plasma generated by combining together two laser beams of different frequencies, i.e., colors. This technique, usually referred to as “two-color” approach, requires powerful, expensive lasers. The

“one-color” approach uses single laser frequency to generate the plasma. Pioneered by Harald Hamster and colleagues in 1993, it required even higher laser energies and therefore it was not explored further until recently by Buccheri and Zhang.