In a paper published 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 underlying physics to reduce the necessary laser power for plasma generation.
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 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.
A microplasma is created by focusing intense laser pulses in ambient air with a microscope objective in the lab of University of Rochester Institute of Optics Ph.D. student Fabrizio Buccheri and his advisor, Professor Xi-Cheng Zhang. 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 courtesy of J. Adam Fenster, University of Rochester.