Lasers Produce High-Energy Terahertz Pulses

From the colour difference of two slightly delayed laser flashes a non-linear crystal generates an energetic terahertz pulse. (Source: DESY / Lucid Berlin)

A team of scientists from DESY and the Uni­versity of Hamburg has achieved an important milestone in the quest for a new type of compact particle accelerator. Using ultra-powerful pulses of laser light, they were able to produce parti­cularly high-energy flashes of radiation in the terahertz range having a sharply defined wavelength. Terahertz radiation is to open the way for a new generation of compact particle acce­lerators that will find room on a lab bench.

Terahertz radiation might also be used to build compact particle acce­lerators. “The wavelength of terahertz radiation is about a thousand times shorter than the radio waves that are currently used to acce­lerate particles,” says Franz Kärtner from the Hamburg Center for Free-Electron Laser Science CFEL. “This means that the components of the accelerator can also be built to be around a thousand times smaller.” The generation of high-energy terahertz pulses is therefore also an important step for the AXSIS – frontiers in Attosecond X-ray Science: Imaging and Spectro­scopy – project at CFEL, funded by the European Research Council (ERC), which aims to open up completely new applications with compact terahertz particle acce­lerators.

However, chivvying along an appre­ciable number of particles calls for powerful pulses of terahertz radiation having a sharply defined wavelength. This is precisely what the team has now managed to create. “In order to generate terahertz pulses, we fire two powerful pulses of laser light into a non-linear crystal, with a minimal time delay between the two,” explains Andreas Maier from the Univer­sity of Hamburg. The two laser pulses have a kind of colour gradient, meaning that the colour at the front of the pulse is different from that at the back. The slight time shift between the two pulses therefore leads to a slight difference in colour. “This difference lies precisely in the terahertz range,” says Maier. “The crystal converts the difference in colour into a terahertz pulse.”

The method requires the two laser pulses to be precisely syn­chronised. The scientists achieve this by splitting a single pulse into two parts and sending one of them on a short detour so that it is slightly delayed before the two pulses are eventually superimposed again. However, the colour gradient along the pulses is not constant, in other words the colour does not change uniformly along the length of the pulse. Instead, the colour changes slowly at first, and then more and more quickly, producing a curved outline. As a result, the colour difference between the two staggered pulses is not constant. The difference is only appro­priate for producing terahertz radiation over a narrow stretch of the pulse.

“That was a big obstacle towards creating high-energy terahertz pulses,” as Maier reports. “Because straightening the colour gradient of the pulses, which would have been the obvious solution, is not easy to do in practice.” It was Nicholas Matlis who came up with the crucial idea: he suggested that the colour profile of just one of the two partial pulses should be stretched slightly along the time axis. While this still does not alter the degree with which the colour changes along the pulse, the colour dif­ference with respect to the other partial pulse now remains constant at all times.

“The changes that need to be made to one of the pulses are minimal and sur­prisingly easy to achieve: all that was necessary was to insert a short length of a special glass into the beam,” reports Maier. “All of a sudden, the terahertz signal became stronger by a factor of 13.” In addition, the scientists used a parti­cularly large non-linear crystal to produce the terahertz radiation, specially made for them by the Japanese Institute for Molecular Science in Okazaki.

“By combining these two measures, we were able to produce terahertz pulses with an energy of 0.6 milli­joules, which is a record for this technique and more than ten times higher than any terahertz pulse of sharply defined wavelength that has previously been generated by optical means,” says Kärtner. “Our work demonstrates that it is possible to produce suffi­ciently powerful terahertz pulses with sharply defined wavelengths in order to operate compact particle acce­lerators.” (Source: DESY)

Reference: S. W. Jolly et al.: Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation, Nat. Commun. 10, 2591 (2019); DOI: 10.1038/s41467-019-10657-4

Link: Ultrafast Optics and X-Rays UFOX,  Center for Free-Electron Laser Science CFEL, DESY, Hamburg, Germany

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