Laser lathe produces high energy terahertz pulses

A team of scientists from DESY and the University of Hamburg has taken an important step in the search for a new type of compact particle accelerator. Using ultra-powerful pulses of laser light, they have been able to produce particularly high energy radiation flashes in the terahertz range with a clearly defined wavelength (color). The terahertz radiation will pave the way for a new generation of compact particle accelerators that will find room on a laboratory bench. The team led by Andreas Maier and Franz Kärtner of the Hamburg Free Electron Laser Science Center (CFEL) presents its results in the journal Nature Communications. The CFEL is jointly managed by DESY, the University of Hamburg and the Max Planck Society.

The terahertz electromagnetic radiation range is between the infrared and microwave frequencies. Air travelers can be familiar with the radiation terahertz body scanners used by airport security to search for hidden objects under a person's clothes. However, the radiation in this frequency range could also be used to build compact particle accelerators. "The wavelength of terahertz radiation is about a thousand times shorter than the radio waves currently used to accelerate particles," said Kärtner, one of DESY's leading scientists. "This means that the accelerator components can also be built to be about a thousand times smaller." The generation of high-energy terahertz pulses is therefore also an important step for the CFES-funded AXSIS project (Frontiers in Attosecond Science: Imaging and Spectroscopy), funded by the European Research Council (ERC), which aims to open completely new technologies. applications with compact terahertz particle accelerators.

However, selecting an appreciable number of particles requires powerful terahertz radiation pulses having a clearly defined wavelength. This is precisely what the team has managed to create. "In order to generate terahertz pulses, we emit two powerful pulses of laser light into a so-called nonlinear crystal, with minimal delay between the two," says Maier of the University of Hamburg. The two laser pulses have a kind of color gradient, which means that the color at the front of the pulse is different than the one at the back. The slight time difference between the two pulses therefore causes a slight difference in color. "This difference is precisely in the terahertz range," says Maier. "The crystal converts the color difference into a terahertz pulse."

The method requires the two laser pulses to be precisely synchronized. Scientists do this by splitting a single pulse into two parts and sending one of them a short detour so that it is slightly delayed before the two pulses are finally superimposed. However, the color gradient along the pulses is not constant, in other words, the color does not change evenly over the entire length of the pulse. Instead, the color changes slowly at first, then more quickly, producing a curved outline. As a result, the color difference between the two staggered pulses is not constant. The difference is only suitable for producing terahertz radiation on a narrow part of the pulse.

"It was a major impediment to creating high energy terahertz pulses," said Maier. "Because straightening the color gradient of pulses, which would have been an obvious solution, is not easy to do in practice." It's his co-author, Nicholas Matlis, who came up with the crucial idea: he suggested that the color profile of only one of the two partial impulses be slightly stretched along the line. axis of time. Although this still does not change the degree of color change along the pulse, the color difference from the other partial pulse now remains constant. "The changes to make to one of the pulses are minimal and surprisingly easy to achieve: it was enough to insert a short length of special glass into the beam," says Maier. "Suddenly, the terahertz signal became more powerful by a factor of 13." In addition, scientists used a particularly large non-linear crystal to produce terahertz radiation, specially made for them by the Japan Institute for Molecular Science in Okazaki.

"By combining these two measurements, 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 that of any terahertz pulse of perfectly defined wavelength. , previously generated by optical means., "says Kärtner. "Our work demonstrates that it is possible to produce sufficiently powerful terahertz pulses with well-defined wavelengths to operate compact particle accelerators."