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Oct. 24, 2007 Research Highlight Physics / Astronomy

Laser-driven harmonics hit a high note

A new technique could improve development of high-efficiency light sources

Image of high-frequency lasers Figure 1: High-frequency lasers can help to take snapshots of atomic processes.

RIKEN scientists have developed a way to create incredibly brief bursts of high-frequency light that should help to take better snapshots of atoms (Fig. 1).

Short pulses of radiation are useful for studying very fast processes, such as the way that electrons dance between atoms as they form chemical bonds. This happens in mere attoseconds (10-18 s)—billionths of a billionth of a second—and the light acts like a camera flash, freezing the action in a subatomic snapshot.

Higher frequencies of light can capture briefer events, yet few lasers operate beyond the visible spectrum. Under certain conditions, however, laser frequencies can be multiplied many times.

This relies on the way that blasts of laser light wrench electrons away from their parent atoms. The incoming light has a varying electric field which first accelerates the electron, and then pushes it back towards the atom, where it is recaptured. The electron then releases its energy as a short, intense laser pulse, with a frequency that is a multiple of the original laser frequency. Creating ‘harmonic’ frequencies in this way allows scientists to turn visible laser light into higher-energy ultraviolet, or even soft x-rays.

Normally, higher harmonics are more difficult to access because as the electron becomes increasingly energized, it is more likely to give up some of its energy, rather than absorb even more.

Now, scientists at RIKEN’s Discovery Research Institute, Wako, and colleagues at the University of Tokyo, have developed a technique to dramatically enhance the amount of high-frequency light generated by high-order harmonics1.

“A key strategy of our experiment is the use of a mixture of two kinds of rare gases as harmonic emission medium—helium and xenon,” says Eiji Takahashi, part of the RIKEN team.

First, an infrared laser excites electrons in the xenon atoms up to the 23rd harmonic, so that they emit pulses of extreme ultraviolet light. These photons then hit the helium atoms, acting as a booster for the harmonic generation, helping it to push helium’s electrons up to the 27th harmonic together with the infrared laser. These electrons finally emit soft x-ray light at an even higher frequency, producing 4,000 times more photons than a system that uses helium alone.

“Dramatic enhancement of high harmonic generation is an attractive phenomenon because it may be crucial for further development of high-efficiency soft x-ray light sources,” says Takahashi. This should help to push back the frontiers of precision spectroscopy and ultrafast science, he adds.

References

  • 1. Takahashi, E. J., Kanai, T., Ishikawa, K. L., Nabekawa, Y. & Midorikawa, K. Dramatic enhancement of high-order harmonic generation. Physical Review Letters 99, 053904 (2007). doi: 10.1103/PhysRevLett.99.053904

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