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Development of XFEL Technique for Generating Monocycle X-Ray Pulse - Control of “optical slippage” by light wave interference - (Press Release)

Release Date
27 Jan, 2015
  • SACLA

RIKEN

   Takashi Tanaka (chief scientist) of Advanced X-ray Laser Laboratory, RIKEN SPring-8 Center, developed a method of generating a monocycle X-ray pulse, an X-ray with a pulse width*3 having theoretically the shortest wavelength, using X-ray free electron laser (XFEL)*2 facilities such as SACLA.*1

   A monocycle pulse is a light wave that vibrates only once during optical emission. The method of generating a monocycle pulse in visible-light and infrared regions (wavelength, ~1 µm) is already mature. Stroboscopic imaging of chemical reaction processes such as photocatalysis is possible utilizing the characteristics of short pulses [pulse width, several femtoseconds (1 femtosecond = 10-15 s)]. Recently, monocycle pulses with a width of several hundreds of attoseconds (1 attosecond = 10-18 s) in the extreme ultraviolet region of shorter wavelengths (wavelength, approximately several tens of nanometers) have become available owing to high-order harmonic generation.*4 However, no principles or methods of generating a monocycle pulse in XFEL, which has a different laser oscillation principle from visible lasers and high-order harmonics, have been developed. In particular, an optical slippage effect (the phenomenon of a light wave moving slightly more forward than electrons), observed when an electron beam passes through a periodic magnetic field, makes the generation of monocycle X-ray pulses in XFEL impossible.

   Dr．Tanaka (chief scientist) developed a method of controlling the pulse width by eliminating the optical slippage effect by light wave interference and theoretically confirmed that the monocyclic X-ray pulses can be generated in XFEL.

   If this method can be applied to the hard X-ray region (wavelength, ~0.1 nm), an ultimate light source with a pulse width of several hundreds of zeptoseconds (1 zeptosecond = 10-21 s) will be realized. The development method is expected to lead to zeptosecond science, a totally new scientific field to pursue ultrafast phenomena at the zeptosecond level, in the future.

   The achievements of this research were published online on 26 January 2015, prior to their publication in the American Scientific journal Physical Review Letters.

Publication:
Physical Review Letters
Title: "Proposal to generate an isolated monocycle X-ray pulse by counteracting the slippage effect in free electron lasers"
Authors:  Takashi Tanaka
DOI: 10.1103/PhysRevLett.114.044801

<<Figures>>

Optical emissions from an electron beam with a bunch length shorter than the wavelength and an electron beam with regularly aligned microbunches
Fig.1 Optical emissions from an electron beam with a bunch length shorter than the wavelength
and an electron beam with regularly aligned microbunches

(a)Electrons contained in an electron beam with a bunch length shorter than the wavelength meander and are left behind the light wave emitted by the electrons, resulting in the light wave travelling ahead of the electrons. One light wave is generated by one meandering motion and travels ahead of the electrons; N light waves are generated by N meandering motions, leading to an increase in pulse width.
(b) A monocycle pulse is generated by adjusting the magnetic field so that the slippage length of the n-th period emission from the electron beam with regularly aligned microbunches is equal to the interval of the n-th microbunch.

Fig. 2 Mechanism behind generation of monocycle pulse
Fig.2 Mechanism behind generation of monocycle pulse

(a) Waveform of current
(b) Optical waveform after the passage of the first period. An optical waveform similar to the waveform of current shifts forward by a distance of λ1.
(c,d,e) Optical waveforms after the passage of second, third, and fourth periods.
(f) Optical waveform after the passage of tenth period. The light waves constructively interface with each other to increase in their intensities at the position of the resonant pulse indicated by the arrow; however, they attenuate as they deviate from the position of the resonant pulse.

<<Glossary>>
*1 SACLA
Japan’s first X-ray free electron laser (XFEL) facility constructed jointly by RIKEN and Japan Synchrotron Radiation Research Institute (JASRI). SACLA generates X-rays from a bunch of electrons that are simultaneously vibrated under precise control in an accelerator. As one of the five national key technology centers, the facility was constructed and improved in a five-year project starting from FY 2006. It was completed in March 2011 and named SACLA after the initial letters of SPring-8 Angstrom Compact Free Electron LAser.

*2 X-ray free electron laser (XFEL)
Laser in the X-ray region. Unlike conventional lasers using semiconductors and gases as the oscillation medium, XFEL uses electron beams that travel in vacuum at a high speed and have theoretically no limit in terms of wavelength.

*3 Pulse width
Pulse width is the duration of optical pulse emission and is represented in units of length or time. The conversion factor is the speed of light in vacuum (300,000 km/s). For example, the width of a 1 fs pulse is 10-15 (s) × 3 × 108 (m/s) = 3 × 10-7 (m) = 0.3 µm.

*4 High-order harmonic generation
When a target such as a noble gas is irradiated with a high-intensity laser, light with a wavelength one-nth of that of the laser is generated (n, integer).


For more information, please contact:
(Presenter) *Please contact the presenter regarding the research content.
Takashi Tanaka (chief scientist), Advanced X-ray Laser Laboratory, RIKEN SPring-8 Center
TEL: +81-(0)791-58-0802 (ext. 3806)
E-mail：mail1

(Contact office)
Press Office, Public Relations Division, RIKEN
TEL：+81-(0)48-467-9272　FAX：+81-(0)48-462-4715
E-mail：mail4