Observation of superfluorescenceEin helium gas following excitation with extreme ultraviolet free-electron laser pulses
Oct. 21, 2011
A team of researchers from JAEA, RIKEN, the Institute for Molecular Science (IMS), and JASRI, have been successful in observing superfluorescence in helium atoms using the extreme-ultraviolet free-electron laser (FEL) facility at RIKEN/SPring-8. Superfluorescence is a phenomenon in which a large number of atoms in excited states decay simultaneously, resulting in an extremely short and intense pulse of radiation. The research team was led by Mitsuru Nagasono (RIKEN), James Harries (JAEA), and Hiroshi Iwayama and Eiji Shigemasa (IMS). As well as being of fundamental interest as one of the first demonstrations of quantum optical effects at short wavelengths, there are potential applications in fine-tuning the characteristics of (X)FEL radiation, and increasing the sensitivity of analytical fluorescence techniques.
An atom in an excited state is usually unstable, and decays to a state of lower energy by emitting radiation. This happens on a timescale defined by the transition ratesEout of the upper state. However when the distance to neighbouring atoms is comparable to the wavelength of the emitted radiation the atoms can decay collectively rather than individually, with a much increased rate and consequently a much larger peak intensity. The resulting burst of radiation is termed superfluorescenceE and shares many characteristics with laser emission, including a high level of coherence. In this work superfluorescence was observed on the 1s3p to 1s2s helium transition, at a rate (number of photons emitted per second) around 75 000 that which occurs for a single atom decaying on the same transition.
The researchers focussed free-electron laser radiation with a wavelength of 53.7 nm to a spot inside a helium gas cell, and observed intense superfluorescence pulses at a wavelength of 501.6 nm (a blueish green colour) from about 1011 (100 000 million) atoms. They were successful in determining the duration of the emitted pulse for different gas pressures, and also in measuring the characteristic delay which occurs following the excitation pulse. This delay time can be thought of as the time for the collective transition to develop, and is in stark contrast to the behaviour of single atoms. Superfluorescence has previously been observed at visible wavelengths (400 nm E700 nm) and longer, but this work is the first to use excitation in the extreme ultraviolet region (<~ 100 nm). The work hints at the possibility of observing X-ray superfluorescence using the new X-ray free-electron laser SACLAE which will begin user operations in March 2012.
The results of the research have been published in the journal Physical Review Letters and highlighted as an Editors SuggestionEEan indication that the paper is of significant interest to readers from a wide range of fields.
Reference: Observation of Free-Electron-Laser-Induced Collective Spontaneous Emission (Superfluorescence)E M Nagasono, J R Harries, H Iwayama, T Togashi, K Tono, M Yabashi, Y Senba, H Ohashi, T Ishikawa, and E Shigemasa. Physical Review Letters 107, 193603 (2011) [5 pages].
For more information:
Original press release (Japanese)
Video of superfluorescence pulses
SACLA XFEL at RIKEN
Related article on physorg.comEbr>
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