Observation of free-electron-laser-induced collective spontaneous emission (superfluorescence)

Nagasono, Mitsuru*; Harries, J.; Iwayama, Hiroshi*; Togashi, Tadashi*; Tono, Kensuke*; Yabashi, Makina*; Semba, Yasunori*; Ohashi, Haruhiko*; Ishikawa, Tetsuya*; Shigemasa, Eiji*

We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 113 excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ (10 photons) are observed, corresponding to a photon number conversion efficiency of up to 10%. We observe the peak intensity to scale as , and the pulse width and delay with respect to the pump pulse to scale as , where is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75000 times faster than the spontaneous 1312 decay rate. To our knowledge this is the first observation of superfluorescence following pumping in the extreme ultraviolet wavelength region, and extension of the technique to the generation of EUV and X-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.