title: Observation of free-electron-laser-induced collective spontaneous emission (superfluorescence).
abstract: We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 1s(2) → 1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ are observed, corresponding to a photon number conversion efficiency of up to 10%. We observe the peak intensity to scale as ρ(2) and the emitted pulse width and delay to scale as ρ(-1), where ρ is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75,000 times faster than the spontaneous 1s3p → 1s2s 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 extreme ultraviolet and x-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.
authors.affiliation: RIKEN/SPring-8, Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan. nagasono@spring8.or.jp
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Nagasono
authors.forename: James R
authors.initials: JR
authors.lastname: Harries
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Iwayama
authors.forename: Tadashi
authors.initials: T
authors.lastname: Togashi
authors.forename: Kensuke
authors.initials: K
authors.lastname: Tono
authors.forename: Makina
authors.initials: M
authors.lastname: Yabashi
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Senba
authors.forename: Haruhiko
authors.initials: H
authors.lastname: Ohashi
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Ishikawa
authors.forename: Eiji
authors.initials: E
authors.lastname: Shigemasa