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Report No.

Superfluorescence observed following EUV-FEL excitation of helium gas

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

We are currently witnessing rapid progress in the extension of effects now commonplace with visible light lasers towards shorter and shorter wavelengths. Highlights include XFEL-pumped lasing, and electromagnetically-induced transparency. Superfluorescence shares similarities with ASE lasing, but occurs only if several specific conditions are met. We have observed and characterised superfluorescence at a wavelength of 501.7 nm following 1s1s$$rightarrow$$1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ (10$$^{11}$$) photons were observed, corresponding to a photon number conversion efficiency of up to 10%. We observed the peak intensity to scale as $$rho^2$$, and the pulse width to scale as $$rho^{-1}$$, where $$rho$$ is the atom number density. Emitted pulses as short as 1 ps were observed.



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