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Superfluorescence (collective spontaneous emission) observed from helium atoms following excitation using an extreme-ultraviolet free-electron laser

ハリーズ　ジェームズ, ハリーズ ジェームズ

Published Jul 31, 2017• 0 citations
OpenAlex

Abstract

Using numerical simulations, and experiments at a free-electron laser, we have studied superfluorescence emission in helium atoms at visible wavelengths, and also observed strong directional emission at extreme ultraviolet wavelengths.

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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.
We have studied in detail the emission of superfluorescence at a wavelength of 469 nm (4p→3s) from helium ions created and excited using free-electron laser pulses from SACLA’s BL1. In earlier work we observed superfluorescence at visible wavelengths in neutral helium [1-5]. To observe superfluorescence from helium ions we used a wavelength of 24.3 nm, resonant with the He + 1s→4p transition and sufficiently short to ionise neutral helium atoms. The density of excited He + ions created by this sequential 2-photon absorption was sufficient to allow for the development of superfluoresence.
Using multilevel (161-level) numerical simulations beyond the rotating-wave approximation, we investigate the single-atom response of helium to intense pulses of radiation with properties characteristic of a self-amplified spontaneous emission (SASE) free-electron laser (FEL) source, and central wavelengths resonant with 1snp1snp1snp excitation (n=3n=3n=3,4). The results show a complete breakdown of the two-level approximation and suggest that, under typical experimental conditions, levels other than the resonant level are significantly populated. We discuss the implications for the subsequent development of superfluorescence (a collective effect) from the excited atomic medium and compare the results to recent experiments.