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Journal of Physics B Atomic Molecular and Optical Physics

Time-resolved observation of superfluorescence at a wavelength of 164 nm from helium ions

James Robert Harries, Yoshiro Azuma, Arisa Iguchi, Ichiro Inoue, Hiroshi Iwayama, Shigeki Owada, E. Shigemasa, Tadashi Togashi, Susumu Kuma

Published Mar 11, 2026• 0 citations
Publisher

Abstract

Abstract We report the results of an experiment to study the temporal properties of superfluorescence at a wavelength of 164 nm emitted from helium ions excited to either the 3p state or 4p state by pulses from a free-electron laser. The two different processes (direct 3p–2s superfluorescence; two-step cascade 4p–3s–2p or 4p–3d–2p superfluorescence) are compared to each other and to the results of numerical simulations.

DOI
10.1088/1361-6455/ae50bf

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Abstract We report the results of an experiment to study the temporal properties of superfluorescence at a wavelength of 164 nm emitted from helium ions excited to either the 3p state or 4p state by pulses from a free-electron laser. The two different processes (direct 3p–2s superfluorescence; two-step cascade 4p–3s–2p or 4p–3d–2p superfluorescence) are compared to each other and to the results of numerical simulations.
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.
Abstract We report the observation of superfluourescence at wavelengths (in air) of 501.6 nm, 667.8 nm, and 728.1 nm following the excitation of helium atoms with free-electron laser pulses at wavelengths of 53.7 nm ( n = 3 excitation) and 52.2 nm ( n = 4 excitation). The observed wavelengths of the superfluorescence pulses correspond to 1s3p–1s2s, 1s3d–1s2p, and 1s3s–1s2p transitions. Observation of superfluorescence on these transitions implies either competing cascade decays, or direct excitation of non-dipole allowed transitions. We have studied the time structure of the emitted pulses using a streak camera, and the results cannot be explained by straightforward considerations using the usual model for two-level superfluorescence.
We report the probable observation of superfluorescence at a wavelength of 164 nm from helium ions created and excited using pulses from the EUV free-electron laser at SACLA BL1. The emission is the second step of a superfluorescence cascade from excited 4p states. To our knowledge this is the shortest wavelength at which superfluoresence has ever been demonstrated. The high intensity of the emission and the narrow angular divergence strongly support the interpretation as superfluorescence, and numerical simulations will also be presented which support this.