Using EUV pulses from SACLA BL1 free-electron laser, we have successfully observed superfluorescence at wavelengths of 469 nm and 164 nm from a dense sample of excited helium ions, created by (1+1)-photon ionisation/excitation. The results of related numerical simulations will also be reported.
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.
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.