Physical Review Letters

Amplification of Spontaneous Emission from Doubly Excited He Atoms

Janez TurnŠEk, Špela Krušič, A. Mihelič, K. Bučar, Laura Foglia, Riccardo Mincigrucci, Marija Krstulović, Marcello Coreno, Gabriele Bonano, Kevin Charles Prince, Carlo Callegari, Alberto Simoncig, Zeinab Ebrahimpour, Ettore Paltanin, Andrei Benediktovitch, Roberto Osellame, Anna Gabriella Ciriolo, Rebeca Martìnez Vàzquez, C. Vozzi, Emiliano Principi, M. Žitnik

Published Aug 26, 2025• 0 citations

Publisher

Abstract

We have observed amplification of weak fluorescence from He atoms in the 3a ^{1}P^{o} autoionizing state, which decay by spontaneous emission of 40.74 eV photons in one out of 1600 cases. On average, a 4.1±1.6% conversion of 63.67 eV photons was detected in the forward direction at 20 μJ pump pulse energy and 40 mbar He gas pressure. A comparison with state-of-the-art theoretical simulations shows an unexpected spectral line shift at high gas pressure that is ascribed to transient molecular and electron scattering effects.

DOI: 10.1103/mg5s-xvct

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Physical Review Letters

Amplification of Spontaneous Emission from Doubly Excited He Atoms

Published Aug 26, 2025• 0 citations

Publisher

Abstract

DOI: 10.1103/mg5s-xvct

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Physical Review ANov 23, 200518 citations

We present a theoretical study of self-amplification of extreme ultraviolet light in a helium gas target. Resonant excitation to the

3a\phantom{\rule{0.16em}{0ex}}^{1}P^{o}

doubly excited state below the

N=2

threshold by

100\phantom{\rule{0.28em}{0ex}}\mathrm{fs}

free-electron laser pulses is followed by fluorescence at

40.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}

photon energy. The process is modeled by a three-level atomic system coupled to the continuum and described by the Maxwell-Bloch equations. The evolution of state populations and radiation fields is studied over a wide range of pump intensities and target pressures, clearly showing the regions of spontaneous emission, amplification, and saturation of light emitted in the forward direction. The treatment includes both coherent and SASE pump pulses. In both cases the amplified pulses are temporally coherent and exhibit a duration similar to that of the pump pulses.

We present a theoretical study of He

2\phantom{\rule{0.16em}{0ex}}^{1}P--2\phantom{\rule{0.16em}{0ex}}^{1}S

emission from a helium gas, where the upper atomic state is prepared by a 100 fs laser pulse resonant with the

1\phantom{\rule{0.16em}{0ex}}^{1}S--2\phantom{\rule{0.16em}{0ex}}^{1}P

transition. The fluorescence signal emitted in the forward direction at 2059 nm and the transmitted signal at

58.4\phantom{\rule{0.28em}{0ex}}\mathrm{nm}

are followed in time starting with the low pump intensity regime where the single-atom spontaneous emission results in a characteristic exponential decay. By increasing the gas density and pump intensity, collective effects such as self-amplified spontaneous emission and superfluorescence modify the temporal dependence of the radiation field by amplifying the radiation pattern as well as changing its delay and intensity distribution. The extended Maxwell-Bloch simulation of the corresponding three-level $\mathrm{\ensuremath{\Lambda}}$ system in one spatial dimension covers emission delays of up to 2.4 ns, reaching beyond the natural lifetime of the

2\phantom{\rule{0.16em}{0ex}}^{1}P

state.

3a{\phantom{\rule{0.16em}{0ex}}}^{1}{P}^{o}

resonance in helium with

63.66\phantom{\rule{0.28em}{0ex}}\mathrm{eV}

excitation energy is presented for realistic parameters of seeded light pulses from the FERMI FEL facility and a recently developed high-pressure gas cell. Results of numerical simulations show that both the spectral width and angular divergence of emitted radiation vary with gas pressure and pump pulse intensity in a complex way.

The doubly excited singlet states of He below the

N=2

threshold may decay by autoionization or fluorescence. In the fluorescence decay channel, most decay cascades consist of emission of three photons, of which the first is a VUV photon, the second is in or near the visible range, and the last is another VUV photon. We have studied the fluorescence channel decay dynamics of the

(2,{0}_{\mathrm{n}})

(2,{1}_{\mathrm{n}})

and $(2,\ensuremath{-}{1}_{\mathrm{n}})$

^{1}\mathrm{P}

n=3--7

, states by wavelength dispersed photon-induced fluorescence spectroscopy. We have detected the photons in the second step of the cascade and determined the branching ratios for the strongest lines in this step. From these data we are able to calculate the branching ratios of the first step in the cascade. The results are in good agreement with calculations of the main decay channels of the higher resonances, but about 20--30 % lower, and so we are able to describe quantitatively the whole fluorescence cascade of the above-mentioned doubly excited states.