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Physical Review A

Amplification of fluorescence from the 3aPo1 doubly excited state in helium

Špela Krušič, K. Bučar, A. Mihelič, M. Žitnik

Published Jan 23, 2019• 4 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.99.013429

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We present a theoretical study of self-amplification of extreme ultraviolet light in a helium gas target. Resonant excitation to the 3a1Po3a\phantom{\rule{0.16em}{0ex}}^{1}P^{o}3a1Po doubly excited state below the N=2N=2N=2 threshold by 100fs100\phantom{\rule{0.28em}{0ex}}\mathrm{fs}100fs free-electron laser pulses is followed by fluorescence at 40.7eV40.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}40.7eV 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 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.
We present the results of a theoretical study of XUV superfluorescence from doubly excited states of helium resonantly pumped by free-electron laser (FEL) pulses. Our model allows us to predict both the spectrum and angular distribution of emitted XUV radiation in a wide range of experimentally accessible parameters. This is achieved by going beyond two key deficiencies of most previous models: The one-dimensional treatment in space is upgraded to three dimensions with electromagnetic fields treated in the paraxial approximation and spontaneous emission is modeled by a recently developed approach that avoids the unrealistic delayed response but preserves the expected characteristics of the emitted field in the spontaneous emission limit. The case study of 3a1Po3a{\phantom{\rule{0.16em}{0ex}}}^{1}{P}^{o}3a1Po resonance in helium with 63.66eV63.66\phantom{\rule{0.28em}{0ex}}\mathrm{eV}63.66eV 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.
We present a theoretical study of He 21P−−21S2\phantom{\rule{0.16em}{0ex}}^{1}P--2\phantom{\rule{0.16em}{0ex}}^{1}S21P−−21S emission from a helium gas, where the upper atomic state is prepared by a 100 fs laser pulse resonant with the 11S−−21P1\phantom{\rule{0.16em}{0ex}}^{1}S--2\phantom{\rule{0.16em}{0ex}}^{1}P11S−−21P transition. The fluorescence signal emitted in the forward direction at 2059 nm and the transmitted signal at 58.4nm58.4\phantom{\rule{0.28em}{0ex}}\mathrm{nm}58.4nm 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 21P2\phantom{\rule{0.16em}{0ex}}^{1}P21P state.
We have observed amplification of weak fluorescence from He atoms in the 3a 1 𝑃𝑜 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 μ⁢J 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.
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.
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.