Physical Review A

Collective effects in the radiative decay of the 2P1 state in helium

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

Published Jul 20, 2018• 13 citations

Abstract

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.

DOI: 10.1103/physreva.98.013416

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Physical Review A

Collective effects in the radiative decay of the 2P1 state in helium

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

Published Jul 20, 2018• 13 citations

Publisher

Abstract

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.

DOI: 10.1103/physreva.98.013416

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Physical Review AJan 24, 20234 citations

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}

\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.

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.

1snp

excitation (

n=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.

ns

np

Rydberg states with principal quantum numbers $26\ensuremath{\le}n\ensuremath{\le}40$. Time-delayed state-selective field ionization is used to directly monitor the population in the initial and neighboring Rydberg levels. The measured time dependence of the Rydberg population is well described by numerical simulations which consider only spontaneous emission and population transfer by blackbody radiation. No evidence for collective decay is found at atom densities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This result is in contrast to a previous study [Wang et al., Phys. Rev. A 75, 033802 (2007)], in which superradiant decay was theoretically predicted and experimentally inferred for atom density and laser focal volume conditions very similar to those considered here. Suppression of collective emission is likely due to variations in transition energies within the atom sample, dominated by inhomogeneities in dipole-dipole exchange interactions for initial

s

states, or by a combination of dipole-dipole and electric field inhomogeneities for the initial

p

states.

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.