Observing Superradiant Decay of Excited-State Helium Atoms Inside Helium Plasma

Abstract

Using femtosecond transient absorption spectroscopy on excited-state helium atoms in a plasma created through optical field ionization, we measured the decay of 2(3)S-2(3)P excitation with sub-ps temporal resolution. The population evolution shows that initial decay is significantly faster than the electron-atom collisions and three orders of magnitude faster than the single atom spontaneous decay rate. This indicates on superradiant coherent behavior of the atomic system inside the plasma.

Ultrafast dynamic evolution of multilevel systems in medium-strength laser fields

“…In the probe-pump scenario, there is a phase lag e iω τ between the emitted field and probe pulse, because the probe pulse comes first, but emission of the third-order polarization starts only after the last interaction. This phase lag will cause time-dependent patterns in the absorption spectrum, which is referred to as perturbed free induction decay (PFID) [33][34][35][36]. In the probe-pump scenario, t 1 =τ, t 2 =0, and t 3 =t will be used.…”

The Dawn of Quantum Biophotonics

“…A small population in the excited state can thus lead to net gain, and this was the subject of substantial theoretical work by us several groups in the 1980s [24][25][26]. We have carried out the first LWI demonstrations in the mid-1990s [54,55], and have recently continued in theoretical and experimental fronts to investigate the possibility of coherence driven lasing and lasing in XUV and soft X-ray regions [56,57].…”

Enhanced fluorescence emission of helium atoms by seeded optically driven impact excitation

“…Recently, the excited-state lifetime with subpicosecond temporal resolution was measured by using femtosecond transient absorption spectroscopy on excited-state helium atoms in plasma, and thereby demonstrated the superradiant coherent behavior of the helium atomic system [4]. However, the superfluorescence intensity was limited by the longitudinal length of plasma, typically on the scale of Rayleigh distance.…”

“…Alternatively, femtosecond laser filamentation could dramatically extend plasma channels [5]. On the other hand, certain atomic coherence is anticipated to enhance XUV-like lasing gain [4]. Nevertheless, this effect may be greatly affected by electron-ion collisions in plasma channels.…”