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

We present a theoretical study of He 2 P-1-2 S-1 emission from a helium gas, where the upper atomic state is prepared by a 100 fs laser pulse resonant with the 1 S-1-2 P-1 transition. The fluorescence signal emitted in the forward direction at 2059 nm and the transmitted signal at 58.4 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 A system in one spatial dimension covers emission delays of up to 2.4 ns, reaching beyond the natural lifetime of the 2 P-1 state.