Narrowing the absorption linewidth and its limitation in a four-level inverted-Y atomic system

Abstract

Depending on a four-level inverted-Y atomic system, we demonstrate the limitation of linewidth-narrowing for the probe absorption spectrum in the electromagnetic induced absorption platform. Thanks to the use of an auxiliary control field which couples one hyperfine ground state and one middle-excited state we show that the linewidth limitation can be constrained by a coherence decay rate between two hyperfine ground states, rather than by the decay rate between the ground and the excited states as in previous Ladder schemes. That fact makes the theoretically-predicted absorption linewidth at least two orders of magnitude narrower. By using a suitable adjustment for the control-field amplitude and the detuning we numerically show that an extremely-narrowed probe absorption spectrum accompanied by a higher spectra contrast can be obtained, which confirms well with our theoretical predictions. We study the transient time response to the absorption spectrum and show that a relatively longer response time arises due to the small coherence decay rate between two hyperfine ground states. Furthermore, we reduce the influence on linewidth-narrowing from the Doppler effect via an optimized design of lasers, and reveal that no Doppler-free effect exists due to the lack of three-photon process. Our results may pave a route to the development of high-resolution spectroscopy in current experiments.