Comparison of Ultranarrow Fluorescence and Four-Wave Mixing in Electromagnetically Induced Transparency Windows

We simultaneously observe the probe transmission, four-wave mixing (FWM) and fluorescence signals under dressing effects in a four-level -type atomic system for the first time. The physical mechanisms of three types of signals are explored by changing the frequency detuning and power of incident laser beams. Especially, the ultranarrow fluorescence signals have been experimentally obtained due to twice shearing by electromagnetically induced transparency (EIT) window, which are much narrower than the Doppler-free EIT window. Such ultranarrow fluorescence signals with very high coherence and monochromaticity have potential applications in the quantum correlation and narrow linewidth laser. Studying the FWM processes can find applications in optical switch and optical communication.