Electromagnetically induced absorption spectra depending on intensities and detunings of the coupling field in Cs vapour

We have observed electromagnetically induced absorption (EIA) spectra depending on intensities and detunings of the coupling field in the cycling transition (6S_1/2, F = 4↔6P_3/2, F' = 5) of the Doppler-broadened Cs atomic system. With a weak coupling field, we were able to see an EIA spectrum with a subnatural linewidth. On increasing the intensity of the coupling laser, we found that a transmission dip produced by an electromagnetically induced transparency appeared at the centre of the EIA structure, which is the first time that this has been observed. By detuning the frequency of the coupling laser, we were able to obtain a clear dispersion-like spectrum for blue-side detuning, but we obtained a complicated shape for red-side detuning because of the influences of other hyperfine levels. These results were compared qualitatively with theoretical curves obtained from a simple model.