Six-wave mixing spectroscopy in a Doppler-broadened cascade four-level system

Abstract

We study six-wave mixing (SWM) spectroscopy based on electromagnetically induced transparency in a Doppler-broadened cascade four-level system. It is found that the SWM spectra are extremely sensitive to the configuration of the incident beams, where the linewidth can be either Doppler-free or very broad, due to the polarization interference of atoms of different velocities. This polarization interference can be controlled in the presence of a strong coupling field. Moreover, SWM can be employed as a new type of Doppler-free Autler–Townes (AT) spectroscopy, which has better spectral resolution than conventional AT spectroscopy. Finally, we also reveal the intrinsic connection between frequency-domain SWM spectroscopy and time-domain photon echoes through studying the time-domain correspondence of SWM.

Effects of Doppler broadening on Autler-Townes splitting in six-wave mixing

“…As mentioned in Ref. [22], in the case of 1 = 2 = 0, the SWM spectrum exhibits a doublet structure as 3 is scanned when ζ 2 < 1; however, there is only a single peak in the SWM spectrum when ζ 2 > 1 (see Fig. 2 in Ref.…”

“…2 in Ref. [22]). This can be explained by using the frequency-detuning dependence of the resonant velocities v ± .…”

“…(9), to observe the resonance originating from v + = v − it is required that ζ 2 > 1. Also, in a cascaded four-level system, as 3 is scanned the SWM spectrum exhibits a doublet structure only when ζ 2 < 1 [22]. To reveal the underlying physics, in this section we shall employ the dressed-state model to study how the Doppler frequency shift affects the AT splitting.…”

“…Moreover, higherorder wave mixing has attracted much attention [17][18][19][20]. In our previous works, we studied the AT splitting in electromagnetically-induced-transparency-based (EIT-based) six-wave mixing (SWM) in a Doppler-broadened system [21,22]. An AT doublet appears in the SWM spectrum when atoms with specific velocities have double resonance with one of the dressed states and with a third level.…”

“…We have also employed the dressed-state model to explain the strong wave-number-ratio dependence of the AT splitting. For example, in the presence of a strong coupling field the SWM spectrum exhibits either a single peak or an AT doublet, depending on the ratio between the magnitudes of the wave vectors [22]. In this paper, we analyze the velocities at which atoms are in resonance with the dressed states through Doppler frequency shifting.…”