RESUMO
In this paper, we study the formation of optical bistability (OB) and optical multistability (OM) in a degenerate four-level atomic system by an external magnetic field that is excited by a probe laser field, a coupling laser field and a signal laser field. The coupling field can cause electromagnetically induced transparency (EIT) for the probe field in the atomic medium, while the signal field and/or external magnetic field can switch between single-EIT and two-EIT regimes. Based on these properties, OB and OM effects can be formed at two different frequency regions of the probe field (two channels). By adjusting the magnetic field or the intensity and the frequency of laser fields, the threshold intensity and the width of OB or OM can also be changed simply. The model can be useful for experimental observations and applications in modern photonic devices.
RESUMO
We report an experimental measurement of giant group index in the 85Rb atomic medium at room temperature via a four-level V-type scheme on the D2 line. Our experiment uses co-propagation configuration of probe and coupling lasers through an atomic sample. In this configuration, three sharp EIT windows with significant transparency depths are observed on the probe absorption spectrum. By establishing a Mach-Zehnder interferometer, we measure the dispersive curve and hence obtain the group index curve with three enhanced positive peaks at the locations of the EIT windows, interspersed with negative peaks. The amplitude of the group index curve is increased as the temperature decreases and is decreased as the temperature increases. We estimate from our experimental results the good values of the group index to be ng = 5.8 × 104 (slow light regime) and ng = -4.0 × 104 (fast light regime). We also show that the experimental measurements are in good agreement with the theoretical results.