Coherent control of group index and magneto-optical anisotropy in a multilevel atomic vapor

We study electromagnetically induced transparency (EIT) in a heated potassium vapor cell, using a simple optical setup with a single free-running diode laser and an acousto-optic modulator. Despite the fact that the Doppler width is comparable to the ground state hyperfine splitting, transparency windows with deeply sub-natural line widths and large group indices are obtained. A longitudinal magnetic field is used to split the EIT feature and induce magneto-optical anisotropy. Using the beat note between co-propagating coupling and probe beams, we perform a heterodyne measurement of the circular dichroism (and therefore birefringence) of the EIT medium. The observed spectra reveal that lin∥lin polarizations lead to greater anisotropy than lin⊥lin. A simplified analytical model encompassing sixteen Zeeman states and eighteen Λ subsytems reproduces the experimental observations.