Circular Dichroism Analysis and Ellipticity Measurement in SERF Atom Ensemble

In the research of the interaction between light and atoms, ellipticity is an important characterization of the pump and absorption rates. In previous researches, it was generally believed that ellipticity only originated from the interaction between magnetic fields and polarized alkali metal atomic ensembles. In this article, we derive and analyze the circular dichroism based on density matrix and Maxwell equations during the interaction between magnetic fields, probe light, and alkali metal atomic ensembles in spin-exchange relaxation-free (SERF) regime. We construct an optical path for measuring ellipticity based on photo elastic modulator (PEM) to measure ellipticity. Under different experimental conditions, we compare the measurement results of ellipticity from circular dichroism effect and rotation angle from circular birefringence effect. The results indicates that the interaction between magnetic field and alkali metal atomic ensemble which is polarized by probe light is a new source of ellipticity. Then, we research the ratio of rotation angle to ellipticity, and the results indicates that ellipticity generated by the interaction between the magnetic field and the alkali metal atomic ensemble is an intrinsic characteristic, which is independent of external conditions. The confidence level of the fitting between the experimental results and the theoretical equation is 99.7%. This research can be applied to SERF magnetometers and co-magnetometers.