Grating magneto-optical trap of cesium atoms with an additional retroreflected laser beam

Abstract: A magneto-optical trap of cesium atoms was generated by applying a circularly polarized cooling laser beam onto a reflective two-dimensional diffraction grating with an aperture and by retroreflecting the incident beam passing through the aperture while reversing the circular polarization. The cooling laser beams comprised the incident, retroreflected, and four diagonally diffracted beams at an angle of 50{\deg}. The intensity of the retroreflected beam was carefully adjusted to balance the radiation forces acting on the atoms. Despite the challenges posed by cesium atoms with high nuclear spin, a significant number of cold atoms ($7.0 {\times} 10^6$) were captured when the detuning and power of the incident beam were -10 MHz and 131 mW, respectively, with the intensity of the retroreflected beam set to 69 % of that of the incident beam. The importance of the retroreflected beam in the trapping process was highlighted when the intensity ratio was reduced to 24 %, resulting in the absence of trapped atoms. This underscores the significance of the retroreflected beam in the trapping process. Notably, the distribution of the cold atom cloud differed from other magneto-optical traps, as it was not centered in the region where the cooling beams overlapped. Instead, numerous cold atoms were observed when the cloud was positioned near the apex closer to the grating side and an edge line of the overlapping region. Therefore, trapping can be achieved with the assistance of attractive dipole forces exerted by the diffracted beams, which exhibits high intensities at these positions.