Continuous loading of a magneto-optical trap of Rb using a narrow transition

We report continuous loading of ${}_{\text{87}}\mathrm{Rb}$ atoms in a magneto-optical trap (MOT) at narrow linewidth, 420-nm $5S_{1/2}$, $F=2\to 6P_{3/2}$, $F=3$ blue transition (blue MOT). Continuous loading of the blue MOT is achieved by superimposing the blue laser beam inside a hollow core of infrared (IR) laser beam driving the broad $5S_{1/2}$, $F=2\to 5P_{3/2}$, $F=3$ transition at 780 nm. We typically load $\sim {10}^8$ atoms in the blue MOT in 2.5 s. We characterize the continuous loading of the blue MOT with various parameters such as magnetic-field gradient, detuning, power and diameter of the blue MOT beam, and diameter of the hollow core (spot) inside the IR MOT beam. We observe that the blue laser beam should overfill the spot of the IR laser beam. This is because the blue laser cools the atoms to a lower temperature even in the presence of the broad IR laser, i.e., in the overlapped region, and hence helps in loading. To support the experimental result, we also present the theoretical framework for cooling atoms in the presence of two lasers driving two different transitions simultaneously. This method of continuous loading of the blue MOT can be useful to produce a continuous atomic beam of cold Rb atoms.