This study is based on a two-diamond energy level system of cesium atoms described as 6S_1/2→6P_3/2→6D_3/2→7P_3/2→6S_1/2 and 6S_1/2→6P_3/2→6D_3/2→7P_1/2→6S_1/2. When two high-power pump laser beams with wavelengths of 852 nm and 921 nm simultaneously act on the 6S_1/2→6P_3/2 and 6P_3/2→6D_3/2 ultrafine transition lines, a double four-wave mixing process occurs in a heated cesium-atom vapor chamber, and two coherent blue light beams with wavelengths of 455 nm (7P_3/2→6S_1/2) and 459 nm (7P_1/2→6S_1/2) are generated through frequency conversion. The two beams are experimentally studied in the absence of gravity-pump light. The dependence of pump laser detuning, power, and cesium-atom gas chamber temperature on the intensity of the two generated blue light beams is also studied.

In this experiment, the 921 nm pump light was generated by a continuously tunable titanium-doped sapphire laser (with a maximum output power of 1 W), while the 852 nm pump light was provided by an external cavity diode laser. After amplification, the maximum output power was 1.2 W. Two sets of half wave plates (HWPs) and polarizing beam splitters (PBS) were used to process these two laser beams. Subsequently, the 921 nm and 852 nm pump lights were reflected by mirror M3 and coincided at the dichroic mirror (DM), propagating in the same direction. After passing through a quarter-wave plate (QWP), both laser beams changed from linearly to circularly polarized light with the same rotation direction, ensuring the highest efficiency in blue light generation. Telescopic systems composed of two lenses with a focal length of 100 mm were placed in front of and behind the cell, with the cell placed at the confocal center to increase the power density acting on the medium. Finally, the angles of the two pump lights were adjusted to satisfy the angle phase-matching condition, and the temperature control system of the Cs atomic vapor chamber was adjusted to satisfy the phase-matching condition.

First, we analyze the influence of the pump laser power and temperature of the cesium vapor cell on the power of the two generated blue lights. Figures 2(a) and (b) show the relationship between the power of the 852 nm and 921 nm pump lasers and the power of the generated blue light, respectively, while Fig. 4 shows the curves of the power of the two blue lights changing with the temperature of the cesium vapor cell. We conclude that when the power of the pump lasers is high and there is no 895 nm repumping light, the power of the generated blue light depends on temperature, which first increases and then decreases. It also depends on the power of the pump lasers, which first increases and then reaches a specific threshold.

The detuning of the pump laser is varied to observe its effect on the blue light. Figure 3 shows the dependence of the intensity of the coherent blue lights at 455 nm and 459 nm on the frequency detuning of the 852 nm and 921 nm pump lasers. We conclude that the frequency detuning range of the pump laser corresponding to 455 nm is wider than that of the pump laser corresponding to 459 nm, and the intensity is higher. The small frequency detuning range of the pump laser corresponding to 459 nm indicates that under this condition, it is easier to generate 455 nm blue light than 459 nm blue light.

In this experiment, based on the two energy level systems of cesium atoms, 852 nm and 921 nm pump lasers were used to generate two 455 nm and 459 nm blue light beams through a four-wave mixing process under phase-matching conditions in a cesium vapor cell. The intensity of blue light was measured under angle and temperature phase-matching conditions, with the maximum power of 455 nm and 459 nm blue laser light reaching 3.03 mW and 4.5 mW, respectively, when the temperature of the cesium vapor cell was 110 ℃, the power of the 921 nm pump light was 150 mW, and the power of the 852 nm pump light was 460 mW. The conversion efficiency was 10.91%/W. We also compared the experimental results with the addition of a single 895 nm repumping light, which showed the same trend of light intensity change as the pump light and provided a more straightforward experimental operation to verify the generation of blue light by the four-wave mixing process in cesium atoms.