Objective Recently, the study of integrated photonic devices is an important field of investigation, especially the photonic chip loaded with an atomic medium is one of the research hotspots. Based on the atomic medium, the integration of optical non-reciprocity, optical storage, and atomic clocks can be achieved. This requires an efficient interaction between light and atoms, which means that a certain optical depth is necessary and it is difficult to achieve at room temperature. The optical depth of an atomic medium can be increased by heating, but it is limited to the size of the photonic chip, which is not suitable in this scheme. Therefore, the use of light-induced atomic desorption (LIAD) is necessary, which does not require a heating system and it is beneficial to the integration of photonic chips.

LIAD is an impressive phenomenon observed in a sodium-vapor glass cell, whose inner surface is coated with a thin siloxane film. It has been observed in experiments that an incoherent light illuminates a cell filled with alkali atoms and these atoms adsorbed on the inner surface fall off. Thus, LIAD can effectively increase the atomic concentration in the vapor cell, which is completely different from the thermal effect, and many parameters influence the effect of light-induced atomic desorption, such as the intensity and frequency of desorption light, the atom species, the geometry of the cell, and the inner surface morphology. In this paper, it is found that the atomic concentration increases due to the LIAD effect and photothermal effect. The influences of two mechanisms are experimentally studied, which is of great significance to the research of photonic chips.

Methods A 150 mm diameter sphere with a highly reflective coating on the inner surface is used to enhance the LIAD effect, and a 460 nm light-emitting diode (LED) array mounted on the sphere is used as the illumination source. The power of the desorption light is changed by adjusting the driving current of the LED. When LIAD is carried out in a rubidium vapor cell, it is found that the temperature of the cell rises, which means that the atomic concentration increases due to the LIAD effect and the photothermal effect. By measuring the photothermal temperatures under different desorption light powers, the increase of atomic concentration caused by the photothermal effect is distinguished, and thus the relative influences of pure LIAD and photothermal effect are obtained. In order to verify whether the heat conduction caused by the LED array affects the cell or not, a water cooler is used to cool the LED array in the experiment. Besides, a glass plate is used to isolate the heat convection, which can reduce the effect of heat radiation on the cell.

Results and Discussions With the increase of desorption light power, the effects of LIAD and photothermal effect are both increased (Fig. 4). When the intensity of desorption light is 15 mW/cm², the contribution of pure LIAD is close to 6 times that of the photothermal effect. In this case, the increase in atomic concentration caused by photothermal effect is basically negligible, and when the intensity reaches 80 mW/cm², the ratio of atomic concentration increase caused by pure LIAD to that caused by photothermal effect is close to 6:4, which means that the effects of these two mechanisms are comparable (Fig. 5).

By cooling the LED array with a water cooler and isolating the heat convection, the temperature of the rubidium cell is 1--2 ℃ higher than that without water cooling (Fig. 6), which means that the heat radiation caused by the LED array has little effect on the cell. Besides, the effect of heat convection is even negligible, which means that the increase of temperature in LIAD is mainly caused by the photothermal effect.

Conclusions LIAD is a useful method to increase alkali atomic concentration without heating. In this paper, it is found that when using desorption light with high power density, the combined action of pure LIAD and photothermal effect further enhances the increase in atomic concentration. Besides, when increasing the wavelength of desorption light, the LIAD effect is reduced, while the photothermal effect is undoubtedly increased. Therefore, by changing the wavelength of desorption light, the contribution of these two mechanisms can be adjusted. In addition, since the desorption light with different power densities corresponds to different temperatures, the temperature of the atom cell can be controlled by adjusting the power density of the desorption light. These results have important reference value for the research of integrated photonic chips.