Recently, an increasing number of frequency-stabilized deep ultraviolet (DUV) lasers have been used in research on the laser cooling of atoms and ions, such as H, Hg, Cd, Mg, Be⁺, In⁺, and Mg⁺. Mercury atoms, which are the heaviest stable laser-cooled atoms, have been widely studied owing to their unique nature. Mercury atoms have a low sensitivity to blackbody radiation, and the mercury lattice clock is one of the recommended secondary representations of the second in the international system of units. It is also a good candidate to test the variation in the fine-structure constant and to measure the permanent electric-dipole moment of the electron. For the laser cooling of neutral mercury atoms, it prefers to adopt a two-dimensional magneto-optical trap (2D-MOT) plus three-dimensional magneto-optical trap (3D-MOT) configuration to improve the loading rate. This configuration requires a higher deep ultraviolet laser power, which is limited by DUV damage to optical elements. Here, we present a frequency stabilization laser system based on an optical phase-locked loop (OPLL) between infrared seed lasers, which can easily adjust the laser frequency over a wide range and efficiently use DUV laser power.

The ¹S₀→³P₁ transition at 253.7 nm is used for the laser cooling^{of mercury atoms. In the 2D-MOT plus 3D-MOT configuration, the frequencies of the 2D-MOT, push beam, and 3D-MOT must be independently adjustable. Therefore, in this study, two 253.7 nm frequency-quadrupled DUV lasers are used, and a frequency-stabilized DUV laser system with optical phase-locked loop technology is developed for the laser cooling of neutral mercury atoms. One DUV laser is locked on the saturated absorption spectroscopy and is used as the cooling laser of the 2D-MOT. Three acousto-optic modulators (AOMs) are used to set the detuning of each beam. Another DUV laser is frequency-stabilized by an OPLL between the semiconductor seed lasers at 1014.9 nm and is used as the cooling laser of the 3D-MOT without passing through any additional frequency shifter. A feed-forward method is adopted to reduce the frequency switch time because the cooling laser for 3D-MOT is also used for fluorescence detection.}

Two DUV cooling lasers are phase-locked by the OPLL method (Fig. 3), and the frequency drift in the long term is markedly suppressed. After being locked on the saturated absorption spectroscopy, the frequency fluctuation of the DUV laser is less than 350 kHz, and the relative frequency instability between the two DUV lasers is reduced to 30 mHz at an average time of 1000 s (Fig. 5) by the OPLL. A phase frequency detector (PFD) is used as the phase discriminator of the OPLL. It has a broad capture range, and the loop bandwidth can be easily controlled. Therefore, the frequency offset between the two DUV lasers can be adjusted within 2 GHz, and the DUV laser linewidth is measured with weak frequency locking (Fig. 4). Using the feed-forward method, the frequency switch time of the 3D-MOT cooling laser is found to be 0.15 ms with 100 MHz frequency shift, which is 1/23 of the original (Fig. 7). Therefore, the frequency stability and fast tunability of this cooling laser system meet the requirements of our 2D-MOT plus 3D-MOT configuration and the loading rate is 1×10⁶/s.

In our frequency-stabilized laser system, the frequency of the DUV laser can be easily adjusted over a wide range, the DUV laser power is efficiently used, and the system complexity is reduced. This frequency-stabilization scheme not only satisfies the laser cooling requirements of neutral mercury atoms but is also applicable to other atoms, such as Cd and Mg. Moreover, it can produce coherent DUV lasers, which can be used in experiments such as electromagnetic induction transparency, Raman sideband cooling, and atom interferometry.