NOVEL LASER COOLING TECHNIQUE EFFICIENT IN OPTICAL TRAPS

为了将87Rb原子光学冷却到低温以下，使其被限制在光阱中，我们制作了两个频率稳定的单模连续波激光器，其频率接近于5s 2S1/2（F=2）→5p 2P3/2（F=3）和5s 2S1/2（F=1）→5p 2P3/2（F=2）跃迁。两种激光器的激光频率宽度均小于1 MHz，这是铷原子成功激光冷却的必要条件。我们还制作了一种新的超高真空（UHV）室，用于长时间捕获冷原子，以观察光阱中的蒸发冷却过程。利用这台新的实验装置，我们操作了双磁光阱系统。首先，在室温下从原子蒸气中磁光收集Rb原子，并在低于2 mK的磁光阱（MOT）中进行光学冷却。冷原子从MOT中释放出来，并在推射激光束的辐射压力下转移到UHV室中的另一个MOT中。重复这一加载过程约200次，我们在1 mK的温度下将大约8×10_8原子限制在UHV-MOT中。为了有效地将冷原子限制在光阱中，我们进一步通过偏振梯度冷却对原子进行光学冷却，并获得了100 μK的温度，这与典型的光阱深度相当。通过监测磁阱的寿命和热化率，我们还估计了由于背景气体引起的原子加热和损失率。得到的寿命约为1 min，通过rf蒸发冷却，捕获的原子在2 min内从1 μK冷却到10 μK。从这些事实我们成功地证实，即使在光阱中，进一步的激光冷却也可以用来对抗阱的加热和损耗。

In order to optically cool 87Rb atoms below such a low temperature as to be confined in an optical trap, we made two frequency-stabilized single-mode cw lasers whose frequencies were nearly resonant to the 5s 2S1/2(F=2)→5p 2P3/2(F=3) and 5s 2S1/2(F=1)→5p 2P3/2(F=2) transitions. The laser frequency width less than 1 MHz, which is necessary for the successful laser cooling of Rb atoms, was achieved for both lasers. We also made a new ultra-high vacuum (UHV) chamber for trapping cold atoms for a long enough time to observe evaporative cooling process in the optical trap.With this new experimental apparatus we operated the double magneto-optical trap system. Initially, Rb atoms were magneto-optically collected from the atomic vapor at a room temperature and optically cooled in the magneto-optical trap (MOT) below 2 mK. The cold atoms were released from the MOT, and were transferred into the other MOT in the UHV chamber by the radiation pressure of the pushing laser beam. Repeating this loading procedure about 200 times, we confined about 8×10_8 atoms in the UHV-MOT at a temperature of 1 mK. For efficiently confining the cold atoms into an optical trap, we further optically cooled the atoms by the polarization gradient cooling and achieved a temperature of 100 μK, which was comparable to the typical trap depth of an optical trap.We also estimated atom heating and loss rates due to background gases by monitoring a trap lifetime and a thermalization rate in a magnetic trap. The obtained lifetime was approximately 1 min, and the trapped atoms were cooled from 1 mK to 10 μK in about 2 min by the rf evaporative cooling. From these facts we successfully confirmed that even in an optical trap further laser cooling could be applied against the trap heating and losses.