NOVEL LASER COOLING TECHNIQUE EFFICIENT IN OPTICAL TRAPS

为了将~（87）Rb原子光学冷却到能被囚禁在光阱中的低温以下，我们制作了两台频率与5s 2S_（1/2）（F=2）→ 5 p2P_（3/2）（F=3）和5s 2S_（1/2）（F=1）→ 5 p2P_（3/2）（F=2）跃迁近似共振的单模连续激光器。两种激光器的激光频率宽度小于1 MHz，这是成功的激光冷却Rb原子所必需的。我们还制作了一个新的超高真空（UHV）室，用于长时间捕获冷原子，观察光阱中的蒸发冷却过程，并利用这个新的实验装置运行了双磁光阱系统。最初，Rb原子在室温下从原子蒸气中磁光收集，并在磁光阱（MOT）中光学冷却至低于2 mK。冷原子从MOT中释放出来，并通过推动激光束的辐射压力转移到超高真空室中的另一个MOT中。在1 mK的温度下，重复加载约200次，我们在UHV-MOT中约束了约8×10_8个原子。为了有效地将冷原子限制在光阱中，我们进一步利用偏振梯度冷却对原子进行光学冷却，使原子温度达到100 μK，与光阱的典型阱深相当，并通过监测磁阱中的阱寿命和热化率，估算了原子加热和背景气体引起的原子损失率.所获得的寿命约为1 min，所捕获的原子通过射频蒸发冷却在约2 min内从1 mK冷却到10 μK。从这些事实中，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于防止阱加热和损耗。

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.