Formation of Monochromatic Atomic Beam for Nanolithography

在激光冷却或囚禁、原子光学、原子干涉仪和原子光刻中，光诱导原子力被有效地用来操纵中性原子的运动。在这些实验中，要求强流原子束具有窄的纵向速度分布。在原子干涉仪中，原子束的速度扩展会降低干涉对比度。在原子光刻的情况下，通过使用单色原子束来获得最终窄的线宽。因此，单色原子束的制备在各个科学和技术方面都是非常重要的。本研究发展了一种中性原子束速度谱仪，用以制备单色原子束。为了根据其纵向速度分散相对快的原子束，已经使用具有大磁梯度的磁四极透镜。该装置由NdFeB永磁体制成。四极中的磁场梯度被测量为足够大（1 T/cm）并且沿径向方向沿着均匀。用单模可调谐激光器测量了Li原子束通过磁四极透镜后的共振荧光光谱。从荧光光谱的峰位移来看，磁透镜对Li原子施加的力是重力加速度的8160倍。用该谱仪测量了800℃热Li束流的纵向速度分布，与荧光谱的结果符合得很好。利用所研制的速度谱仪，可以在一个紧凑的实验装置上容易地形成速度展宽小于0.1%的1000 m/s的单色Li原子束。

Light induced force on atoms has been effectively used to manipulate neutral atomic motion in laser cooling or trapping, atom optics, atom interferometer, and atom lithography. In these experiments, an intense atomic beam with narrow longitudinal velocity distribution has been required. For the atom interferometer, the interference contrast is reduced by the velocity spread of the atomic beam. In the case of atom lithography, the ultimately narrow linewidth is obtained by using a monochromatic atomic beam. Therefore, preparation of a monochromatic atomic beam is quite important in various scientific and technological aspects. In this study, a velocity spectrometer for a neutral atomic beam has been developed to prepare a monochromatic atomic beam. In order to disperse a relatively fast atomic beam according to its longitudinal velocity, a magnetic quadrupole lens with a large magnetic gradient has been used. The device was made by NdFeB permanent magnets. The magnetic gradient in the quadrupole was measured to be sufficiently large (1T/cm) and uniform along radial direction. The resonance fluorescence spectra of the Li atomic beam after passing through the magnetic quadrupole lens have been measured by using a single mode tunable laser. From the peak shift of the fluorescence spectrum, the exerted force on Li atoms by the magnetic lens was 8160 times as large as the acceleration of gravity. Using the present spectrometer, we have measured the longitudinal velocity distribution of the thermal Li beam at 800℃, which was in good agreement with the result from the fluorescence spectra. It is expected that a monochromatic Li atomic beam of 1000m/s with the narrower velocity spread than 0.1% can be easily formed with a compact experimental arrangement using the developed velocity spectrometer.