Formation of Monochromatic Atomic Beam for Nanolithography

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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.