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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum simulation of laser cooling for fabrication of nanometer scale pitch standard

Yi-Li Xiao, Yan Ma, Wen Wu, Weigang Gong, Tongbao Li

Published Aug 24, 2010• 0 citations

Abstract

A quantum mechanical Monte Carlo simulation is carried out for one-dimensional laser cooling of Chromium atoms. Chromium atom beam collimation via transverse laser cooling is essential for the fabrication of periodic nanometer scale structures by laser-focused atomic deposition. Such structures can be used as precision pitch standards for nanometer scale measurement and engineering. We explore the roles played by several factors, such as laser intensity and detuning, in the cooling process. Calculation results show different dependencies for cooling rate and ultimate temperature to be reached with given sets of parameters. Also, for atoms with different initial velocities in the transverse direction, the conditions under which optimal collimation effect is achieved can differ. Consequently, the current experimental setup, which utilizes LIF spots from marginal beams to monitor the laser collimation, can produce cooling effects which are not optimal for the end product.

DOI: 10.1117/12.886447

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A quantum mechanical Monte Carlo simulation is carried out for one-dimensional laser cooling of Chromium atoms. Chromium atom beam collimation via transverse laser cooling is essential for the fabrication of periodic nanometer scale structures by laser-focused atomic deposition. Such structures can be used as precision pitch standards for nanometer scale measurement and engineering. We explore the roles played by several factors, such as laser intensity and detuning, in the cooling process. Calculation results show different dependencies for cooling rate and ultimate temperature to be reached with given sets of parameters. Also, for atoms with different initial velocities in the transverse direction, the conditions under which optimal collimation effect is achieved can differ. Consequently, the current experimental setup, which utilizes LIF spots from marginal beams to monitor the laser collimation, can produce cooling effects which are not optimal for the end product.

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To meet the requirement of nanoscale dimensional metrology, lengthy standards with features below 100 nanometers are indispensable instruments. Our group has successfully fabricated length standards through atom lithography. For further improvement of the quality of these standards, laser cooling of Chromium atom beam was studied through a transverse Doppler cooling scheme. Moreover, utilizing the software developing kit (SDK) of CCD camera, we developed an image collecting software, which had functions of real-time display for gray-scale image and image measure. In our experimental setup, with the software, we could detect the laser induced fluorescence spots from marginal beams to monitor and analyze the effect of laser cooling.

We have studied laser collimation of a chromium atomic beam using a transverse polarization gradient cooling scheme. We present detailed measurements of the angular distribution of atoms on the beam axis, over a broad range of laser intensities and detunings, including those that produce significant excitation, and observe collimation angles as small as 0.16\ifmmode\pm\else\textpm\fi{}0.01 mrad (50% quantile). We compare our results with existing calculations based on assumptions of steady-state conditions and low excited-state population.

In this paper,a size detecting system based on Laser induced fluorescence(LIF)is designed to measure the Cr atomic beam divergence of laser collimation by using one-dimensional transverse Doppler cooling scheme.Under the parameters of cooling laser power of 45mW,interaction distance of 24mm and detuning of-0.5Γ,we observe the angular distribution of Cr atoms(50%quantile) on the beam axis is from 1.07±0.10mrad to 0.46±0.10mrad by laser collimation.we also present detailed measurements of collimation angles over different laser powers and compare and analysis our results with numerical calculation based on the semiclassical model.

We developed an atom-optical system that utilized a laser diode (LD) for both laser atom cooling and trapping. The LD is easy to maintain and operate, and is therefore suitable for engineering applications. The neutral atom beam of chromium was collimated by Doppler cooling and focused by a standing wave lens, and then directly deposited onto a silicon substrate. A nanograting structure with periodicity identical to the wavelength of the standing wave, 212.78 nm, was successfully achieved. The atom beam generation, collimating, focusing and deposition processes were experimentally and theoretically investigated.

The technology of laser-focused atomic deposition can be used to develop the nanostructure transfer standard of length.The atomic laser cooling is a key technique in atom lithography.A theoretical investigation of chromium atomic beam in optical traps is reported in this paper.The Doppler and sub-Doppler laser cooling forces are discussed and some characteristics of these forces are shown based on the semi-classical theory.The simulation results indicate that the atomic beam can be collimated by these laser cooling forces,especially by sub-Doppler laser cooling force and can reach the Doppler limit 120 μk,relative horizontal-speed 13.87 cm/s.