Laser cooling of a rubidium atomic beam

: towards a high-brightness ion beam for nm-resolution focused ion beam milling

Abstract

The Atomic Beam Laser-cooled Ion Source (ABLIS) is a new ion source concept that, after integration in a Focused Ion Beam, should enable milling in integrated circuits with nanometer resolution.It is based on photo-ionization of a thermal rubidium atomic beam that is collimated and compressed by near-resonant laser light. This thesis consists of a theoretical study to the so-called repump laser field, followed by laser cooling experiments.Laser cooling of rubidium requires multiple laser frequencies as a result of hyperfine splitting. To understand the role of the so-called repump laser field in laser cooling, a rate equation model has been made to study the population dynamics of the six relevant energy levels of rubidium through the laser cooling stage. The model shows that a repump field with 10% of the intensity of the cooling laser field is sufficient, but vital for successful cooling. This is confirmed by Monte Carlo simulations and experiments.In the experiment, we study the one-dimensional cooling of a Rb atomic beam travelling through two counterpropagating, near-resonant laser beams with orthogonal linear polarizations. We estimate the transverse temperature of the atomic beam by comparing the spatial distribution of its laser-induced fluorescence at two positions along the atomic beam line. We vary the detuning ?c saturation parameter sc, length L of the cooling stage to find the parameters for optimal laser cooling.We observe an atomic beam with a transverse temperature of T = 400 µK for ?c = -g/2, sc = 1.7 and L = 80 mm. This temperature is a factor 1.5 higher than the respective doppler limit, but a factor 4 lower than calculated by Monte Carlo doppler cooling simulations. This is attributed to polarization-gradient cooling, due to the lin-perp-lin polarization scheme. We therefore expect that sub-doppler cooling will occur for higher laser intensities.

Date of Award	31 Oct 2014
Original language	English
Supervisor	S.H.W. Wouters (Supervisor 1), G. ten Haaf (Supervisor 2) & Edgar J.D. Vredenbregt (Supervisor 2)