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Characterizing the AC-MOT

M. Anholm

Published Jan 1, 2014• 0 citations
Publisher

Abstract

Magneto-Optical Traps (MOTs) have long been used to produce samples of cold trapped neutral atoms, which can be used in the measurement of a variety of physical quantities and theories. Until recently, one limitation of this type of trap was the necessity for the presence of a relatively large magnetic field which would decay only slowly after the trapping mechanism was turned off. This residual magnetic field is expected to partially destroy any atomic polarization induced, for example, by optical pumping. As a result, the precision of any physical measurement which requires polarization is limited. We will discuss the construction of our version of a newer type of MOT, the AC-MOT [2], which is designed specifically so as to minimize residual magnetic fields. We have found that our AC-MOT has lifetimes and cloud sizes similar to those we measured in our DC-MOT. We intend to use a trap similar to this in upcoming nuclear beta decay parity-violation measurements. We also discuss the numerical evolution of the optical Bloch equations in the presence of transverse and longitudinal magnetic fields, so as to quantify the effect of a magnetic field on atomic polarization.

DOI
10.14288/1.0165992

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Magneto-Optical Traps (MOTs) have long been used to produce samples of cold trapped neutral atoms, which can be used in the measurement of a variety of physical quantities and theories. Until recently, one limitation of this type of trap was the necessity for the presence of a relatively large magnetic field which would decay only slowly after the trapping mechanism was turned off. This residual magnetic field is expected to partially destroy any atomic polarization induced, for example, by optical pumping. As a result, the precision of any physical measurement which requires polarization is limited. We will discuss the construction of our version of a newer type of MOT, the AC-MOT [2], which is designed specifically so as to minimize residual magnetic fields. We have found that our AC-MOT has lifetimes and cloud sizes similar to those we measured in our DC-MOT. We intend to use a trap similar to this in upcoming nuclear beta decay parity-violation measurements. We also discuss the numerical evolution of the optical Bloch equations in the presence of transverse and longitudinal magnetic fields, so as to quantify the effect of a magnetic field on atomic polarization.
A novel atom trap is described using alternating current to generate the magnetic B field, together with high speed polarization switching of the damping laser field. This combination produces a trap as effective as a standard magneto-optical trap (MOT), with the advantage that the average B field is zero. No net current is hence induced in surrounding conductive elements, and the B field produced by the ac MOT is found to switch off >300 times faster than a conventional MOT. New experiments can hence be performed, including charged particle probing or detection of the cold target ensemble.
The magneto-optical trap (MOT) serves as the culmination of optical cooling and trapping, where by applying a linear magnetic field to orthogonal, circularly polarized, retroreflecting laser beams, one can produce a dense cloud of very cold atoms, on the order of μK. This work set out to create a magneto-optical trap using only visible light with the 6s 2S1/2 → 7p 2P◦ 3/2 transition of cesium-133, which is far weaker than the transition typically employed for a cesium-133 MOT. This transition alone has yet to be used in isolation to create a MOT, and the purpose of this work is to demonstrate the feasibility of such a MOT. While the creation of a MOT with the setup produced has thus far been unsuccessful, the components are in place for one to be made as a demonstration of optical cooling and trapping that should be visible to the naked eye. Additionally, an experimental setup was created for high precision measurements of the absolute energies of the 7p 2P◦ 1/2 and 7p 2P◦ 3/2 states. The theory behind the MOT and high precision spectroscopy is expounded upon within.
The results of characterization of cold cloud of cesium (Cs) atoms trapped in a magneto-optical trap (MOT) have been presented. The MOT is a part of the Physics package of the Cs Fountain Clock being developed at National Physical Laboratory (NPL), India. The number of atoms, size and temperature of the cloud have been measured and calculated. It is also been investigated how the number of trapped atoms changes with the trapping laser’s beam intensity, detuning and magnetic-field gradient of the trap.
This thesis presents details of a new type of cold atom electron source (CAES) that uses an alternating current magneto-optical trap (AC-MOT). Cold atom electron sources produce electron bunches by threshold photoionisation from a population of laser-cooled atoms. The electrons extracted from the atoms are made into a beam which has an ultra-low transverse velocity distribution. The low velocity distribution increases the brightness and reduces the energy spread of the beam, making CAESs potential sources for future electron diffraction and electron spectroscopy studies. The newly commissioned source detailed here produces focused electron bunches with characteristic temperatures as low as 12.6 ± 0.8 K, at a rate of up to 10 Hz, and with a bunch charge of 2.2±0.2fC, comparable to other state-of-the-art cold atom electron sources. The AC-MOT CAES can also produce cold bunches with energies <200 eV, which is lower than has previously been reported for this type of source. The electrons originate from a population of laser-cooled 85Rb atoms held in an AC- MOT. The AC-MOT CAES design allows the magnetic trapping fields to be switched to zero in 32 μs, more than �ˆ�175 times faster than is possible using a comparable DC-MOT source. This means that electron bunches can be created and extracted in a magnetic field-free environment without any reduction in trap density and hence bunch charge, as occurs in conventional DC-MOT sources. The design, development, characterisation, and operation of the systems needed for the new AC-MOT CAES are detailed in this thesis. This includes details of the atom trapping system, the photoionisation system, the electron extraction system and the various timing systems for creation and characterisation of the resultant cold electron bunches. Finally, the characterisation of the AC-MOT CAES itself is presented, and the measured beam metrics are used to compare the new AC-MOT CAES with other CAES designs which are under development.
It has been more than 30 years since the first demonstration of a magneto-optical trap (MOT) using sodium atoms. Since then the MOT has revolutionised the field of atomic physics by facilitating the emergence of a broad range of productive avenues of research using atoms prepared at low temperatures and high densities. This thesis describes the development of a novel kind of magneto-optical trap: the blue-detuned MOT. Unlike in all previous MOTs the light is blue detuned from atomic resonances and drives "type-II" transitions that have dark ground-state sub-levels. A discussion of the position-dependent and velocity-dependent forces experienced by an atom or molecule in a MOT is first used to consolidate recent theoretical work and, in particular, to introduce the concept of a blue-detuned MOT. The design and construction of an experiment that has been built to demonstrate a blue-detuned MOT using \Rb{87} is described. A thorough characterisation of this novel MOT has been performed. At high magnetic field gradients, radiation-pressure-limited densities exceeding 101110^{11}1011~cm−3^{-3}−3 have been reached whilst temperatures are cooled below 30~\si{\micro K} by the efficient and robust sub-Doppler cooling mechanisms. The maximum phase-space density measured is 6×10−66\times10^{-6}6×10−6, which is higher than in most normal atomic MOTs, comparable to the best dark SPOTs, and a million times higher than that reported for red-detuned type-II MOTs. This makes the blue-detuned MOT particularly attractive for molecules where laser cooling and trapping always uses type-II transitions. For the first time, a study of trap loss due to ultra-cold collisions between atoms occurring in the presence of near-resonant blue-detuned light is undertaken. Finally, the experiment is used to demonstrate many new and unreported configurations of MOT for \Rb{87}, showing that a comprehensive understanding of complicated MOTs is now possible, and presenting a clear direction for further research.