Towards atomic physics using spatially structured light

Webster, Jason (2018-12)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT : Structured light has seen many applications in the fields of telecommunications, optical microscopy, and quantum information processing, however, few works exist that explore the interactions of structured light with atoms. In this thesis, we seek to explore these interactions further, both theoretically and experimentally. Of particular interest is the interaction between an atom and a Laguerre-Gauss beam of topological charge l = 1. Such a beam is easily constructed in a lab setting, and also carries an additional unit of orbital angular momentum that has been shown to couple with atomic transitions. Theoretically, we derive a framework upon which to describe atomic transitions due structured light. We find that tailoring light’s structure will have a direct influence on the strength of quadrupole transitions in atoms. A theoretical analysis of the effects of an atom situated in a magnetic field interacting with a Laguerre-Gauss beam of topological charge l = 1 was conducted. It was found that atoms placed in a region of zero intensity, such as the beam centre, can undergo quadrupole transitions – in contrast to dipole transitions that rely on the field intensity. We go on to show the differences between this beam and a gaussian beam, and find that the former can cause transitions between magnetic sub-levels that are typically considered forbidden in quadrupole spectroscopy under certain conditions. We explain this through the principle of conservation of angular momentum. Experimentally, we chose to use a potassium vapour cell as our medium, as the linewidth of its quadrupole transition was relatively large, and could be simply modelled as a three-level system to easily understand its dynamics. We performed an experiment where we indirectly measured a 4 2S1/2 → 3 2Dj quadrupole transition for j = 3/2, 5/2, induced by a custom-built external cavity diode laser operating at ∼ 464.3 nm. The 4 2S1/2 → 3 2Dj transition was measured by monitoring the fluorescence due to a dipole decay 4 2Pj 0 → 4 2S1/2 during the decay cascade from 3 2Dj → 4 2Pj 0 → 4 2S1/2 , where j 0 = 1/2, 3/2. We then used our theoretical framework to fit the sum of two gaussian peaks to our measured fluorescence spectrum, and find an excellent agreement with our data.

AFRIKAANSE OPSOMMING : Gestruktureerde lig is reeds aangewend in verskeie toepassings in telekommunikasie, optiese mikroskopie, en kwantum informasie prosessering. Daar bestaan egter min pogings on die interaksie van gestruktureerde lig met atome te ondersoek. In hierdie tesis loods ons sodanige ondersoek, beide teoreties en eksperimenteel. Van besonder belang is die interaksie tussen ‘n atoom en ‘n Laguerre-Gauss laser straal met topologiese lading l = 1. So ‘n straal kan maklik gekonstrueer word in ‘n laboratorium en dra ‘n addisionele eenheid hoekmomentum wat kan koppel met atomiese oorgange. Teoreties herlei ons ‘n raamwerk waarbinne die atomiese oorgan a.g.v. gestruktureerde lig beskryf kan word. Ons bevind dat die lig se struktuur ‘n direkte invloed het op die sterkte van die kwadrupool oorgange in atome. ‘n Teoretiese analise van die koppeling van ‘n atoom met ‘n Laguerre-Gauss straal met topologiese lading l = 1 terwyl dit in ‘n magneet veld is word uitgevoer. Daar word bevind dat ‘n atoom wat is by ‘n posisie van nul veld-intensiteit steeds ‘n kwadrupool oorgang kan ondergaan – in teenstelling met dipool oorgange. Ons beskryf voorts die verskille tussen sodanige straal en ‘n Gaussiese straal, en bevind dat die voorgenoemde oorgange kan opwek tussen magnetise subvlakke wat tipies as verbode beskou work. Hierdie word verduidelik deur die behoud van hoekmomentum. Eksperimenteel het ons kalium gekies as ons gassel medium, siende dat die kwadrupool oorgang van kalium betreklik wyd is en gemodelleer kan word met ‘n eenvoudige drievlak model. Ons het ook ‘n eksperiment uitgevoer om die 4 2S1/2 → 3 2Dj kwadrupool oorgang indirek te meet, na opwekking met 464.3 nm lig uit ‘n tuisgemaakte eksterne resonator diode laser. Die 4 2S1/2 → 3 2Dj oorgang was waargeneem deur flouresensie te meet vanaf die 4 2Pj 0 → 4 2S1/2 dipool oorgang se verval. Ons gebruik dan die teoretiese raamwerk om twee gaussiese pieke te pas teen die gemete fluoresensie spectrum en vind uitstekende ooreenstemming met ons data. Na die beste van ons wete is sodanige eksperimente nog nie voorheen op kalium uitgevoer nie, hoewel soortgelyke eksperimente al op sesium en rubidium toegepas was.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104968
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