Cooperative optics of low-dimensional clouds of dipolar atoms

Bean, Gabriel and Ruostekoski, Janne and Drummond, Neil (2024) Cooperative optics of low-dimensional clouds of dipolar atoms. PhD thesis, Lancaster University.

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Abstract

The emergence and control of collective optics are studied for dense quasi-1D and 2D collections of dipolar atoms. The collective optical response is studied in the regimes of weak and strong magnetic dipole-dipole interactions both at low and high optical density. The introduction of magnetic dipole-dipole interaction changes the availability of collective eigenmodes in the system and how easily they can be occupied. In general, with greater magnetic interactions, there is a reduction in the number of highly super- and subradiant modes with large collective resonances. However, modes that are either uncommon or absent in non-interacting systems can also become available. The introduction of magnetic interactions also makes the occupation of collective modes more selective in terms of beam detuning. This feature allows an enhanced selective excitation of sub-radiant modes in particular compared to a non-interacting system. We study the optical response of these interacting systems and see an increase in resonant scattering at low density. At high density, multiple collective eigenmodes resolve themselves, which is seen as multiple resonant peaks in the scattered intensity. In principle, because the atoms are magnetic dipoles, each atom in the sample should experience a local zeeman shift due to the magnetic fields of its neighbours. We study how this alters the optical response and find this local shift broadens the lineshape. Finally, we study how the introduction of strong magnetic interactions causes the collections of atoms to crystallise spontaneously, both in the 1D and 2D cases. Once crystallised, the atoms' regular spacing dramatically alters the available collective eigenmodes. In weakly interacting systems, the distribution of available eigenmodes in terms of collective resonance and linewidth is largely continuous. In contrast, when crystallised, the distribution localises into a collection of small `islands'. These modes can be resolved spectrally, allowing for further selective excitation of particular eigenmodes with beam detuning. These modes can also be selected using different beam polarisations and orientations. We can target modes where atoms in the sample are primarily polarised in or out of the plane in the 2D case, or polarised along or normal to the atom chain in the 1D case.