Plasmonic nanocavities and their interaction with quantum emitters

Bedingfield, Kalun (2023). Plasmonic nanocavities and their interaction with quantum emitters. University of Birmingham. Ph.D.

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Abstract

When a photonic environment hosts an emitter, it heavily influences its spontaneous emission. For an emitter placed within a photonic environment that supports extreme field enhancements, their light-matter interactions are also significantly altered. One such example of an extreme photonic environment is that of a plasmonic nanocavity, which is a plasmonic gap of just 1-2 nm formed between two metallic nanostructures. The nanoparticle-on-mirror geometry--formed of a metallic nanoparticle assembled on a flat metallic substrate, separated by a dielectric spacer--is a plasmonic nanocavity that has become particularly prolific in experimental literature, recently used to realise the room temperature light-matter strong coupling between a plasmonic mode and a single molecule. Emitters placed within such plasmonic nanocavities experience sub-wavelength field confinements and extreme light enhancements, due to the intense hybridisation of localised plasmons that are evanescently bound to the metallic surfaces. This thesis explores the optical properties of plasmonic nanocavities and their complex light-matter interactions with emitters hosted within them, aiding in the future potential to facilitate strongly coupled quantum devices that operate in ambient conditions.

The emission and excitation properties of plasmonic nanoantennas are explored initially, addressing the misconception that energy can be coupled into all plasmonic systems just as efficiently as it is out-coupled. Using a Green's function formalism to decompose the modal plasmonic response, the dominant out-coupling of certain plasmonic nanocavities is unveiled through a mode selectivity of the excitational properties--accentuated by non--quasi-static metallic elements of the system. Custom designs of plasmonic systems are then considered to tailor relative in- and out-coupling rates, capable of supporting both enhanced and retarded excitation regimes.

The near-field resonances and complex eigenfrequencies are then obtained for a series of realistic polyhedral nanocavity geometries. Although most theoretical studies of plasmonic nanocavities have focused on spherical nanoparticles, in reality they acquire a polyhedral shape during fabrication due to the crystalline nature of gold. Each polyhedron supports multiple unique facet assemblies whose exact morphology dramatically controls the supported modes, their spectral arrangement and their degeneracies. Projecting the modal response to the far-field, we reveal the paramountcy of the facet shape, neighbouring facets and polyhedral symmetry on their ability to radiate from the system: showing the extraordinarily strong dependence on the position and frequency of the emitter within the nanocavity, as well as the morphology of the system. This work paves the way in identifying the shape and symmetry of nanocavity geometries, as well as in determining the exact position of an emitter placed within a realistic plasmonic nanocavity.

Finally, we consider the quantum nature of emitters to model their cyclic energy exchange with plasmonic nanocavities (i.e. Rabi oscillations). We develop a quantum mechanical description via a mode quantisation scheme for the plasmonic modes that, unlike most methods currently proposed in the literature, is fully consistent in returning the classical limit. The Rabi oscillations are explored for multi-emitter systems--through the dependencies of emitter positioning and arrangement within the cavity--in addition to Rabi splitting and anti-crossing dynamics, when operating in a laser driven excitation regime. Excited emitter longevity is explored through exotic state preparations, uncovering massive lifetime enhancements for emitter initialised systems, where semi-persistent states may be long lived even in these extreme yet lossy nanoantennas systems--capable of aiding in the room temperature strong coupling of quantum plasmonic devices.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):