Engineering the couplings to the continuum : controlling the fundamental properties of radiation and enabling forbidden light-matter

Abstract

In this thesis, I consider the general question: what new dynamics can be realized by engineering the coupling between a discrete state and a continuum of states? In the first part of the thesis, we choose bound states in the continuum (BICs) as our starting point for answering this question. We construct a large class of BICs associated with separable Hamiltonians and show that by designing special perturbations of these systems, the dimensionality and propagation direction of waves can be controlled. We present potential realizations of this physics in potentials for ultracold atoms, optically induced potentials for photons, and lattice systems.Such resonances with easily reconfigurable radiation allows for applications such as the storage and release of waves at a controllable rate and direction and systems that switch between different dimensions of confinement. In the second part of the thesis, we look at the same question in a different physical setting: the coupling of electrons to the electromagnetic fields of polaritons such as plasmon and phonon-polaritons. We consider the potential for 2D materials such as graphene, thin films of SiC, and hBN to enable atomic and molecular transitions that have, to this date been either very difficult to observe, or have not yet been observed. Examples of such transitions include high-order multipolar transitions (as high as E5), multi-photon spontaneous emission, and intercombination processes such as spin-flip phosphorescence transitions. We find that plasmon polaritons in graphene can speed up spin-flip phosphorescence process by 7 orders of magnitude, that they can speed up two-photon spontaneous emission processes by 15 orders of magnitude, and that they can speed up multipolar transitions by over 20 orders of magnitude. This brings the lifetimes of all of these transitions to the nanosecond scale, comparable with the speed of the single-plasmon dipole transitions which have traditionally been thought to be the only transitions worth considering in most circumstances. The potential applications of this work include: spectroscopy for inferring electronic transitions which cannot be determined with photons, sensors based on forbidden transitions, organic-light sources arising from fast singlet-triplet transitions, fast entangled light generation, and fast generation of broadband light with tunable width in the visible or IR. From there, we ask: is it possible to engineer couplings between an electron and its radiative continuum such that it prefers to spontaneously emit via a conventionally forbidden transition? We find an affirmative answer. In particular, we show that in these systems, it is possible to have an electron prefer to change its orbital angular momentum by more than one. Processes that normally take years to happen are typically considered negligible become dominant processes which happen on the scale of nanoseconds. Going beyond processes at first order, we find that it is also possible to have an electron prefer to decay by the emission of two near-field photons, even when it is possible for the electron to decay via the emission of a far-field photon. In the process of showing these results, we arrive at a general result connecting the enhancement of N-photon emission to the Purcell factor, which has been of fundamental importance in quantum nanophotonics. Our results have direct implications for the design of fundamentally new types of emitters in the mid and far IR: ones which prefer to change their angular momentum by large amounts and also ones that prefer to emit a relatively broad spectrum of entangled photons. Our results may allow for the possibility of ushering in new classes of quantum emitters with tunable multi polarity and/or tunable emission spectra, in addition to new materials for broad-band absorption and emission, new capabilities for IR spectroscopy, sensing platforms, and many other applications.