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arXiv:2109.11940

Perfect Chirality with imperfect polarisation

Ben Lang, Dara P. S. McCutcheon, Edmund Harbord, A. B. Young, Ruth Oulton

Published Sep 24, 2021• 0 citations
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Abstract

Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarisation (C points), with elliptical polarisations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor $\sim 30$), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.

DOI
10.48550/arxiv.2109.11940

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Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarisation (C points), with elliptical polarisations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor ∼30\sim 30∼30), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.
Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarization (C points), with elliptical polarizations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles, and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor ∼30), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.
Summary form only given. There has recently been great interest in chiral waveguides, where the polarisation of the forwards and backwards propagating light fields are oppositely circular at certain positions in space (C-points) [1]. This results in new phenomena, such as the one way emission of light from spin-transitions in quantum dots (QDs) or atoms (simulation, fig. b). The emitted photon's direction is determined by the spin state of the emitter.To date calculations on this topic have either been restricted to certain special cases, or have relied on simplifying “rate forwards/backwards” models [2]. Using a Green's function method we have generalised to the case of any polarisable dipole at a point supporting any polarisation. Several interesting physical effects arise here that could not have been predicted using the previous models. For example in the figure we show how perfect chiral behaviour can be recovered at points of elliptical polarisation by using equally elliptical dipoles stretched along the orthogonal axis. This may be realised in QDs with a vector magnetic field.We further generalise our approach to include the population state of a multi-level system. This allows us to calculate the dynamics of complicated multi-transition “quantum dipoles”, such as a Λ-system, recovering results consistent with experiment [3].
Abstract The chiral interaction between light and matter has brought chiral quantum optics into a flourishing stage. Here, we explore the perfect chiral interaction between the atoms with elliptically polarized dipoles and waveguide modes, and find that ideal unidirectional radiation can be obtained by adjusting the position and polarization of atoms. When there are two waveguide modes in the system, by controlling the polarization of the dipoles, the dipoles can radiate not only the unidirectional single waveguide mode, but also two seperated waveguide modes.
High Resolution Image Download MS PowerPoint Slide Spin-dependent, directional light–matter interactions form the basis of chiral quantum networks. In the solid state, quantum emitters commonly possess circularly polarized optical transitions with spin-dependent handedness. We demonstrate numerically that spin-dependent chiral coupling can be realized by embedding such an emitter in a waveguide-coupled nanocavity, which supports two near-degenerate, orthogonally polarized cavity modes. The chiral behavior arises due to direction-dependent interference between the cavity modes upon coupling to two single-mode output waveguides. Notably, an experimentally realistic cavity design simultaneously supports near-unity chiral contrast, efficient (>95%) cavity-waveguide coupling and enhanced light–matter interaction strength (Purcell factor F P > 70). In combination, these parameters enable the development of highly coherent spin–photon interfaces ready for integration into nanophotonic circuits.
The ability to engineer chiral light-matter interactions is a valuable resource for realizing quantum networks and non-reciprocal quantum optics. Here, we demonstrate chiral coupling between light and a negatively charged exciton state, i.e., a four-level system, embedded in a standard photonic-crystal waveguide. By controlling the orientation and strength of an external magnetic field, we tune the polarization of the individual optical transition dipoles relative to the fixed elliptically polarized waveguide mode. In a two-sided waveguide, an optimized oblique magnetic field allows a selected transition to decay exclusively into a single propagation direction, enabling near-unity directional emission (0.99−0.02+0.010.99^{+0.01}_{-0.02}0.99−0.02+0.01​), despite the waveguide exhibiting imperfect polarization for chiral coupling. We further observe two tunable chiral branching ratios, the directional analogue of optical cyclicity, for two ΛΛΛ-systems, with one reaching 134−77+∞134_{-77}^{+\infty}134−77+∞​. The magnetic-field-dependent spectroscopy also allows reconstruction of the local guided-mode polarization at the emitter via the Stokes parameters relative to the emitter's dipole moments. Finally, we demonstrate that this local chirality enables coherent control of an electron spin via an optical Raman process mediated by waveguide driving. Building on the simultaneous realization of a large branching ratio, directionality and spin control achieved at the same oblique magnetic-field angle, we propose a protocol with high tolerance to photon loss for generating high-fidelity remote spin-spin entanglement. Our magnetic control opens a reconfigurable and generic route for enhancing on-chip chiral spin-photon coupling in standard nanophotonic interfaces without the need to carefully engineer the polarization of the waveguide.
Exploring the spontaneous emission control of an excited two-level atom interacting with the vacuum within a circular waveguide surrounded by a perfect electric conductor is the main objective of this work. This type of emission is dependent on the intrinsic properties of the emitter, such as the emitter’s transition frequency and dipole moment direction, as well as the surrounding electromagnetic environment. Due to being statistical in nature, the fields do not have a well-defined phase, and thus the emission is considered incoherent or in an uncontrollable direction. We propose an approach to direct the emission from an atom embedded in a circular waveguide . To achieve this, multimode waveguides should be excluded because the total emission is simply presented in three orthogonal directions. In contrast, with mono-mode waveguides, the single photon transports in a single-mode waveguide coupled to a cavity embedded with an atom. Thus, the electric dipole moment vector is forced to take the direction of the only mode in which it is allowed to propagate. Besides giving rise to a single value for the emission, this also provides a way to control direction. Such waveguides have already been produced by recent advances in microfabrication technology.