title: Time-reversal constraint limits unidirectional photon emission in slow-light photonic crystals.
abstract: Photonic crystal waveguides are known to support C-points-point-like polarization singularities with local chirality. Such points can couple with dipole-like emitters to produce highly directional emission, from which spin-photon entanglers can be built. Much is made of the promise of using slow-light modes to enhance this light-matter coupling. Here we explore the transition from travelling to standing waves for two different photonic crystal waveguide designs. We find that time-reversal symmetry and the reciprocal nature of light places constraints on using C-points in the slow-light regime. We observe two distinctly different mechanisms through which this condition is satisfied in the two waveguides. In the waveguide designs, we consider a modest group velocity of vg≈c/10 is found to be the optimum for slow-light coupling to the C-points.This article is part of the themed issue 'Unifying physics and technology in light of Maxwell's equations'.
authors.affiliation: Department of Electrical and Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK bl9453@bristol.ac.uk.
authors.forename: Ben
authors.identifier: 0000-0002-5602-9166
authors.initials: B
authors.lastname: Lang
authors.affiliation: Centre for Quantum Photonics, H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: Daryl M
authors.identifier: 0000-0002-2231-7514
authors.initials: DM
authors.lastname: Beggs
authors.affiliation: Department of Electrical and Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK Centre for Quantum Photonics, H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: Ruth
authors.identifier: 0000-0003-1206-5357
authors.initials: R
authors.lastname: Oulton
keywords: photonic crystal; polarization; polarization singularities; slow light; waveguide