  Strong light-matter coupling occurs when the coupling strength between a
confined electromagnetic mode and a molecular resonance exceeds losses to the
environment. The study of strong coupling has been motivated by applications
such as lasing and the modification of chemical processes. Here we show that
strong coupling can be used to create phase singularities. Many nanophotonic
structures have been designed to generate phase singularities for use in
sensing and optoelectronics. We utilise the concept of cavity-free strong
coupling, where electromagnetic modes sustained by a material are strong enough
to strongly couple to the material's own molecular resonance, to create phase
singularities in a simple thin film of organic molecules. We show that the use
of photochromic molecules allows for all-optical control of phase
singularities. Our results suggest a new application for strong light-matter
coupling and a new, simplified, more versatile pathway to singular phase
optics.
