Next colloquium

Wednesday, 8 February at 3 pm

Emilio Pisanty (KCL)

Knotted topologies in the polarization state of bichromatic light

The fundamental polarization singularities of light are generally symmetric under coordinated rotations: that is, transformations which rotate the spatial dependence of the fields by an angle θ and the field polarization by a fraction γθ of that angle, as generated by 'mixed' angular momenta of the form L + γ S. Generically, the coordination parameter γ has been thought to be restricted to integer or half-integer values. In this talk I will show that this constraint is an artifact of the restriction to monochromatic fields, and that a much wider variety of optical singularities is available when one considers polychromatic beam combinations that contain more than one frequency.

Building upon this I will show that these new optical singularities present novel field topologies, isomorphic to torus knots, and I will show how they can be characterized both analytically and experimentally. I will also show that the generator for the symmetry group of these singularities, whose algebraic structure is deeply related to the torus-knot topology of the beams, is conserved in nonlinear optical interactions. Finally, I will report on recent work that constructs explicit three-dimensional versions of these topologies, providing an explicit embedding in real space.

Main references:
- E. Pisanty et al. Knotting fractional-order knots with the polarization state of light. Nature Photonics 13 569 (2019).
- E. Pisanty et al. Conservation of torus-knot angular momentum in high-order harmonic generation. Physical Review Letters 122 203201 (2019).

Colloquia 2022/23

2022

2023