8 February 2023

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).

22 March 2023

Thierry Ruchon (KCL)

Extreme ultraviolet attosecond beams with orbital angular momentum: synthesis and applications

Light beams may carry both a spin and an orbital angular momentum (OAM). While the former is associated to their polarization state, the latter stems from the geometrical properties of their wavefront. In their prototypical form, beams with OAM have “donuts-like” intensity profiles and helicoidal wavefronts, carrying integral multiplesof ℏ as angular momenta. Since their “rediscovery” in the late 90’s, beams with OAM of visible wavelengths have found innumerable applications in quantum optics, atomic physics, microscopy or information transfer. A major recent development was the generation of such beams with much smaller wavelengths – in the extreme ultraviolet ( XUV) – using synchrotron sources, free electron lasers as well as high harmonic sources (HHG). In this latter case, it creates ultrashort XUV sources of light with OAM, suited for time-resolved applications at femtosecond and attosecond time scales.

In this seminar, we will summarize our recent progress on the synthesis of such beams with HHG [1, 2, 3, 4], and extensions to non-trivial light topologies such as polarization Möbius strip [5]. It offers an original tool to explore new kind of dichroisms, such as Magnetic Helicoidal Dichroism that we will introduce [6, 7].


[1] Géneaux, R. et al., 2016. Nature Communications, 7, 12583. http://­dx.doi.org/­10.1038/­ncomms12583

[2] Géneaux, R. et al., 2017. Phys Rev A, 95, 051801. http://­dx.doi.org/­10.1103/­PhysRevA.95.051801

[3] Gauthier, D. et al., 2017. Nature Communications, 8, 14971. http://­dx.doi.org/­10.1038/­ncomms14971

[4] Camper, A. et al., 2017. Optics Letters, 42(19), 3769. http://­dx.doi.org/­10.1364/­ol.42.003769

[5] Luttmann, M. et al., 2022. Conservation of a half-integer angular momentum in nonlinear optics with a polarization möbius strip. http://­dx.doi.org/­10.48550/­ARXIV.2209.00454

[6] Fanciulli, M. et al., 2021. Physical Review A, 103(1). http://­dx.doi.org/­10.1103/­physreva.103.013501

[7] Fanciulli, M. et al., 2022. Physical Review Letters, 128(7), 077401. http://­dx.doi.org/­10.1103/­physrevlett.128.077401