Spin and orbital angular momentum dynamics in counterpropagating vectorially structured light

Abstract

It is well-known that electric spin angular momentum and electric orbital angular momentum are conserved under paraxial propagation of travelling waves in free-space. Here we study the electric and magnetic angular momentum in counter-propagating waves and show both theoretically and experimentally that neither component alone is conserved except in special cases. We attribute this non-conservation to spin-spin and orbit-orbit coupling between the electric and magnetic fields. This work generalises previous findings based on travelling waves, explains the apparent spin-orbit coupling in counter-propagating paraxial light, and broadens our understanding of angular momentum conservation in arbitrary structured light waves.

Gouy-phase-mediated propagation variations and revivals of transverse structure in vectorially structured light

“…Recently, the Khoury group discussed more generally the pattern (intensity profile) variations and revivals of scalar structured Gaussian modes, which were enabled by the fractional Gouy phase [18,19]; importantly, they unified the physics behind the pattern revivals and the well-known Talbot effect [20]. In addition, in the vector regime, the generation and application of vector structured light with custom propagation-variant transverse structures have also attracted increasing interest in recent years [21][22][23][24][25].…”

Quantum Structured Light: Non-classical Spin Texture of Twisted Single-photon Pulses

“…We show that beyond the paraxial approximation, the photon spin density of a Bessel single-photon pulse can exhibit rich spatial texture. This builds on previous important work in the field but also marks a significant departure [20][21][22][23][24][25][26][27][28][29][30][31]. Our proposed framework provides a powerful and versatile tool to engineer the local photon spin and OAM densities of a quantum structured light pulse, specifically for spatiotemporal optical vortices [37,38].…”

“…This is significantly beyond the previous semi-classical theory [20,21,31], in which the quantum statistics of the photon spin cannot be studied. Similarly, we can evaluate the mean value of the spin of a coherent-state Bessel pulse with polarization λ and photon numbern = |α| 2 , α ξλ Ŝ obs α ξλ = nλ (0, 0, cos θ c ) .…”

“…In previous sections, we have shown that bothŜ obs andL obs are vector operators. However, in previous studies, usually only their projections on the propagating direction have been fully studied [20,21,31]. Their mean value on the transverse plane and more importantly, their quantum fluctuations have not been investigated.…”

“…Existing theories of quantum light-matter interaction have advanced over the last two decades to capture a plethora of phenomena related to SAM and OAM of light [20][21][22][23][24][25][26][27][28][29][30][31]. Important outstanding questions remain even within this large body of work which is the focus of this manuscript, namely -photon statistics, quantum spin and OAM vector density and the single photon wavefunction.…”