Lights Twist Miles Padgett FRS Kelvin Chair of Natural Philosophy - PowerPoint PPT Presentation

Light’s Twist Miles Padgett FRS Kelvin Chair of Natural Philosophy !

Light beams carry a Linear Momentum • Light carries a linear momentum – Force exerted = Power/c • Laser pointer exerts 10 -11 N • Strong enough to move cells and bacterial • Or (with a very large sail!) a spacecraft … .

Light’s Angular Momentum in History We now refer to this as the SPIN angular momentum of the photon But light has an extra angular momentum in addition to photon spin (this extra momentum is needed for some rare atomic transitions)

Getting started on Orbital Angular Momentum of Light • 1992, Allen, Beijersbergen, Spreeuw and Woerdman Miles Padgett • 1994, Les meets Miles at dinner … ...

Orbital Angular Momentum from helical phase fronts E E B S B S S θ ≠ 0 S θ = 0

Angular momentum in terms of photons • Spin angular momentum σ = +1 ! – Circular polarisation � σ ! per photon • Orbital angular momentum σ = -1 ! – Helical phasefronts � ℓ! per photon

A double-start helix ( ℓ =2) Chambord castle (chateaux de la Loire)

Orbital angular momentum from Skew rays Poynting vector E B S

Orbital angular momentum from skew rays

Angular Momentum Transfer Spin Angular Momentum Orbital Angular Momentum (circular angular momentum) (helical phasefronts)

OAM and SLM control OAM in Phenomenology in micromanipulation OAM in Quantum OAM in Imaging OAM in Communications Optics

The OAM communicator • Every OAM state forms an independent channel

Linear vs. Rotational Doppler shifts Light source ! Light source ! - exerts torque = ℓ P/ ω ! - exerts force = P/c ! Ω ! v ! Doing work on a light beam changes its energy and hence shifts it’s frequency

Doppler shift from a SPINNING surface N photons per second Torque= ℓ N ! ω / ω Work Done= Ω N ℓ! Work per photon Ω≠ 0 ! ! Δω = Ω ℓ! Frequency shift Δω = Ω ℓ v=0 !

Illuminate with OAM at +/- ℓ and measure Δω Δω' A B 0 - 5 Rel. Power H dB L - 10 - 15 - 20 - 25 - 30 2200 2250 2300 2350 2400 2450 2500 f mod H Hz L C 500 450 W H rad. s - 1 L 400 350 300 250 200 1200 1400 1600 1800 2000 2200 2400 2600 f mod H Hz L

Me Johannes Courtial Sonja Franke-Arnold Students/RAs, Richard Bowman especially Graham Gibson Martin Lavery Jonathan Leach Kevin O’Holleran Jackie Romero Les Allen Steve Barnett Collaborators Bob Boyd External Jon Cooper Mark Dennis Mervyn Miles Monika Ritsch-Marte Wilson Sibbett