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

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May 05, 2023 481 likes •669 views

Lights 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

SLIDE 1

Light’s Twist Miles Padgett FRS

Kelvin Chair of Natural Philosophy

SLIDE 2

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….

SLIDE 3

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)

SLIDE 4

Getting started on Orbital Angular Momentum of Light

Miles Padgett

1992, Allen, Beijersbergen, Spreeuw and Woerdman
1994, Les meets Miles at dinner…...

SLIDE 5

E B S Orbital Angular Momentum from helical phase fronts

Sθ = 0 Sθ ≠ 0

E B S

SLIDE 6

Angular momentum in terms of photons

Spin angular momentum

– Circular polarisation σ! per photon

Orbital angular momentum

– Helical phasefronts ℓ! per photon σ = +1! σ = -1!

SLIDE 7

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

SLIDE 8

Orbital angular momentum from Skew rays Poynting vector E B S

SLIDE 9

Orbital angular momentum from skew rays

SLIDE 10

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

SLIDE 11

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

SLIDE 12

The OAM communicator

Every OAM state

forms an independent channel

SLIDE 13

SLIDE 14

Linear vs. Rotational Doppler shifts

v! Light source!

exerts force = P/c!

Ω! Light source!

exerts torque = ℓP/ω !

Doing work on a light beam changes its energy and hence shifts it’s frequency

SLIDE 15

Doppler shift from a SPINNING surface

v=0! Ω≠0!

Torque= ℓN!ω/ω Work Done= ΩNℓ! Work per photon !Δω = Ωℓ!

Δω = Ωℓ

N photons per second Frequency shift

SLIDE 16

Illuminate with OAM at +/- ℓ and measure Δω Δω'

1200 1400 1600 1800 2000 2200 2400 2600 200 250 300 350 400 450 500 fmod HHzL W Hrad. s-1L A C B

2200 2250 2300 2350 2400 2450 2500

fmod HHzL

Rel. Power HdBL

SLIDE 17

Sonja Franke-Arnold Johannes Courtial Me

Richard Bowman Graham Gibson Martin Lavery Jonathan Leach Kevin O’Holleran Jackie Romero Les Allen Steve Barnett Bob Boyd Jon Cooper Mark Dennis Mervyn Miles Monika Ritsch-Marte Wilson Sibbett