1. Elbert van Putten
Complex Photonic Systems
Wave front shaping
Controlling light in disordered materials

2. Acknowledgements
Duygu Akbulut
Ivo Vellekoop
Allard Mosk
Ad Lagendijk
Willem Vos
Photon Scattering Group (AMOLF, Amsterdam)
Complex Photonic Systems

3. Light and Disorder
Scattering obstructs view

4. Scattering is a nasty problem
target
Today I’ll show you that scattering can be extremely useful

5. Outline
○ Scattering in optics
○ Wave front shaping
○ Focusing through opaque materials
○ Focusing inside opaque materials

6. Speckle
- Light still interferes after it is scattered
- Transmitted light has random phase: laser speckle
- Low intensity, no resolution.
LASER

7. How to describe this scattering?
See the sample as a waveguide…
Channels in x = Channels out

8. How to describe this scattering?
…and add disorder to mix the channels
Channels in x = Channels out

9. Field at an outgoing channel
Outgoing channel
Ingoing channels
random, uncorrelated
scattering coefficients
=1

10. Controlling channels
What happens if we could control
the incident channels…
=1

11. Controlling channels
We choose the phase to
maximize the amplitude of Eb
=1

12. Controlling channels
We choose the phase to
maximize the amplitude of Eb
=1

13. Controlling channels
By controlling the incoming channels
we control how light propagates!
=1

14. Outline
● Scattering in optics
● Wave front shaping
○ Focusing through opaque materials
○ Focusing inside opaque materials

15. Speckles
sample
target
Divide incoming
wavefront in
N segments
total field in target

16. Basic idea
Digital
feedback system
sample
target
Divide incoming
wavefront in
N segments

17. Basic idea
samplephase modulator
with N segments
target
total field in target

18. Outline
● Scattering in optics
● Wave front shaping
● Focusing through opaque materials
○ Focusing inside opaque materials

19. LCD
Experimental setup
Spatial amplitude and phase
modulation using commercial TN
LCDs
van Putten, Vellekoop & Mosk,
Appl. Opt. 47, 2076 (2008).

20. LCD
Sample: 10 µm layer of TiO2
pigment
Light source: 633 nm HeNe laser
Feedback: 1 pixel of CCD camera
Experimental setup

21. Focusing light through paint
Focus is more than 1000 times brighter than the background
Vellekoop & Mosk. Opt. Lett. 32, 2309 (2007)

22. Perfect focusing
Vellekoop, Lagendijk & Mosk, Nat. Photon. 5, 320 (2010)
Wave front
shaping
Diffraction
limit of the
sample

23. Teeth (ex vivo) Paint Daisy petals
Scotch tapePrepared
chicken
Egg shell
Works on a variety of materials

24. Outline
● Scattering in optics
● Wave front shaping
● Focusing through opaque materials
● Focusing inside opaque materials

25. Focusing inside
• Open problem in science, technology and
biomedical imaging
• Highly relevant for near-field
superresolution with metamaterials.
• Need feedback signal from inside

26. Focusing inside
Use fluorescent probe particle (150 nm) for
feedback.
shaped
wavefront

27. Enhanced fluorescence
Size of the sphere (same scale)
20x more light!
Plane wave
illumination
Shaped wave
illumination

28. Intensity vs. depth
Conventional optics
corrected for all aberrations
(classical adaptive optics)
Wave front shaping
Vellekoop, van Putten, Lagendijk & Mosk, Opt. expr. (2008)
Wave front shaping works independent of depth

29. Size of the focus
Resolution illumination

30. Size of the focus
Resolution illumination
Wave front shaping

31. Size of the focus
Resolution illumination
Wave front shaping
Resolution detection
van Putten, Lagendijk & Mosk, submitted

32. Conclusions
• Disorder no longer a nasty problem
• Light can be focused at any location in and
outside a disordered material
• Wave front shaping is a powerful tool to
control light propagation

33. As discussed in Nature Physics 4, 91 (2008)

35. Resolution and contrast
Normal lens:
Wavefront error deteriorates focus size
Opaque lens:
Wavefront error scattered into background
(deteriorates focus contrast)

36. Graphical representation
Re E
Im E
contribution of
segment 1
contribution of
segment 2
contribution of
segment N
Maximize Eb

37. Global maximum
before after
global maximum

38. Finding the optimal wavefront
Adjust phase of individual segments
until contribution is in phase with total field

39. Intensity of focus