Wavefrontshaping: Controlling light in disordered materials

Elbert van PuttenComplex Photonic Systems

Wave front shapingControlling light in disordered materials

Acknowledgements

Duygu AkbulutIvo VellekoopAllard MoskAd LagendijkWillem Vos

Photon Scattering Group (AMOLF, Amsterdam)

Complex Photonic Systems

Light and Disorder

Scattering obstructs view

Scattering is a nasty problem

target

Today I’ll show you that scattering can be extremely useful

Outline

○ Scattering in optics

○ Wave front shaping

○ Focusing through opaque materials

○ Focusing inside opaque materials

Speckle

- Light still interferes after it is scattered

- Transmitted light has random phase: laser speckle

- Low intensity, no resolution.

LASER

How to describe this scattering?

See the sample as a waveguide…

Channels in x = Channels out

How to describe this scattering?

…and add disorder to mix the channels

Channels in x = Channels out

Field at an outgoing channel

Outgoing channel Ingoing channels

random, uncorrelated scattering coefficients

=1

Controlling channels

What happens if we could control the incident channels…

=1

Controlling channels

We choose the phase to maximize the amplitude of Eb

=1

Controlling channels

We choose the phase to maximize the amplitude of Eb

=1

Controlling channels

By controlling the incoming channelswe control how light propagates!

=1

Outline

● Scattering in optics

● Wave front shaping

○ Focusing through opaque materials

○ Focusing inside opaque materials

Speckles

sample

target

Divide incomingwavefront in N segments

total field in target

Basic idea

Digital feedback system

sample

target

Divide incomingwavefront in N segments

Basic idea

samplephase modulatorwith N segments

target

total field in target

Outline

● Scattering in optics

● Wave front shaping

● Focusing through opaque materials

○ Focusing inside opaque materials

LCD

Experimental setup

Spatial amplitude and phase modulation using commercial TN LCDsvan Putten, Vellekoop & Mosk,Appl. Opt. 47, 2076 (2008).

LCD

Sample: 10 µm layer of TiO2 pigmentLight source: 633 nm HeNe laserFeedback: 1 pixel of CCD camera

Experimental setup

Focusing light through paint

Focus is more than 1000 times brighter than the background

Vellekoop & Mosk. Opt. Lett. 32, 2309 (2007)

Perfect focusing

Vellekoop, Lagendijk & Mosk, Nat. Photon. 5, 320 (2010)

Wave front shaping

Diffraction limit of the sample

Teeth (ex vivo) Paint Daisy petals

Scotch tapePrepared chicken

Egg shell

Works on a variety of materials

Outline

● Scattering in optics

● Wave front shaping

● Focusing through opaque materials

● Focusing inside opaque materials

Focusing inside

• Open problem in science, technology and biomedical imaging

• Highly relevant for near-field superresolution with metamaterials.

• Need feedback signal from inside

Focusing inside

Use fluorescent probe particle (150 nm) forfeedback.

shapedwavefront

Enhanced fluorescence

Size of the sphere (same scale)

20x more light!

Plane wave illumination

Shaped wave illumination

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

Size of the focus

Resolution illumination

Size of the focus

Resolution illumination

Wave front shaping

Size of the focus

Resolution illumination

Wave front shaping

Resolution detection

van Putten, Lagendijk & Mosk, submitted

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

As discussed in Nature Physics 4, 91 (2008)

Resolution and contrast

Normal lens:

Wavefront error deteriorates focus size

Opaque lens:

Wavefront error scattered into background

(deteriorates focus contrast)

Graphical representation

Re E

Im E

contribution ofsegment 1

contribution ofsegment 2

contribution ofsegment N

Maximize Eb

Global maximumbefore after

global maximum

Finding the optimal wavefront

Adjust phase of individual segmentsuntil contribution is in phase with total field

Intensity of focus