Federico Capasso

Capasso@seas.harvard.edu

Harvard School of Engineering and Applied Sciences

Zeiss Lecture, Jena March 7, 2013

1

Classical Optics in a New Light:

Flat Photonics based on Metasurfaces

Nanfang Yu Zeno Gaburro

Romain Blanchard

Guillaume Aoust

Patrice Genevet

Mikhail Kats

Francesco Aieta

Key contributors

Recent developments in field of optics

Metamaterials and Transformation Optics

Viktor G Veselago 1968 Sov. Phys. Usp. 10 509 A.J. Ward and J.B. Pendry, J. Mod. Opt. 43 (1996) J.B. Pendry, D. Schurig and D.R. Smith, Science 312, 1780 (2006) Optical cloaking

Propagation of light is controlled by considering artificial 3D materials with designed permittivity and permeability.

What can we do in 2D ? “metasurfaces”

Negative refraction

• Planar technology is central to Integrated Circuit technology ( $ 300 B industry):

Technology platform.

• Because of fabrication complexity 3D optical materials (metamaterials etc.) don’t

have a good chance of a major technology impact at optical wavelengths.

• Lessons of photonic (PC) crystals: beautiful physics but very limited technology penetration due to fabrication complexity\

So we should look at what we can do in 2D with metasurfaces:

METASURFACES FOR FLAT OPTICS • Optically thin engineered metasurfaces for Wave Front Engineering (phase control): • Refractive index is not a useful quantity when we think about metasurfaces: local

phase and amplitude control of light along the surface using optical antennas

• New class of flat, compact and broadband components:(lenses, polarizers, etc.), be-

yond conventional diffractive optics

Optical phased arrays for high speed wavefront control

The Vision of Flat Optics with Metasurfaces

5

What if we introduce in the path a distribution of phase jumps?

CONVENTIONAL OPTICAL COMPONENTS

How to impart an abrupt phase profile to a wavefront

Requirements : - deep subwavelength thickness - subwavelength separation

- - 2 π phase coverage

- - high and uniform scattering amplitude

-Flat (2D) photonic components: - Lenses and Axicons - - Polarizers

- -Vortex plates - =optical phased arrays

By using subwavelenth optical resonators on a metasurface with suitably designed geometry in order to scatter light with the desired

phase distribution

Light propagation with phase discontinuities

Generalized reflection and refraction of light N. Yu, P. Genevet , M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, Z. Gaburro, Science 334,333 (2011)

A sin(ωt - kx x)

x

A sin(ωt - kx x + Φjump)

Φjump

Secondary wavelets

Primary wavefront

Interface

z

Interface

z Huygens principle

Light propagation with phase discontinuities

What if could have a spatial distribution of different phase discontinuities along the entire interface? can make any desired wave front !

How? Optically thin array of sub-wavelength spaced antennas

Fermat’s principle: Imposing the stationary phase conditions: konidxSin(θi) +(Φ+dΦ) = kontdxSin(θt)+Φ

Generalized reflection and refraction of light Suppose interface with a constant gradient of phase delay

dΦ/dx

Similarly for reflection:

Generalized Snell’s law

Reflection and refraction from a metasurface

10

Optically thin: 50nm Subwavelength phase resolution: ~λ/5 Instant imprinting of a linear phase distribution

anomalous reflection

anomalous refraction

λ = 8 µm

incident wave

silicon

N. Yu et al., Science 334, 333 (2011)

3 4 5 6 7 8 9

-0.5

0

0.5

1

Wavelength (µm)

Phas

eπ

3 4 5 6 7 8 90

0.2

0.4

0.6

0.8

1

Scat

terin

g In

tens

ity (a

.u.)

Wavelength (µm)

V-shaped antenna I

This antenna has a broader resonance and greater phase coverage than a linear one

V-Shape Antenna II

π/4 0 7π/4 6π/4 5π/4 4π/4 3π/4 2π/4

✓ uniform amplitude ✓ 2π phase coverage

-45o

0o

45o

90o

-45o 0o 45o 90o 135o 180o 225o 270o

Eight-element unit cell: full 2 π coverage with equal scattering amplitude

Mirror structure

Anomalous refraction

The antenna operate as secondary scatterers with a tailorable phase response, re-directing a normally-incident beam away from the normal

Uniform scattering amplitude Controlled phase responses between 0 to 2π

Arrays of Au antennas on Silicon

Experimental results: anomalous refraction

N. Yu et al. Science 334,333 (2011)

Γ

• Linear gradient of phase discontinuities creates modified refraction and reflection behavior

θt = arcSin(- λo/Γ)

Anomalous reflection and refraction

- Negative reflection - Nonlinear relationship - Critical angle for reflection

- Negative refraction - New total internal reflection angles

Reflection Refraction

Reflected and refracted beams

ordinary refraction

incidence λ= 8µm

ordinary reflection

anomalous refraction

anomalous reflection

iθ

silicon

~10% ~20%

~20%

~40%

antenna absorption ~10%

in-plane component

out-of-plane component

F. Aieta et al. Nano Letters 12, 3 (2012)

• Refraction and reflection out of the plane of incidence!

Out-of-plane refraction and reflection

Polarization properties of the anomalous beam

â ŝ Einc

α

Incident wave

e-ray (scattered

light)

Quarter wave plate

o-ray (background

)

Design based on phased optical antennas Background free

Arbitrary orientation of incident E-field vector

Broadband operation: quarter-wave plate ∆λ=5-14µm

2 µm

z

x

y

45o

Einc π/8

Ea Eb d π/8

3π/8

3π/8

45o

Unit a

Unit b

Creating beams with arbitrary polarization

The two rows of antennas in the unit cell create the two orthogonal polarized components with 90 degree phase difference

d = Γ/4 ↔ Ψ = π/2

P=E×H

Propagation direction

Poynting vector

OPTICAL VORTICES

Φ∝ A eikz eiLθ

L=1

M. Padgett et al. Physics Today, May 2004 issue

Intensity distribution in the

cross section

Equal-phase wavefront

• Doughnut-shaped intensity • Corkscrew-shaped wavefront (at center phase undefined leading to phase singularity) • Can impart orbital angular momentum • Can encode optical information

How can we create twisted beams ?

• Spiral phase plate

M.W. Beijersbergen Optics Comm, 112 (1994) 321- 327

Both are often implemented in a programmable spatial light modulator (SLM)

• Fork diffraction gratings

www. wikipedia.org

VORTEX PLATES

Linear phase gradient leads to the generalization of the laws of reflection

and refraction

Phase discontinuities are achieved by considering the scattering of light

by sub-wavelength resonators (example V-shape nano-antennas)

Go beyond the basics manipulation of

reflection and refraction of light ?

6π/4

5π/4

4π/4 3π/4

2π/4

π/4

0o 7π/4 • Angular arrangements of antennas create

optical vortices • Radial arrangements create flat lenses …

DETECTING THE ORBITAL ANGULAR MOMENTUM OF LIGHT

How can we detect those twisted light beams?

Intersection between the two wavefronts follows a spiral line

L=1

How can we detect those twisted light beams?

Intersection between the two wavefronts

Dislocated fringe

Generation of multi-charged vortex beam

20 µm

2π -2π

-4π 4π 20 µm

WHAT’S THE BIG DEAL ?

Amplitude

Phase

0.2 µm 1 µm 2 µm 3.5 µm 8 µm

Distance from interface

Desired phase distribution is imprinted on the incident wavefront at the interface.

n1 n2

Why Lenses are thick ? Can we make a flat lens

Propagating Wavefront

Flat Lens

To focus at a certain focal f the interface must compensate for the distance of every point from a spherical surface centered in the focus and with radius f.

F. Aieta et al Nanoletters ,Aug. 15 , 2012 (on line)

No spherical aberration and large numerical aperture

Flat Lens

High N.A.

No Spherical Aberration for flat lens

sinϑi ≠ ϑi High N.A. = 0.8

Wavefront = Envelope of Secondary Waves with radii R(x) = λ/2π *φ(x)

x z

Metasurface

φ(x) -0.5 0.5 -0.5 0.5 0 x/f x/f 0

Spherical Hyperboloidal

Lens

φ(x)

Flat Lens

Multi-beam collimated Mid-IR Quantum Cascade Laser

SEM images

Experimental far-fields Modeled far-fields

Diffractive optics based on metasurfaces

• Technology simplification: a single digital pattern (one mask level) creates arbitrary analog phase profile • Lithography: nanoimprint and soft lithography

Thin film optical coatings Last half century: a lot of work on optical coatings and filters using thin film

interference effects

Nearly all thin film optical coatings use dielectric layers with thickness on the order of a wavelength, where the dielectrics have low optical loss

Thin film interference: Coloring by reflection resonance in λ/4 thick films

Nanometer thickness optical coatings We were able to make “blue gold”, “pink gold”, and other colors

utilizing 5-20 nm semiconductor layers much thinner than λ/4

Polished substrate

Rough substrate (still works!)

Kats et al, Nature Materials (published online, 2012)

Bare Au

Au + 7nm Ge

Au + 15nm Ge

Nanometer thickness optical coatings Patterning ultra-thin coatings to create images, labels, etc

The difference between blue and purple, and purple and pink is only ~4 nm of germanium (~ 8 atomic layers)!

Kats et al, Nature Materials (published online, 2012)

Reflection phase shifts at dielectric-dielectric interfaces are either 0 or π and reflection from an ideal metal has π phase shift

Metals with finite conductivity and lossy dielectrics have “weird” interface reflection phase shifts (i. e. not 0 or π) Resonances can exist for films significantly thinner than λ/(4n)

To achieve high absorption, the dielectric must be very lossy

Punchline: it is possible to have an absorption resonance at specific wavelength (and even a perfect absorber) resulting from coherent effects in a system involving an ultra-thin, highly-lossy layer

This leads to coloring

Im[r]

Re[r] h << λ/4n

r0

r1 r2

r3

Absorption resonance in ultrathin (<< λ) highly absorbing films

Absorption resonances lead to coloring

Lossy dielectric Germanium Lossy metal Gold

Ellipsometry Experiment

Calculation Calculation

Physical concept

Why was this simple geometry be overlooked? 1. Coherent effects are not discussed in highly-lossy materials 2. Without lossy materials, reflection phase shifts are fixed to

either 0 or π

Lossy materials introduce nontrivial reflection phase shifts (> π) Resonant cavities can be made thinner than λ/4 Short propagation lengths allow the use of highly

absorbing media

π

n1 < n2

0

n2 > n1 n1 PEC

π

n1

??

2n

Ultra-thin perfect absorber

To appear in Nov. 26 issue of Applied Physics Letters

M. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M. Qazilbash, D. Basov, S. Ramanathan, F. Capasso, “Ultra-thin perfect absorber using a tunable phase change material”

Tunable perfect absorber

Our experimental system comprises a thin (180nm) film of vanadium dioxide (VO2) on sapphire VO2 serves as highly-absorbing layer Sapphire appears “metallic” at λ ≈ 10 – 15 μm and 20 – 25 μm due to strong

phonon activity

Vanadium dioxide (VO2) VO2 is a correlated metal oxide which experiences phase change upon heating

to past ~70 °C (reversible, but with hysteresis) Conductivity changes by >10,000 from the insulating to conducting state

Transition can be triggered thermally, optically, and electrically Ongoing research to make various electronic devices (switches, etc) with VO2

Yang et al, Ann. Rev. Mat. Res. (2011)

VO2 in the transition region

What happens in the transition state of VO2?

Nanoscale islands of metal-phase VO2 begin to form within a background of dielectric-phase VO2, which then grow and connect The mixture can be viewed as a disordered, natural metamaterial

The ratio of co-existing phases can be controlled tunable medium

Qazilbash, Basov, et al, Science (2007)

Tunable perfect absorber

Temperature control of VO2 allows us to significantly change the sample reflectivity!

Reflectivity tuning from ~80% to 0.25% at 11.6um on off ratio of more than 300 entire effect due to just 180nm of VO2 on sapphire

5 10 150

0.2

0.4

0.6

0.8

1

Wavelength (µm)

Ref

lect

ivity

297K343K346K347K348K351K360K

Tunable perfect absorber

Used textbook 3-layer equation

5 10 150

0.2

0.4

0.6

0.8

1

Wavelength (µm)

Ref

lect

ivity

297K343K346K347K348K351K360K

5 10 150

0.2

0.4

0.6

0.8

1normal incidence, 180nm VO2

Wavelength (µm)

Ref

lect

ivity

295K341K342K343K343.6K346K360K

Perfect absorber: angle-dependent spectra

Calculation of the angle-dependent reflectivity spectra

Reflectivity remains under 0.01 for incident angles of 0° ~ 30° for both s- and p- polarization Striking for a thin-film interference effect Useful for capturing large fraction of incident light

Perspectives: ultra-thin optical coatings

Coloring of metals Smooth and rough metals can be colored by application of ultra-

thin, highly-absorbing films

Modifying reflectivity for optical applications Very black surfaces Reshaping ultra-fast pulses

Patterning colors

New, ultra-flat, single-material labels and patterns

Harnessing the absorption Ultra-thin angle-insensitive photodetectors, solar cells, etc Can use semiconductors or semiconducting polymers

Flat optics

High NA Objective

Non-Invasive Imaging for Biomedical Application

Major opportunity in Midir due to

poor refractory materials

Smart Phones

StrechableMaterials

• Lithography: from Optical to Nanoimprinting and Soft Lithography

• New class of flat, compact and broadband components: lenses, polarizers., filters,

High speed tunable phased array for real-time wavefront control

Thank you!

Schematic experimental set up

ordinary refraction

incidence λ= 8µm

ordinary reflection

anomalous refraction

anomalous reflection

iθ

silicon

~10% ~20%

~20%

~40%

antenna absorption ~10%