Flat Optics and

Metasurfaces

Daniel Wintz

Patrice Genevet, Mikhail Kats, Antonio Ambrosio, Nanfang Yu, Francesco Aieta, Romain Blanchard, Alex

Woolf, Alan She, Zeno Gaburro, Federico Capasso

Andor Academy April 8th, 2015

School of Engineering and Applied Sciences, Harvard University

Overview

Introduction to flat optics

Examples of optical elements using phase discontinuities

Controlled steering of surface plasmon wakes

Active metalens for tunable focusing of surface waves

Conventional Optics

Transformation Optics

Ward, Pendry, J. Mod. Opt. 43 (1996)

Diffractive Optics

Can we replace bulky optical components with

nanoscale, flat ones?

• Goal: control amplitude, phase, and polarization of light

with a low footprint, nanoscale device

• 3 ideals and challenges: high efficiency, high bandwidth,

reconfigurability

• Metasurfaces: Optically thin, subwavelength arrays of

optical elements for engineering wavefronts

The Vision of Flat Optics

Smart Phones Stretchable

Materials Google glass

Wave optics (thin lens example)

Conventional optical elements rely on gradual phase accumulation through dielectric media—phase accumulates through propagation

Wave optics (thin lens example)

Conventional optical elements rely on gradual phase accumulation through dielectric media—phase accumulates through propagation

~𝝀 or thicker

Primary wavefront

Interface

z

Wave optics (Huygens principle)

Secondary wavelets

Primary wavefront

Interface

z

Wave optics (Huygens principle)

Secondary wavelets

Primary wavefront

Interface

z

Wave optics (Huygens principle)

A sin(ωt - kx x)

x

A sin(ωt - kx x + jump)

jump

Secondary wavelets

Primary wavefront

Interface

z

Interface

z

Huygens principle

Light propagation with phase discontinuities

A sin(ωt - kx x)

x

A sin(ωt - kx x + jump)

jump

What can we use to make these discontinuities?

Light propagation with phase discontinuities

Optical Antennas

++ --

L1

• The case of a single rod antenna—behaves as a driven, damped

harmonic oscillator

• Light scattered from an antenna does not always have the same phase

as the incident light!

More complicated antenna structures can be used to achieve full 2𝜋 coverage

Overview

Introduction to flat optics

Examples of optical elements using phase discontinuities

Controlled steering of surface plasmon wakes

Active metalens for tunable focusing of surface waves

Demonstrated optical capabilities of metasurfaces

• Refraction/beam deflection

• Complex beams—Bessel beams, vortex beams, Cosine-Gauss beams

• Full and half wave plates

• Flat lenses

• Dispersionless flat lenses

• Creation and control of surface plasmon wakes

• Metagratings for focusing of surface waves

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)

Refraction and beam deflection The antennas 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

Vortex beams

• Phase profile is an optical ‘vortex’ or helix that carries angular momentum ±𝑚ℏ

• Not just fancy: can be used for information transfer—2.5TB/sec already achieved: Nature Photonics 6, 488–496 (2012)

Applied Physics Letters 100, 13101 (2012)

Non-diffracting beams

• Diffraction is a consequence of the wave nature of light

• Can be turned off for small length scales

Flat Lenses

Nano Lett., 2012, 12 (9), pp 4932–4936

Multiwavelength dispersionless flat lens

Science 347, (2015)

Overview

Introduction to flat optics

Examples of optical elements using phase discontinuities

Controlled steering of surface plasmon wakes

Active metalens for tunable focusing of surface waves

Introduction to Surface Plasmon Polaritons

x

Surface Plasmon Polaritons (SPPs)—Light confined to a metal/dielectric interface Surface: confined to the interface Plasmon: free electrons (plasma) oscillate Polariton: light-like

Surface Plasmon Wakes?

Sonic booms Boat wakes

Cherenkov effect

What will be the disturbance for surface plasmons?

Wakes are a general wave phenomena where a disturbance propagating in the medium travels faster than the phase velocity of the waves it creates

The Disturbance: ‘Running Wave of Polarization’

sin 𝛾 =sin 𝜃

𝑛𝑒𝑓𝑓

𝑅𝑒 𝐸𝑧

Near-field scanning optical microscopy (NSOM)

• AFM tuning fork + Tapered optical fiber

• Probes near-field • Tip diameters ~50 − 100 𝑛𝑚

• Collection mode or scattering mode

• NOT diffraction limited

NSOM Details

Schematic of collection mode operation with metal coated tapered optical fiber.

𝑘𝑆𝑃𝑃

Experimental setup and sample data

Data for 𝜃 = 20°

Interferogram Peculiarity

𝛾 Δ𝑦

𝜆𝑅𝑊𝑃

Data for 𝜃 = 20°

Experimental Data on Slits for different 𝜃

Others have done nanoslit excitation of SPPs already: Lee, S.Y., et al. PRL (2012)

Phased array gives routing configurability

𝑠𝑖𝑛𝛾 =𝑠𝑖𝑛𝜃

𝑛𝑒𝑓𝑓+

1

𝑘𝑠𝑝𝑝

𝜕𝜙

𝜕𝑥

𝟏. 𝟓 𝝁𝒎

𝚫𝒙 𝛾 𝛾 𝚫𝒅

𝝓𝟏 𝝓𝟏 + 𝚫𝝓 𝚪

(a)

𝚫𝒅

𝑘𝑆𝑃𝑃 𝑘𝑆𝑃𝑃

Spin-Angular-Momentum dependent phasing

(a)

x

y

𝑬

Linear Polarization

(b)

x

y

𝑬

Spin-Angular-Momentum dependent phasing

𝜕𝜙

𝜕𝑥= 𝜎±

𝜋

Γ

(a)

x

y

𝑬

Linear Polarization Circular Polarization

𝑠𝑖𝑛𝛾 =𝑠𝑖𝑛𝜃

𝑛𝑒𝑓𝑓±

1

𝑘𝑠𝑝𝑝

𝜕𝜙

𝜕𝑥

Calculated results for the interferogram

3 𝜇𝑚

SPP Intensity

Calculated results for the interferogram

+ =

3 𝜇𝑚

SPP Intensity

Gaussian beam Intensity

Calculated results for the interferogram

+ =

3 𝜇𝑚

3 𝜇𝑚

SPP Intensity

Gaussian beam Intensity

Interferogram Intensity

Experimental Setup and Calculation vs. Experiment

Experimental Calculated

Comparison for different incident angles

• Varying angle of incidence changes angle of the wakes

• Changing from right circularly polarized to left circularly polarized light and completely reverse the direction of the wakes

𝑠𝑖𝑛𝛾 =𝑠𝑖𝑛𝜃

𝑛𝑒𝑓𝑓±

1

𝑘𝑠𝑝𝑝

𝜕𝜙

𝜕𝑥

Applications

• Polarization detection

• Angle of incidence detection

• Testbed for Cherenkov radiation without the need for particle accelerators

• Testbed for Reversed Cherenkov studies

Overview

Introduction to flat optics

Examples of optical elements using phase discontinuities

Controlled steering of surface plasmon wakes

Active metalens for tunable focusing of surface waves

Under review

Goals

• Can we overcome some of the coupling constraints for exciting surface plasmons?

• Can we focus the surface plasmons after coupling? • Can we achieve tunable unidirectionality after coupling?

• Optoelectronics applications

• On-chip spectroscopy

Motivations

Traditional Coupling Methods

Kretschmann-Raether Method

Traditional Coupling Methods

Kretschmann-Raether Method

Grating Coupling Method

• Polarization restraints

• Fixed directionality after coupling

New Age Coupling Methods

L. Yin, C.W. Kimball et al. Nano Letters (2005)

Z. Liu, X. Zhang, et al. Nano Letters (2005)

F. Lopez-Tejeira, A. Dereux, et al. Nature Physics (2007)

New Age Coupling Methods

L. Yin, C.W. Kimball et al. Nano Letters (2005) J. Lin, F. Capasso et al.

Science (2013)

T. Tanemura, D.A.B. Miller, et al. Nano Letters (2011)

Z. Liu, X. Zhang, et al. Nano Letters (2005)

F. Lopez-Tejeira, A. Dereux, et al. Nature Physics (2007)

Nanoslit excitation of SPPs

x

y

Nanoslit excitation of SPPs

±𝒌𝒔𝒑𝒑

Laser light

y

z

Δ𝑦

B x

y

Δ𝑦Δ𝑘~1

Metalens Design Principle a)

Metalens Design Principle a) b)

Metalens Design Principle a) b)

c)

Metalens Design Principle

Experiment and sample results

𝑬

𝑬

𝜆0 = 670 𝑛𝑚

Experimental

Calculated

to APD

Single Wavelength Excitation

d) 750 𝑛𝑚

a) 632 𝑛𝑚

b) 670 𝑛𝑚

c) 710 𝑛𝑚 Vertical Slits Horizontal Slits

Same experiment as before…

Experimental

Calculated

to spectrometer

Spectrally resolved wavelength demultiplexing

Full 580 − 700 nm band

𝜆0 [nm]

No

rmal

ized

Inte

nsi

ty

a) b)

632 𝑛𝑚 band 670 𝑛𝑚 band c) d)

Spectrally resolved wavelength demultiplexing

Full 580 − 700 nm band

𝜆0 [nm]

No

rmal

ized

Inte

nsi

ty

a) b)

632 𝑛𝑚 band 670 𝑛𝑚 band c) d)

Spectrally resolved wavelength demultiplexing

Full 580 − 700 nm band

𝜆0 [nm]

No

rmal

ized

Inte

nsi

ty

a) b)

632 𝑛𝑚 band 670 𝑛𝑚 band c) d)

640 𝑛𝑚

650 𝑛𝑚

660 𝑛𝑚

e)

Focusing Nature

Experiment Calculation

Polarization Selectivity

on off

For 710 nm

• Goal: control amplitude, phase, and polarization of light

with a low footprint, nanoscale device

• 3 ideals and challenges: high efficiency, high bandwidth,

tunability

• Metasurfaces: Optically thin, subwavelength arrays of

optical elements for engineering wavefronts

The Vision of Flat Optics

Smart Phones Stretchable

Materials Google glass

Funding: Support/Instrumentation:

Patrice Genevet, Mikhail Kats, Antonio Ambrosio, Nanfang Yu, Francesco Aieta, Romain Blanchard, Alex

Woolf, Alan She, Zeno Gaburro, Federico Capasso