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Applications and Sustainability �–

Functionality in Nanophotonics

Daniel Erni

General and Theoretical Electrical Engineering (ATE), Faculty of Engineering, University of Duisburg-Essen, and CENIDE �– Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, D-47048 Duisburg

The Interface Problem How is a functional nanophotonic device accessed by its environment ?

Large scale differences.

How to bridge the gap between the nano and

the micro/macro?

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The Functionality-vs-Volume Problem

Functionality is provided by optical signal processing within a nano volume (respective a sub-wavelength volume).

Open question: How does the complexity of the functionality scales with decreasing volume?

How is nanoscopic functionality implemented and exploited?

Is there a degradation of functionality for decreasing nanophotonic device volumes ?

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The Implementation Problem

Optical nanostructure: Photonic crystal device: Metamaterial structure:

Functionality ( ) confined to a single site (volume).

Functionality ( ) encoded into an anomaly (defect).

Functionality ( ) dispersed over the structure (dilution).

How is the nanophotonic functionality actually provided ?

There are 3 typical paradigms of implementation characterized by the structural length scale relative to the operating wavelength .

Ldevice

Lunit cell Lmicro structure

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Agenda Squeezing light into the nanoscale.

On tight light guiding.

Light confinement to metal surfaces: «Plasmonics».

Optical nanoantennas: «Nantennas».

Photonic crystal devices.

Electromagnetic / optical metamaterials.

Few concluding remarks.

«On the implementation of functionality in nanophotonic device design»

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Squeezing Light Into Small Scales

Example: Tight light guiding for dense optical integration.

The very first task: light confinement

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straight wave guide

R = 200 µm

= 1.3 µm / n = 0.1

R = 50 µm

R = 10 µm

On Tight Light Guiding I

20 µm

(A) Electronic chip

100 µm

(B) Photonic chip (conventional)

1250 couplers/cm2 (R = 200 µm)

5.7·108 transistors/cm2 (8-core Itanium, 32 nm)

456�‘000 : 1 Minimal radius of curvature fully determines the integration density.

Comparing integration densities

On Tight Light Guiding II Photonic wires

Rib waveguide

2D-MMP: T = 6%

Simulation: X. Cui Fabrication: F. Robin (ETH Zürich)

2D-MMP: T = 99%

Photonic wire

Strong horizontal light guiding.

conventional light guiding.

X. Cui, Ch. Hafner et al., Opt. Expr., 14(10), pp. 4351, 2006. X. Cui, Ch. Hafner, F. Robin, D. Erni, et al., Proc. SPIE vol.

6617, pp. 66170D-1-11, June 2007.

5 µm

5 µm

1550nm

1550nm

InGaAsP/InP

T < �– 4dB

Via Evolution Strategies (ES)

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On Tight Light Guiding III

Dielectric waveguides:

Light confinenment is the solution of a boundary value problem (cf. total internal reflection at a boundary interface), which translates into an eigenvalue problem.

Light confinement increases with increasing refractive index contrast.

Tight light guiding needs new confinement respective new guiding mechanisms.

A first conclusion

(1) Metallic boundaries: Plasmonics.

(2) Alternative mechanism: Defect waveguiding in

photonic crystals.

Light Confinement to a Metal Surface

(1) «Field-driven» plasma resonance:

oscillating field (light)

Oscillating carriers

Surface Plasmons

Charge carriers (electrons) have mass and thus inertia. Resonant system between

electric field electrons. The light field is «glued»

to the (lossy) metal surface.

(2) Dispersion relation:

SPP: surface plasmon polariton («glued», i.e. guided Zenneck wave)

SP: surface plasmon (guided slow wave up to localized resonance). plasmon freq. / P : SP = P

1+

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T = 77.2%

Simulation

Transmission in the «Nano» I Plasmonic light guiding

(2) Metallic groove waveguide:

S. I. Bozhevolnyi, et al., Nature, 440, pp. 508-511, March, 2006.

(1) Metallic slot waveguide: L. Liu, et al., Opt. Express, 13(17),

pp. 6645-6650, Aug. 15, 2005.

Measurement

100 nm 20 nm Metall

Metal

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Transmission in the «Nano» II Plasmonic band-stop filter

© A. Krasavin, Imperial College London.

T. Holmgaard, S. I. Bozhevolnyi, et al., Appl. Phys. Lett, 94, pp. 051111, Feb., 2009.

Band-rejection filtering effect by destructive interference at .

Emergence of periodic stop-bands.

PMMA-coated gold

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FEM, EC-FDTD simulations

Transmission in the «Nano» III Ultra-compact directional coupler A. Rennings, J. Mosig, D. Erni, et al., ISSSE 2007,

Montréal, Québec, Canada 2007.

German Patent, No.DE102007031841A1 Jan 15, 2009.

Strong coupling yields short coupler (but scattering loss).

Coupling length: only 36nm ! (@ = 438 nm)

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Transmission in the «Nano» IV A second conclusion

Plasmonic light guiding allows for dense nanophotonic integration.

But: The photonic integration density is still 103 below electronic integration.

But: Dense plasmonic waveguiding is still very lossy (propagation length: µm �– mm).

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Interfacing the «Nano» I Nanoparticle as optical antenna (2) Localized surface plasmon resonance: (1) Resonant scattering enhancement :

Mie scattering at a perfectly conducting metallic sphere.

A resonant lossy metallic sphere has the potential to «attract» the power flux.

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340 nm

401 nm I E I

R = 35 nm g = 10 nm L = 100 nm d = 50 nm

Interfacing the «Nano» II Structured optical antennas

X. Cui, D. Erni, L. Dong, and W. Zhang, NANOMETA 2009, Seefeld, Austria, 2009.

X. Cui, L. Dong, W. Zhang, W. Wu, Y. Tang, and D. Erni, Appl. Phys. B: Lasers and Optics, 101(3), pp.601, 2010.

Cu- or Ag-filled CNTs (nanorobotic spot welding L. Dong, B. Nelson, ETH Zürich).

3D-FEM simulation (~ 58 h, 3 GHZ quad core)

(1) «Fat» dipole:

CNTs provide DoFs for tailoring SP resonance, dipolar tuning multipolar farfield.

Molecular emitter (2 Å): farfield enhancement: ~ 6

R. Kappeler, D. Erni, X. Cui, and L. Novotny, J. Comput. Theor. Nanosci., 4(3), pp. 686 2007.

(2) Charge management: TEM image

Self-similar particle chain (N 4). Intrinsic dipole formation pi

enhances the lightning-rod effect Balancing the SP resonance and

Cu

CNT

@ resonance

charge density

p1

p2

p3

Au

2r = 20 nm

3D-FEM simulation

~ 708 nm ~ 610 nm

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J. Huang, et al., Nano Lett., 9, pp. 1897-1902, 2009.

200 nm antenna

(matching) Rmin = �–16 dB

Interfacing the «Nano» IV Optical antenna systems (2) Dipole «nantennas» with feeding lines:

Spot: /150

M. Schnell, et al., Nature Phot., 5, pp. 283-287, May, 2011.

MIR biosensing @ spot size: /150 = 60 nm, (thermal radiation).

Study with receiving and trans- mitting nantenna, separated by a 1344 nm long gold slot line.

gold wires

= 9.3 µm laser beam

= 830 nm

Si

Interfacing the «Nano» III Optical antenna systems (1) Dipole «nantenna» with DC bias:

Dipole antenna with DC electrical connection for future voltage-tunable gap emitters. Connections at

intensity minima. Gap width: 30 nm E-field: 108 V/m Resonance: Q = 6

© O. J. F. Martin, EPFL.

J. C. Prangsma, et al., Nano Lett., 12, pp. 3915-3919, July , 2012.

200 nm

280 nm 380 nm

resonant near field intensity

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Nanowave Engineering A third conclusion

Metal-based optical nanodevice design defines a realm on its own: «Plasmonics».

We are dealing with transmission lines, antennas, directional couplers, matching�…

Both RF engineering as well as microwave engineering is undergoing a «renaissance» at optical frequencies.

Plasmonics could be therefore termed as «nanowave engineering».

Photonic Crystals I PhC defect waveguide

TE polarization

© IBM

W1 crystal defect

(1) 2D band diagram of the bulk PhC:

(2) Dispersion diagram of the defect waveguide:

Planar crystal: strong periodic 2D perturbation. No propagation states allowed within the PBG. Introduction of a defect confined field states.

Line defect «encodes» a channel waveguide.

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Photonic Crystals II Optimal design of a PhC waveguide bend Wrestling around even with simple device designs

modeling

end-fire spectra

P. Strasser, D. Erni, et al., J. Opt. Soc. Am. A., vol. 25, no. 1, pp. 67, Jan. 2008.

Lossy 2D model (FEM) for the hole-type PhC waveguide bend.

Optimization of the bending area in 2D.

Verification in 3D (FDTD). Fabrication in InP/InGaAsP; end-fire characterization. Transmission: �– 8 dB �– 3 dB , bandwidth doubled.

A: upper single-mode region

425 nm

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1

2

92.7 %

90.7 %

Photonic Crystals III Filtering T-junction (diplexer)

J. Smajic, Ch. Hafner, and D. Erni, Opt. Express, vol. 11, no. 6, pp. 567-571, March 24, 2003.

E. Moreno, D. Erni, and Ch. Hafner, Phys. Rev. E, vol. 66, no. 3, pp. 036618-1-12, Sept. 27, 2002.

Size: 7.5 µm × 5.0 µm (@ = 1.55 µm). Smallest diplexer topology at that time.

Si rod in air / a = 575 nm

Si rod in air a = 575 nm

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K. Rauscher, D. Erni, W. Bächtold, OWTNM 2005, April 8-9, Grenoble, France, 2005. P. M. Nellen, P. Strasser, V. Callegari, R. Wüest, D. Erni, and F. Robin, Microelectronic

Engineering (MEE), vol. 85, no. 5-8, pp. 1244-1247, 2007. Photonic Crystals IV Compact functional devices (1) Power splitter:

(2) 4-channel demultiplexer:

close up (resonant cavity)

2D-FEM / 3D-FDTD simulations

end-fire measurement T 40/40 % ( - 4 dB) @ 1550 nm

numerical optimization, 3D-FDTD simulation, T = 42/42 %

17 µm

13 µm 1610 nm 29 %

1510 nm 44 %

1560 nm 34 %

1455 nm 19 %

hardware trimming via

FIB milling of the central hole area.

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Photonic Crystals V A forth conclusion

Photonic crystal structures are currently best suited for ultra-high Q resonators, compact filters, dispersion compensators,

and microlasers.

Photonic crystal structures are too complex and technologically too demanding for high- volume production of nanophotonic devices.

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EM Metamaterials I Validity range of metamaterials

© H. Giessen, Uni Stutgart.

p

The wave «feels» an averaged functionality provided by the mixture of nanostructures.

In the long wavelength limit (red area below) the periodic metamaterial is homogenized.

The wave behaves there as in a homogeneous effective material with characteristics inherited from the nanoscale.

2D band diagram of the periodic structure

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2 µm

direct laser writing into polymer with electro- chemical gold plating

EM Metamaterials II Complex broadband polarizers

H. Giessen, et al., Adv. Materials, vol. 23, pp. 3018-3021, 2011.

Metamaterial as broadband circular polarizer (for normal incidence).

M. Wegener, et al., Science, vol. 325, pp. 1513-1515, 2009.

silver coated polymer helical «knots»

Bi-chiral plasmonic MIR meta- material supporting LH or RH

circular polarization depending on the exposed symmetry of the helical «knots».

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EM Metamaterials III On transformation optics

N. I. Landy, et al, Opt. Express, vol. 17, no. 17, pp. 14872-14879, Aug. 2007.

Metamaterials provide a degree of freedom to shape the metric of an (effective) space.

A light wave will then follow the geodesics of that space.

Designing tailored space metrics to mold the flow of light defines a field on its own called:

«transformation optics» (TO).

Cloacking is the most popular vision.

TO is a narrowband technique.

without transformation optics

with transformation optics

Light waves travel around the US east coast !

True optical metamaterials are difficult to realize because the condition p << yields challenging feature sizes.

I didn�‘t talk about active and nonlinear nanophotonic devices like e.g. nanolasers, solar cells, quantum dot-based emitters.

Nevertheless, the main implementation strategies still apply: confine, encode or dilute «nano features».

Future nanophotonic systems will «hybridize» or combine all three strategies (cf. below).

Conclusion True optical metamaterials are difficult to realize because the condition p << yields challenging feature sizes.

I didn�‘t talk about active and nonlinear nanophotonic devices like e.g. nanolasers, solar cells, quantum dot-based emitters.

Nevertheless, the main implementation strategies still apply: confine, encode or dilute «nano features».

Future nanophotonic systems will «hybridize» or combine all three strategies (cf. below).

An optical phased array as a «directive metamaterial» that

consists of 4096 nantennas in 0.3 mm2 silicon.

© J. Sun, MIT.

J. Sun, et al, Nature, vol. 493, pp. 195-199, Jan. 2013.

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Thanks. Further Information:

www.ate.uni-due.de

Check our site on «publications»

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Appendix

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Photonic Crystals II Is the PhC scheme apt for functional device design?

PhC T-junction: (maximal BW)

J. Smajic, Ch. Hafner, and D. Erni, J. Opt. Soc. Am. A, vol. 21, no. 11, pp. 2223-2232. Nov. 2004.

Evolutionary algorithms Sensitivity-based gradient search There are enough degrees of

freedom in the small PhC lattice volume to implement functionality !

(a) binary:

(b) continuous:

Si rod in air a = 1 µm

Active PhC Devices Organic PhC laser

1st order 2nd order

lasing 494 nm (TM)

PL spectrum (pulsed pump at 355 nm)

R. Harbers, P. Strasser, D. Caimi, R. F. Mahrt, N. Moll, D. Erni, W. Bächtold, B. J. Offrein, and U. Scherf, J. Opt. A: Pure Appl. Opt., vol. 8, S273-S277, 2006.

active polymer MeLPPP

SiO2

35 nm TiO2 210 nm

295 nm

dispersion analysis based on a tailored 2D/3D-PWM

TiO2 quartz

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