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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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