-1 15

Surface plasmon waveguides

dielectric waveguide

~ 10 λ

CMOS transistor:

Photonic integrated system with subwavelength scale components

Medium-sized molecule

Size Mismatch between Scaled CMOS Electronics and Planar Photonics

Introduction

Silicon Photonics?

A World of Nanophotonic Devices

Could such an Architecture be Realized with

Metal rather than Dielectric Waveguide Technology?

Harry Atwater, California Institute of Technology

On-chip light source

Short-range(~ nm) waveguides

Nano-photonics

~ cm

Long-range(~ cm) waveguides

Nano-electronics

Photonic integrated circuit

Could such an Architecture be Realized with

Metal rather than Dielectric Waveguide Technology?

Metal Optics: An introduction

Photonic functionality based on metals?!

Surface-plasmon-polariton waveguides

Dispersion relation of surface plasmon polaritons excited on very- thin metal strips

Modes of very-thin(~ 10 nm) metal strips

Experimental results on SPP waveguide devices

50nm

A World of Nanophotonic Devices

Could such an Architecture be Realized with

Metal rather than Dielectric Waveguide Technology?

Harry Atwater, California Institute of Technology

On-chip light source

Short-range(~ nm) waveguides

Nano-photonics

~ cm

Long-range(~ cm) waveguides

Nano-electronics

Photonic integrated circuit

0 20 40 60 80 100 0

2 4 6 8

ω =c k

λ =337 nm; ε

1

= -1

ω (10

s

)

k

( μ m

)

Plasmons at Planar Metal-Dielectric Interfaces

surface plasmons are longitudinal charge density fluctuations on the

surface of a conductor Surface Plasmon dispersion relation for Ag in air

surface plasmon dispersion relation:

Plasmons are highly localized at metal-dielectric interfaces, so potential for:

• Ultrasmall Optical Devices

• “2D-Optics” on metal surfaces

(Light line)

Plasmon Dispersion Relation

λ = 337 nm

λ << 337 nm

Harry Atwater, California Institute of Technology

ε ₁ : metal ε ₂ : dielectric

x

1 2

1 2

k x

c ω ε ε

= ε ε

+

'

1 2

At large (

), z

1 .

x

k k

ε → − ε  ≈

Strong confinement at the interface Æ Nano focusing

'

At low ( k

ε

>> 1),

'

1

in air :

z x

E i

E = − ε

( : , : - )

z x

E = ± iE air + i metal i

' 1

1 in metal :

z x

E i

E ⁼ ε

0 20 40 60 80 100

0 2 4 6 8

ω =c k

x

λ =337 nm; ε

1

= -1

ω (10

s

)

k

( μ m

)

Broad dispersion

Low loss at the interface Æ Wave guiding

Nano Focusing & Wave Guiding

Surface Plasmons

excited on thin metal films

Dielectric – ε ₃

Dielectric – ε ₁

Metal – ε ₂

Several 1 cm long, 15 nm thin and 8 micron wide gold stripes guiding LRSPPs 3-6 mm long control electrodes

low driving powers (approx. 100 mW) and high extinction ratios (approx. 30 dB) response times (approx. 0.5 ms)

total (fiber-to-fiber) insertion loss of approx. 8 dB when using single-mode fibers

When the film thickness becomes finite.

mode

overlap

Possibility of Propagation Range Extension

frequency

in-plane wavevector

Long-Range SP:

weak surface confinement, low loss

Short-Range SP:

strong surface confinement, high loss

Symmetric mode (long-range SPP)

Anti-symmetric mode (short-range SPP)

E

H

SPP modes at a very thin metal film

Introduction:

Dependence of dispersion on film thickness

practically forbidden

200 400 600 800

-1 -0.75 -0.5 -0.25 0.25 0.5 0.75 1

200 400 600 800

-1 -0.75 -0.5 -0.25 0.25 0.5 0.75 1

6 0 h = n m

250 500 750 1000 1250 1500

-1 -0.75 -0.5 -0.25 0.25 0.5 0.75 1

250 500 750 1000 1250 1500

-1 -0.75 -0.5 -0.25 0.25 0.5 0.75 1

1 0 h = n m

0 20 40 60 80 100 0

5 10 15 20 25 30 35 40

W s/W a

thickness of metal film [nm]

Field solution and dispersion relation of coupled SPP’s

Symmetric Asymmetric

Propagation loss and field confinement of SPP’s

z

z

0 z=

h

ε

ε

ε

[ ]

( , ) x z =

L f z

( ) exp i x β

L e

Magnetic field : L=H/Z

2 2 2

0

j j

s = β − ε k

.

[ ] [ ] [ ] ( )

[ ] [ ] ( )

[ ] ( )

2 1

2 2 3

1 2

2 1

2 2

1 2

1

cosh sinh exp ( )

( ) cosh sinh 0

exp 0

s h s s h s z h z h

s

f z s z s s z z h

s

s z z

ε ε ε ε

⎧⎛ ⎞

+ − − ≥

⎪⎜ ⎟

⎝ ⎠

⎪⎪⎪

=⎨ + ≤ ≤

⎪⎪ ≤

⎪⎪⎩

Confinement

Propagation Loss Asymmetric mode

Symmetric mode

18nm 20nm

14nm 16nm

T

Fundamental symmetric mode of a metal stripe : thickness (T)

W=10um

LR-SP WG

P. Berini, PhotonicWest 2005.

(Spectalis Co.)

Fundamental asymmetric mode of a metal stripe : Δn

1.68 1.68+ Δn

Δn = 0.001 Δn = 0.002 Δn = 0.003

T=16 nm, W=10um

Symmetric mode guided by a metallic channel waveguide

silicon 9μm

fiber 20nm

Au (-96+i11) Polymer (n=1.47) @ 1.55μm

15mm

Propagation loss : 21dB/ cm

~10μm

Y-branch

Channel-1 Channel-2

1 2

Wavelength shifts by direct heating a metal wire

1 5 4 0 1 5 4 5 1 5 5 0 1 5 5 5 1 5 6 0

- 7 0 - 6 5 - 6 0 - 5 5 - 5 0 - 4 5 - 4 0

Transmittance (dB)

W a v e le n g th ( n m )

1544.1 1558.3

INPUT OUTPUT

Polymer 1 Substrate

+ -

Polymer 2

Tunable Wavelength Filter

Vertical directional couplers

H. Won, APL vol.88, 011110 (2006)

Vertical directional couplers

0 2 4 6 8 10 12 14 16

1.470 1.471 1.472 1.473 1.474 1.475 1.476 1.477

εmetal=-116+11.58i (gold) εdielectric=2.16

λ0=1550nm t=20nm

symmetric even mode symmetric odd mode I R/k 0

distance(d : distance between two slabs)

ε ^d = 2.16

ε ^m = −116 +11.58i t= 20nm d

d=4um Æ 254 um

d=6um Æ 558 um

Even mode and odd mode : directional couplers based on LRSPP

4μm, even mode 4μm, odd mode

7μm, even mode 7μm, odd mode

21μm, even mode 21μm, odd mode

0.08μm, even mode 0.08μm, odd mode

3μm, even mode 3μm, odd mode

23μm, even mode 23μm, odd mode

Vertical Lateral

D

D

Vertical directional couplers

even mode odd mode

odd odd

Lateral DC

Extinction ration at 400um : 27dB

Channel 1 Channel 2

Vertical directional couplers

Variable optical attenuator based on LR-SPP

Submitted to EL, S. Park & S. Song

Extremely long-range SPP ?

in-plane wavevector

frequency

Symmetrically coupled LRSP

Anti-symmetrically coupled LRSP

D. Sarid (PRL, 1981) J. J. Burke (APL, 1986)

Extension of SPP propagation length

Thin metal film

P. Berini (PRB, 2000)

Finite-width metal strip

metal

n4 n3

n2

n0 n1 > n0~n4

n1

F. Y. Kou et al (OL, 1987)

LR SPP

G. I. Stegman et al (APL, 1983) Metal

n1

n1 n2

n2 > n1

Double metal films Metal-dielectric films

0 1000 2000 3000 4000 0.1

1 10 100

1 1.4 1.45 1.46

1.47

1.48 1.49 1.5

1.6

propagation length(mm)

separation distance(D : μm)

Extended Long-Range SPPs

Metal

n1

n1

n2 D

metal

n5 n4

n3 n2

n0

n5 ~n1> n0~n2~n4

n1

Range extension with finite-width metal stripes

D < D

n

< n

No good

Two fundamental modes Even mode only

n ₁

n ₂ D t

w

0 1 2 3 4 1

10 100

1.45 1.46 1.40

1.48

1.50 1.47

propagation length (mm)

separation distance (D: μm) ^{0 1 2 3 4}

1.470 1.472 1.474 1.476 1.478 1.480 1.482

1.4 1.45 1.46

1.47 1.48 1.50

β r/k 0

separation distance (D: μm)

Propagation length and effective index

1 1.47 5 20

n = w = μ m t = nm

n

1.40 1.45 1.46

Cutoff (D: μm ) 0.23 0.78 1.78

P-length (mm) 240 230 60

Propagation length of a single stripe is only about 11mm.

Propagation length of double stripes can be extended more than 10 times!

118 11.58 ,

1550

m

i nm

ε = − + λ =

10μm

D=100nm, t= 20nm D=300nm, t= 20nm

D=500nm, t= 20nm D=780nm, t= 20nm

t= 20nm

t= 16 nm

Mode profile & Mode size

1

1.47 ,

1.45, 5 n = n = w = μ m

Propagation length = 230 mm Propagation length = 46 mm

Both of two modes have mode size of ~ 10 μm

Double metal stripe Single metal stripe

0 100 200 300 400 500 600 700 800 2.0x10^-4

3.0x10^-4 4.0x10^-4

fraction of the field confined metal area (%)

separation distance ( D : nm )

0 100 200 300 400 500 600 700 800 2

3 4 5 6 7 8 9 10

fraction of the field confined n2 area (%)

separation distance ( D : nm )

Fraction of field energy in metal and area

-2 0 2

0.0 0.2 0.4 0.6 0.8 1.0

metal stripe

D = 780nm

Abs(Ey)

vertical distance(μm)

n 2

In metal stripes In n2 dielectric

Butt-coupling efficiency with a SM fiber

-10 -8 -6 -4 -2 0 2 4 6 8 10

0.0 0.2 0.4 0.6 0.8 1.0

Abs(E y)

vertical distance ( μm )

double stripe single stripe

-10 -8 -6 -4 -2 0 2 4 6 8 10

0.0 0.2 0.4 0.6 0.8 1.0

Abs(E y)

lateral distance ( μm )

single stripe double stripe 1 double stripe 2

Vertical profile

Lateral profile

20 18 16 14 12 10

0.40 0.45 0.50 0.55 0.60 0.65 0.70

coupling loss wtih fiber ( dB )

thickness of metal ( t : nm)

100 200 300 400 500 600 700 800

0.50 0.55 0.60 0.65

coupling loss wtih fiber ( dB )

separation distance ( D : nm )

Mode profile Coupling loss with fiber

Single metal strip

Double metal strips

Jung (ETRI), 40 Gbit/s light signal transmission on a long-range SPP waveguide, APL, PTL, 2007.

14 nm-thick, 2.5 μm-wide gold stripes

0.6 dB/cm : World best record in propagation loss.

(Previous world record : 3.2 dB/cm by Berini, 2006)

0.5 1.0 1.5 2.0 2.5

0 1 2 3 4 5 6

Loss (dB)

Waveguide length (cm) λ= 1310 nm

Plasmonic Flexible-wires for 40 GHz interconnections

LR-SPP waveguide VCSEL array

Drive IC

TIA &

Pre amp IC

SMA SMA

PD array

Rx Tx

40 Gb/s

World best

ε

ε

ε

D

D SPP mode w

metal strip

metal slab

core cladding

ε

Y-branch S-band

metal strip metal slab

Double-electrode metal waveguides : Lines, S-band, Y-branch

Joo, Long-range surface-plasmon--polaritons on asymmetric double-electrode structures, APL, 2008.

Localized Surface Plasmons :

Nanofocusing and Nanolithography

'

1 2

At large (

), z

1 .

x

k k

ε → − ε  ≈

Strong confinement at the interface Æ Nano focusing

( : , : - )

z x

E = ± iE air + i metal i

0 20 40 60 80 100

0 2 4 6 8

ω =c k

λ =337 nm; ε

1

= -1

ω (10

s

)

k

( μ m

)

Broad dispersion

Beam radius -> zero!

Propagation Loss (asymmetric mode) High

Propose metal nanowires.

Asymmetric mode : field enhancement at a metallic tip

E _r ^E

E _z

E

* See MOVIES : SPP propagation through a metallic tip

M. I. Stockman, “Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides,” Phys. Rev. Lett. 93, 137404 (2004)]

50nm

2007/5/1 ~

an optical range resonator based on single mode metal-insulator-metal plasmonic gap waveguides.

A small bridge between the resonator and the input waveguide can be used to tune the resonance frequency.

FDTD with the perfectly matched layer boundary conditions

Plasmonic Crystal Demultiplexer and Multiports

the realization of two-dimensional optical wavelength demultiplexers and multiports for surface plasmons

polaritons (SPPs) based on plasmonic crystals, i.e., photonic crystals for SPPs.

Slow Propagation, Anomalous Absorption, and Total External Reflection of Surface Plasmon Polaritons in Nanolayer Systems

n=0 n=2 n=1

we show how the dispersion relation of surface plasmon polaritons (SPPs) propagating along a perfectly conducting wire can be tailored by corrugating its surface with a periodic array of radial grooves.

Importantly, the propagation characteristics of these spoof SPPs can be controlled by the surface geometry, opening the way to important applications such as energy concentration on cylindrical wires and superfocusing using conical structures.

Summary : Plasmonic Waveguides for Photonics

* Long-range (symmetric modes) : Low loss is achievable!

-> Trade-off between Localization and Loss

* Short-range (asymmetric modes) : Nano localization is achievable!

Plasmonics: the next chip-scale technology

Plasmonics is an exciting new device technology that has recently emerged.

A tremendous synergy can be attained by integrating plasmonic, electronic, and conventional dielectric photonic devices on the same chip and taking advantage of the strengths of each technology.

Plasmonic devices,

therefore, might interface naturally with similar speed photonic devices and similar size electronic components. For these reasons, plasmonics may well serve as the missing link between the two device

technologies that currently have a difficult time communicating. By increasing the synergy between these technologies, plasmonics may be able to unleash the full potential of nanoscale functionality and

become the next wave of chip-scale technology.

Summary : Plasmonic Photonics