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 sourceShort-range(~ nm)

waveguides

Nano-photonics

~ cm

Long-range(~ cm) waveguides

Nano-electronics

Photonic integrated circuit

0 20 40 60 80 1000

2

4

6

8ω=c kx

λ=337 nm; ε1= -1

ω (1

015 s

-1)

kx (μm-1)

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

ε1 : metal

ε2 : dielectric

x

1 2

1 2xk

cε εω

ε ε=

+

'1 2

1At large ( ), z .ixx

kk

ε ε→ − ≈

Strong confinement at the interface

Nano focusing

'1At low ( 1),xk ε >>

'1 in air :z

x

Ei

Eε= −

( : , : - )z xE iE air i metal i= ± +

'1

1 in metal :z

x

Ei

E ε=

0 20 40 60 80 1000

2

4

6

8ω=c kx

λ=337 nm; ε1= -1

ω (1

015 s

-1)

kx (μm-1)

Broad dispersion

Low loss at the interface

Wave guiding

Nano Focusing & Wave Guiding

Surface Plasmons excited on thin metal films

Dielectric – ε3

Dielectric – ε1

Metal – ε2

Several 1 cm long, 15 nm thin and 8 micron wide gold stripes guiding LRSPPs3-6 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.

modeoverlap

Possibility of Propagation Range Extensionfr

eque

ncy

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

60h nm=

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

10h nm=

0 20 40 60 80 1000

5

10

15

20

25

30

35

40

Ws/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’syz

z

0z =

h

1ε

2ε

3ε

[ ]0( , ) ( )expyx z L f z i xβ=L e

Magnetic field : L=H/Zo

2 2 20j js kβ ε= −

.

[ ] [ ] [ ] ( )

[ ] [ ] ( )

[ ] ( )

2 12 2 3

1 2

2 12 2

1 2

1

cosh sinh exp ( )

( ) cosh sinh 0

exp 0

ss h s h s z h z hs

sf z s z s z z hs

s z z

εε

εε

⎧⎛ ⎞+ − − ≥⎪⎜ ⎟

⎝ ⎠⎪⎪⎪= + ≤ ≤⎨⎪⎪ ≤⎪⎪⎩ Confinement

Propagation Loss

Asymmetric mode

Symmetric mode

18nm 20nm14nm 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

20nmfiberAu (-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

Tran

smitt

ance

(dB

)

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

1544.1 1558.3

INPUT OUTPUT

Polymer 1Substrate

+ -

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

1.477εmetal=-116+11.58i (gold)εdielectric=2.16λ0=1550nmt=20nm

symmetric even mode symmetric odd mode

R/k0

distance(d : distance between two slabs)

εd = 2.16

εm = −116 +11.58id

t= 20nm

d=4um 254 umd=6um 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

oddodd

Lateral DC

Extinction ration at 400um : 27dB

Channel 1 Channel 2

Vertical directional couplers

Variable optical attenuator based on LR-SPPSubmitted to EL, S. Park & S. Song

Extremely long-range SPP ?

in-plane wavevector

freq

uenc

y

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

n3n2

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

1001 1.4 1.45 1.46

1.47

1.48

1.49

1.5

1.6

prop

agat

ion

leng

th(m

m)

separation distance(D : μm)

Extended Long-Range SPPs

Metal

n1

n1

n2 D

metal

n5

n4n3

n2

n0

n5 ~n1> n0~n2~n4

n1

Range extension with finite-width metal stripes

D < Dcutoff

n2 < n1

No good

Two fundamental modes Even mode only

n1

n2 Dt

w

0 1 2 3 4

1

10

100

1.46

1.451.40

1.48

1.50

1.47

prop

agat

ion

leng

th (m

m)

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 20n w m t nmμ= = =

n2 1.40 1.45 1.46Cutoff (D: μm ) 0.23 0.78 1.78P-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!

0118 11.58 , 1550m i nmε 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 21.47 , 1.45, 5n n w mμ= = =

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

fra

ctio

n of

the

field

con

fined

met

al a

rea

(%)

separation distance ( D : nm )0 100 200 300 400 500 600 700 800

2

3

4

5

6

7

8

9

10

fract

ion

of th

e fie

ld c

onfin

ed n

2 are

a (%

)

separation distance ( D : nm )

Fraction of field energy in metal and area

-2 0 20.0

0.2

0.4

0.6

0.8

1.0

metal stripe

D = 780nm

Abs

(Ey)

vertical distance(μm)

2n

In metal stripes In n2 dielectric

Butt-coupling efficiency with a SM fiber

-10 -8 -6 -4 -2 0 2 4 6 8 100.0

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 8 100.0

0.2

0.4

0.6

0.8

1.0

Abs

(Ey)

lateral distance ( μm )

single stripe double stripe 1 double stripe 2

Vertical profile

Lateral profile

20 18 16 14 12 100.40

0.45

0.50

0.55

0.60

0.65

0.70

coup

ling

loss

wtih

fibe

r ( d

B )

thickness of metal ( t : nm)

100 200 300 400 500 600 700 800

0.50

0.55

0.60

0.65

coup

ling

loss

wtih

fibe

r ( d

B )

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 2.50

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

εd3

εd3

ε2D

D

wSPP mode

metal strip

metal slab

core

cladding

εd1

Y-branchS-band

metal stripmetal slab

Double-electrode metal waveguides : Lines, S-band, Y-branchJoo, Long-range surface -plasmon--polaritons on asymmetric double-electrode structures, APL, 2008.

Localized Surface Plasmons : Nanofocusing and Nanolithography

'1 2

1At large ( ), z .ixx

kk

ε ε→ − ≈

Strong confinement at the interface

Nano focusing

( : , : - )z xE iE air i metal i= ± +

0 20 40 60 80 1000

2

4

6

8ω=c kx

λ=337 nm; ε1= -1

ω (1

015 s

-1)

kx (μm-1)

Broad dispersion

Beam radius -> zero!

Propagation Loss(asymmetric mode)

High

Propose metal nanowires.

Asymmetric mode : field enhancement at a metallic tip

Er EzEr

Ez

* 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=1n=2

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 devicesand similar size electronic components. For these reasons, plasmonicsmay well serve as the missing link between the two devicetechnologies that currently have a difficult time communicating. Byincreasing the synergy between these technologies, plasmonics may beable to unleash the full potential of nanoscale functionality andbecome the next wave of chip-scale technology.

Summary : Plasmonic Photonics