Surface plasmon waveguides

Introduction Size Mismatch between Scaled CMOS Electronics and Planar Photonics Photonic integrated system with subwavelength scale components CMOS transistor: Medium-sized molecule dielectric waveguide ~ 10 λ

Silicon Photonics?

Harry Atwater, California Institute of Technology A World of Nanophotonic Devices On-chip light source Long-range(~ cm) waveguides ~ cm Short-range(~ nm) waveguides Photonic integrated circuit Nano-photonics Nano-electronics Could such an Architecture be Realized with Metal rather than Dielectric Waveguide Technology?

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 verythin metal strips Modes of very-thin(~ 10 nm) metal strips Experimental results on SPP waveguide devices 50nm

Plasmons at Planar Metal-Dielectric Interfaces surface plasmons are longitudinal charge density fluctuations on the surface of a conductor surface plasmon dispersion relation: k x ε 2 : dielectric ε 1 : metal ω = c ε1ε 2 ε + ε 1 2 x ω (10 15 s -1 ) Surface Plasmon dispersion relation for Ag in air 8 6 4 2 Harry Atwater, California Institute of Technology λ = 337 nm ω=c k x (Light line) λ=337 nm; ε 1 = -1 Plasmon Dispersion Relation 0 0 20 40 60 80 100 k x (μm -1 ) λ << 337 nm Plasmons are highly localized at metal-dielectric interfaces, so potential for: Ultrasmall Optical Devices 2D-Optics on metal surfaces

Nano Focusing & Wave Guiding 8 ω=c k x ' 1 At large kx ( ε1 ε2), z i. kx E =± ie ( air : + i, metal :- i) z x 6 λ=337 nm; ε 1 = -1 Strong confinement at the interface ω (10 15 s -1 ) 4 2 Broad dispersion Nano focusing 0 0 20 40 60 80 100 k x (μm -1 ) At low k ( ε >> 1), E E z x x = i ε ' 1 ' 1 in air : E E z x = i 1 ε ' 1 in metal : Low loss at the interface Wave guiding

Surface Plasmons excited on thin metal films Dielectric ε 3 Metal ε 2 Dielectric ε 1

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 Long-Range SP: weak surface confinement, low loss frequency Short-Range SP: strong surface confinement, high loss in-plane wavevector

SPP modes at a very thin metal film Symmetric mode (long-range SPP) H E Anti-symmetric mode (short-range SPP)

Introduction: Dependence of dispersion on film thickness 1 1 0.75 0.75 0.5 0.5 0.25 0.25 250 500 750 1000 1250 1500 200 400 600 800 250 500 750 1000 1250 1500 200 400 600 800-0.25-0.25-0.5-0.75-1 h = 10 60nm -0.5-0.75-1 practically forbidden

Field solution and dispersion relation of coupled SPP s 40 35 30 W s /W a 25 20 15 10 5 0 0 20 40 60 80 100 thickness of metal film [nm] Symmetric Asymmetric

Propagation loss and field confinement of SPP s z ε 3 Magnetic field : L=H/Z o h z = 0 z y ε 2 ε 1 0 [ β ] L( x, z) = e L f( z)exp i x y s = β ε k 2 2 2 j j 0. ε2s 1 cosh[ sh 2 ] + sinh[ sh 2 ] exp [ s3( z h) ] ( z h) ε1s2 ε s f ( z) = cosh s2z + sinh s2z 0 z h ε1s2 exp[ sz 1 ] ( z 0) 2 1 [ ] [ ] ( ) Confinement Asymmetric mode Symmetric mode Propagation Loss

Fundamental symmetric mode of a metal stripe : thickness (T) W=10um T LR-SP WG 14nm 16nm 18nm 20nm

P. Berini, PhotonicWest 2005.

(Spectalis Co.)

Fundamental asymmetric mode of a metal stripe : Δn 1.68 1.68+ Δn T=16 nm, W=10um Δn = 0.001 Δn = 0.002 Δn = 0.003

Symmetric mode guided by a metallic channel waveguide fiber Polymer (n=1.47) @ 1.55μm silicon Au (-96+i11) 9μm 20nm 15mm ~10μm Propagation loss : 21dB/ cm

Y-branch Channel-1 Channel-2 1 2

Wavelength shifts by direct heating a metal wire Polymer 2 INPUT OUTPUT + - Polymer 1 Substrate -40-45 -50 Transmittance (db) -55-60 -65-70 1544.1 1558.3 1540 1545 1550 1555 1560 Wavelength (nm)

Tunable Wavelength Filter

Vertical directional couplers H. Won, APL vol.88, 011110 (2006)

Vertical directional couplers d=4um 254 um d=6um 558 um εm = 116 +11.58i t= 20nm d εd = 2.16 R /k 0 1.477 1.476 1.475 1.474 1.473 ε metal =-116+11.58i (gold) ε dielectric =2.16 λ 0 =1550nm t=20nm symmetric even mode symmetric odd mode 1.472 1.471 1.470 0 2 4 6 8 10 12 14 16 distance(d : distance between two slabs)

Even mode and odd mode : directional couplers based on LRSPP Vertical D Lateral D 4μm, even mode 4μm, odd mode 0.08μm, even mode 0.08μm, odd mode 7μm, even mode 7μm, odd mode 3μm, even mode 3μm, odd mode 21μm, even mode 21μm, odd mode 23μm, even mode 23μm, odd mode

Vertical directional couplers odd odd even mode odd mode Lateral DC

Vertical directional couplers Channel 1 Channel 2 Extinction ration at 400um : 27dB

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

Extremely long-range SPP? Symmetrically coupled LRSP frequency Anti-symmetrically coupled LRSP in-plane wavevector

Extension of SPP propagation length LR SPP Thin metal film Finite-width metal strip D. Sarid (PRL, 1981) J. J. Burke (APL, 1986) P. Berini (PRB, 2000) Double metal films n2 > n1 Metal-dielectric films n1 > n0~n4 Metal n1 n2 n1 G. I. Stegman et al (APL, 1983) n4 metal n3 n2 n1 n0 F. Y. Kou et al (OL, 1987)

Extended Long-Range SPPs n1 Metal n2 n1 100 1 1.4 1.45 1.46 D n5 ~n1> n0~n2~n4 n5 n4 metal n3 n2 n1 n0 propagation length(mm) 10 1 0.1 1.47 1.48 1.49 1.5 1.6 0 1000 2000 3000 4000 separation distance(d : μm)

Range extension with finite-width metal stripes t n 1 n 2 D No good w Two fundamental modes Even mode only D<D cutoff n 2 < n 1

propagation length (mm) Propagation length and effective index 100 1.40 1.45 1.46 10 1.47 1.48 1 1.50 0 1 2 3 4 separation distance (D: μm) n1 = 1.47 w= 5μm t = 20nm ε m = 118+ 11.58 i, λ0 = 1550nm β r /k 0 1.482 1.50 1.480 1.478 1.476 1.48 1.474 1.47 1.472 1.4 1.45 1.46 1.470 0 1 2 3 4 separation distance (D: μm) n 2 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!

Mode profile & Mode size n = 1.47, n = 1.45, w= 5μm 1 2 Double metal stripe Single metal stripe D=100nm, t= 20nm D=300nm, t= 20nm t= 20nm 10μm D=500nm, t= 20nm D=780nm, t= 20nm t= 16 nm Both of two modes have mode size of ~ 10 μm Propagation length = 230 mm Propagation length = 46 mm

Fraction of field energy in metal and area n 2 1.0 D = 780nm 0.8 Abs(Ey) 0.6 0.4 metal stripe 0.2 0.0-2 0 2 vertical distance(μm) In metal stripes In n2 dielectric fraction of the field confined metal area (%) 4.0x10-4 3.0x10-4 2.0x10-4 0 100 200 300 400 500 600 700 800 separation distance ( D : nm ) fraction of the field confined n 2 area (%) 10 9 8 7 6 5 4 3 2 0 100 200 300 400 500 600 700 800 separation distance ( D : nm )

Butt-coupling efficiency with a SM fiber 0.70 Abs(E y ) 1.0 0.8 0.6 0.4 0.2 Vertical profile double stripe single stripe coupling loss wtih fiber ( db ) 0.65 0.60 0.55 0.50 0.45 Single metal strip 0.0-10 -8-6 -4-2 0 2 4 6 8 10 vertical distance ( μm ) 0.40 20 18 16 14 12 10 thickness of metal ( t : nm) Abs(E y ) 1.0 0.8 0.6 0.4 0.2 Lateral profile 0.0-10 -8-6 -4-2 0 2 4 6 8 10 lateral distance ( μm ) single stripe double stripe 1 double stripe 2 coupling loss wtih fiber ( db ) 0.65 0.60 0.55 0.50 Double metal strips 100 200 300 400 500 600 700 800 separation distance ( D : nm ) Mode profile Coupling loss with fiber

Plasmonic Flexible-wires for 40 GHz interconnections Jung (ETRI), 40 Gbit/s light signal transmission on a long-range SPP waveguide, APL, PTL, 2007. Tx Drive IC TIA & Pre amp IC LR-SPP waveguide SMA VCSEL array PD array SMA Rx 14 nm-thick, 2.5 μm-wide gold stripes 6 5 40 Gb/s World best Loss (db) 4 3 2 λ= 1310 nm 1 0 0.5 1.0 1.5 2.0 2.5 Waveguide length (cm) 0.6 db/cm : World best record in propagation loss. (Previous world record : 3.2 db/cm by Berini, 2006)

Double-electrode metal waveguides : Lines, S-band, Y-branch Joo, Long-range surface -plasmon--polaritons on asymmetric double-electrode structures, APL, 2008. D metal strip SPP mode ε d3 w ε 2 ε d1 D core metal slab ε d3 cladding S-band metal strip metal slab Y-branch

Localized Surface Plasmons : Nanofocusing and Nanolithography 8 ω=c k x ' 1 At large kx ( ε1 ε2), z i. kx E = ± ie ( air : + i, metal :- i) z x 6 λ=337 nm; ε 1 = -1 Strong confinement at the interface ω (10 15 s -1 ) 4 2 Broad dispersion Nano focusing 0 0 20 40 60 80 100 k x (μm -1 )

Propose metal nanowires. Propagation Loss (asymmetric mode) High Beam radius -> zero!

Asymmetric mode : field enhancement at a metallic tip E r E r E z E z M. I. Stockman, Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides, Phys. Rev. Lett. 93, 137404 (2004)] 50nm * See MOVIES : SPP propagation through a metallic tip

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=2 n=1 n=0

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 * Short-range (asymmetric modes) : Nano localization is achievable! * Long-range (symmetric modes) : Low loss is achievable! -> Trade-off between Localization and Loss

Summary : Plasmonic Photonics 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.