Broad-band nano-scale light propagation in plasmonic structures

Shanhui Fan, G. VeronisDepartment of Electrical Engineering and Ginzton Laboratory

Stanford University

In collaborations with Professors Mark Brongersma and Peter PeumansSupported by the Stanford-GCEP, and NSF-NIRT

Organic Solar Cell

• Silicon and Compound Semiconductor Cells• High efficiency (~30%), but high cost.

• Organic Solar Cell• Low cost, but low efficiency (<5%)

Operational Principle of Organic Solar Cells

Photon absorption Exciton diffusion Charge-transfer

Charge-separation Charge-collection

D

A

Exciton Diffusion Process

D

A

Optical absorption~ 100nm

Exciton Diffusion~ 10nm

• Deliver light directly to the DA interface.• Enhance light absorption at the DA interface.• Efficiently extract carriers once they are generated.

Nanoscale manipulation of light and electrons using metals.

From single-wavelength to deep sub-wavelength scaleFrom single-wavelength to deep sub-wavelength scale

Core of a single mode fiber: ~ 10 m

1 m

Scale of a transistor, < 100nm

Vlasov et al, IBM, 2004

Kobrinsky et al, Intel, 2004

Scale of SOI waveguide

Nanoscale photodetector or latch

Micron scale dielectric waveguide

The need for nano-photonics in optical interconnectThe need for nano-photonics in optical interconnect

Stanford MURI on Plasmonics (Brongersma, Miller, Fan)

•The relevant length scales here:modal diameter ~ 50-100 nanometer; propagation distance ~ 10 micron

• Broadband width.

Two-conductor configuration: perfect metalTwo-conductor configuration: perfect metal

Perfect metal

d

Air

0

0.5

1

1.5

0 0.5 1 1.5 2Wavevector k/kp

Fre

que

ncy

/

p

Two-conductor configuration: plasmonic metalTwo-conductor configuration: plasmonic metal

Plasmonic metal

d

Air

0

0.5

1

1.5

0 0.5 1 1.5 2

Wavevector k/kp

Fre

que

ncy

/

pε ( ) =1 −

ω p2

ω ω + iγ( )

Band 1

Band 2

E. N. Economu, Physical Review B, 182, 539 (1969)

Band 1Band 1

Magnetic field Electric field

Low Frequency

IntermediateFrequency(infrared and visible)

HighFrequency

(ultra-violet)

Band 1Band 1

Magnetic field Electric field

Low Frequency

IntermediateFrequency(infrared and visible)

HighFrequency

(ultra-violet)

Plasmonic slot waveguidePlasmonic slot waveguide

SiO2 (n=1.5)

metal metal

air (n=1)

• The corresponding microwave structure does not support a true bound mode in this asymmetric geometry.

• Intermediate regime showing both microwave and plasmonic behaviors.

slot dimension: 50~100 nm

G. Veronis and S. Fan, Optics Letters, 30, 3359 (2005)

Bound-mode in plasmonic slot waveguidesBound-mode in plasmonic slot waveguides

w = 50 nm

=1550nm =1550nm

• Calculated using tabulated experimentally determined dielectric function of silver at all frequencies.

• True bound mode.• Guiding bandwidth exceeding 100THz.

Modal Diameter << WavelengthModal Diameter << Wavelength

=1550nm

• Mode diameter is small even when the phase index approaches that of silica.

• Mode diameter ~ 90 nm at 1.55 micron wavelength. • Mode diameter weakly dependent upon frequency.

y

G. Veronis and S. Fan, Optics Letters, 30, 3359 (2005)

Far field v.s. near fieldFar field v.s. near field

• Modal size determined by the near field. • Exponential decay only appears far from waveguide, where the field

amplitudes are already negligible.

y

0Z

Z0

Nano-scale waveguide bends

•Complete transmission through sharp bends from microwave to optical wavelength range

Ag

air

Ag

50 nm

G. Veronis and S. Fan, Applied Physics Letters, 87, 131102 (2005).

Coupling between dielectric and MDM guide

G. Veronis and S. Fan, Optics Express (submitted)

• Non-adiabatic taper.• Designed with micro-genetic algorithm. • 93% Coupling efficiency.

• Direct butt coupling. • 70% Coupling efficiency.

Summary

• Proper design of metallic nano-structures leads to sub-wavelength propagating modes with very broad bandwidth.

• Such modes might be exploited for nano-scale manipulation of light in energy and information applications.

• Plasmonic crystals may also be used to substantially modify optical absorption and thermal emission properties.