Surface PlasmonsWhat They Are, and Their Potential

Application in Solar Cells

Martin Kirkengen, AMCS, UiO

Collaboration with Joakim Bergli, Yuri Galperin, Alexander Ulyashin

Definitely work in progress...

What is a Plasmon

• Surface plasmons = surface plasmon polaritons• Fluctuation in density of electrons• Solution to Maxwell equations at metal/dielectric

interface• Exponentially decaying in z-direction

+++ _ _ _ +++ _ _ _ x

z1

2

Dispersion Relation

• Faster phase velocity than in air• Coupling light -> plasmon only possible in special

geometries• Grating (discreet, periodic sampling)

• Roughness• Spheres

(H. Räther, Surface Plasmons, Springer 1988)

Plasmons on a Sphere

• Lowest mode: dipole

• Higher modes

Quadrupole-

+

-

+etc.

n=1

n=3n=2 n=6

Resonance Condition

• General for all modes:m=n2/n1

• Lowest (dipole) mode:n1=1, n=1

• What – negative

2 1nm

n

n22 = m2 = -2,

= -2

n2n1

Bohren&Huffman, Absorption &Scattering of Light by Small Particles (Wiley 1983)

Dielectric Constant of Metals

• 0 at plasmon frequency• Negative dielectric

constant=> imaginary index of refraction => reflection

• High frequencies => transparent

Ag

Free Electrons(Drude model)

Bound electrons(Lorentz model)

Oldest Known Applications

• Colloidal gold in glasses:

Lycurgus Cup (400 AD)Red color due to gold (1400 AD)

Main Plasmon Applications

Today

Measuring dielectric constant of solutions

Guided plasmons -> plasmonics

Forschungszentrum Jülich

What about the Solar Cells?

• Proposed architecture (UNSW):

• Increased spread of incident light – can replace texturing

• Possibly direct transitions from high-k Fourier components of the dipole field

Challenges in Solar Cells

• Avoid reflection

• Shift resonance to visible region

• Cheap method of fabrication

Emission From Dipole Near Surface

• Reciprocity principle+ frustrated total reflection=>emission into”impossible angles”,guided modes

• Emission to front (reflection) reduced.

-

+

Mertz, J. 2002. Radiative abs.etc. J. Opt. Soc. Am. B. 17:1906–1913.

Shifting the Resonance

• SizeLarger particles, higher modes contribute, each mode red-shifted

• ShapeElliptic shape, flatter particles have red-shifted resonance (and stronger coupling to guided modes?)

• Coating/substrateResonance is given as a relative refraction index, changing surroundings changes resonance

• Arrays/clustersLoads of opportunities

How to Make?

• Requirements: Shape, size & position control + price

• Our hope:Deposit oxide + hydrogenation=> hydrogen removes oxygen from structure,remaining metal forms nanoparticles.

• PLEASE HELP!

Thank you for your attention!

-

+