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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)
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!