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Taming light with plasmons –theory and experimentsPowerPoint Presentation

Taming light with plasmons –theory and experiments

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### Taming light with plasmons –theory and experiments

### Observation of plasmon enhanced absorbtion in Apfo3/PCBM and the boundary conditions:

Aliaksandr Rahachou, ITN, LiU

Kristofer Tvingstedt, IFM, LiU

2006.10.19, Hjo

OUTLINE

- Introduction to plasmonics
- Optical excitation of plasmons
- Plasmons in organic solar cells
- Experimental results for APFO3:PCBM on Al gratings
- Theoretical results for APFO3:PCBM on Al gratings

s-polarization:E-field is perpendicular to the plane of incidence (German senkrecht = perpedicular)

p-polarization:E-field is parallel to the plane of incidence

Ez

Hz

E

H

Hy

Ey

Ex

Hx

q1

q1

e1

e1

z=0

z=0

y

y

e2

e2

x

x

q2

q2

z

z

Boundary condition:(a) transverse component of E is conserved,

(b) normal component of D is conserved

creation of the polarization charges

if one of the materials is metal, the electrons will respond to this polarization. This will give rise to surface plasmon modes

p-polarized incident radiation will create polarization charges at the interface. We will show that these charges give rise to a surface plasmon modes

E1z

E1

H1y

e1

E1x

z=0

E2

E2z

y

e2

x

H2y

E2x

z

Polarization charges are created at the interface between two material.

The electrons in metal will respond to this polarization giving rise to surface plasmon modes

compare with p-polarization: two material.

s-polarized incident radiation does not create polarization charges at the interface. It thus can not excite surface plasmon modes

Boundary condition(note that E-field has a transverse component only):

transverse component of E is conserved,

H1z

H1

E1y

e1

H1x

z=0

H2

H2z

y

e2

x

E2y

no polarization charges are created no surface plasmon modes are excited!In what follows we shall consider the case of p-polarization only

H2x

z

intensity two material.

we are looking for a localized surface mode, decaying into both materials

wave propagating in x-direction

z

Thus, the solution can be written as

More detailed theory

Let us check whether p-polarized incident radiation can excite a surface mode

dielectric e1

E1z

E1

H1y

E1x

z=0

y

x

z

metall e2

components of E-, H-fields: E = (Ex, 0, Ez); H = (0, Hy, 0)

Let us see whether this solution satisfies Maxwell equation and the boundary conditions:

condition imposed on k-vector

+

solution for a surface plasmon mode:

dielectric e1

E1z

E1

H1y

E1x

z=0

y

x

z

metall e2

substitute and the boundary conditions:

kx

The surface plasmon mode always lies beyond the light line, that is it has greater momentum than a free photon of the same frequency

light cone = c k

w

k

What is the wavelength of the surface plasmon ?

let us find k:

Ideal case: and the boundary conditions:r1 and r2 are real (no imaginary components = no losses)

Dielectric: r1 >0

kx is real

Metal: r2 < 0, |r2| >> r1

resonant width = 0 lifetime =

k

Realistic case: and the boundary conditions:r1 is real, and r2 is complex,

imaginary part describes losses in metal

resonant width (gives rise to losses)

k

Dielectric functions of Ag, Al

surface plasmon length scales: and the boundary conditions:

metall e2

decay into metal

propagation length

decay intodielectric

dielectric e1

z

dielectric and the boundary conditions: e1

kx

metall e2

The surface plasmon mode always lie beyond the light line, that is it has greater momentum than a free photon of the same frequency .

This makes a direct excitation of a surface plasmon mode impossible!

light cone = c k

w

k

OPTICAL EXCITATION OF PLASMONS

is it possible to excite a plasmon mode by shining light directly on a dielectric/metal interface?

prism and the boundary conditions:

prism

Grating

q1

q1

coupling gap

metal

metal

Kretschmann-Raether

geometry

Otto geometry

METHODS OF PLASMON EXCITATION

Introduction and the boundary conditions:

- Prescence of periodic metal gratings in a dielectric environment triggers surface plasmons and creates an intense optical near field
- An absorbing layer on top of the grating should therefore be exposed to a strong field
- Plasmons are traveling along the interface (not perpendicular as the impinging light)
- Introducing Surface plasmons in solar cells may hence increase the absorption

1 and the boundary conditions:

2

3

Grating manufacturing- Optical diffraction gratings are replicated via PDMS replica molding
- The PDMS replica is subsequently imprinted in a photocureable resin.
- Very high replication throughput

Grating Manufacturing and the boundary conditions:

Grating is metallized by thermal evaporation of ~90 nm Al

Samples and the boundary conditions:

*Metal gratings coated with ~150 nm Apfo3/PCBM 1:4 mixture

*Planar mirror reference samples manufactured

*Reflectance measured in integrating sphere (all angels collected)

Grating mirror reflectance and the boundary conditions:

Different orientation/polarization shows very different reflectance in the UV region.

*Polarized reflection

*Air metal SP

Initial results: and the boundary conditions:Photocurrent from inverted cells

Al-air plasmonic peak and the boundary conditions:

CLEAN GRATING MIRRORnormal incidence and the boundary conditions:

where d is a period of grating (sinusoidal, tiranglar or step-like)

ESTIMATING THE POSITION OF A PLASMON PEAKAPF03:PCBM 1:4-Al dispersion relation

Dielectric function of APFO3:PCBM 1:4 in direction normal to the surface

TE (P)-polarized light and the boundary conditions:

Ey

Hz

Ex

NUMERICAL RESULTS (Green’s function method)~120nm

Flat surface…

APFO3:PCBM

1:4

Al

Air

Air

Flat surface and experiment once again... and the boundary conditions:

~120nm and the boundary conditions:

TE (P)-polarized light

Ey

277nm

Hz

APFO3:PCBM

1:4

Ex

Air

Al

Air

46nm

THEORETICAL RESULTS (Ideal sinosoidal surface)THEORETICAL RESULTS (Sinusoidal surface) and the boundary conditions:

Roughness ~ 6x4nm and the boundary conditions:

~120nm

Smooth surface variation

TE (P)-polarized light

Ey

277nm

Hz

APFO3:PCBM

1:4

Ex

Air

Al

Air

46nm

Realistic surface25 and the boundary conditions:nm

Realistic surfaceCONCLUSIONS and the boundary conditions:

- We demonstrated both experimentally and theretically enchanced absorptance of light in APFO3:PCBM 1:4 solar-cells with Al gratings
- Easy manufacturing with soft lithography.
- The theoretical and experimental data agree very well!

THANKYOU!

Acknowledgements and the boundary conditions:

- Nils-Christer Persson for optical characterization of the materials
- Chalmers for materials

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