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Taming light with plasmons –theory and experiments. 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

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taming light with plasmons theory and experiments

Taming light with plasmons –theory and experiments

Aliaksandr Rahachou, ITN, LiU

Kristofer Tvingstedt, IFM, LiU

2006.10.19, Hjo

outline
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
slide3

INTRODUCTION TO PLASMONICS

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

slide4

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

slide5

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

slide6

compare with p-polarization:

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

slide7

intensity

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)

slide8

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

slide9

substitute

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:

slide10

Ideal case: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

slide11

Realistic case: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

slide12

surface plasmon length scales:

metall e2

decay into metal

propagation length

decay intodielectric

dielectric e1

z

slide13

dielectric 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?

slide14

prism

prism

Grating

q1

q1

coupling gap

metal

metal

Kretschmann-Raether

geometry

Otto geometry

METHODS OF PLASMON EXCITATION

introduction
Introduction
  • 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
grating manufacturing

1

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 manufacturing1
Grating Manufacturing

Grating is metallized by thermal evaporation of ~90 nm Al

grating characterization
Grating Characterization

Period: 277 nm

Depth: ~48 nm

Rougness ~5 nm

samples
Samples

*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
Grating mirror reflectance

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

*Polarized reflection

*Air metal SP

sample reflectance
Sample reflectance

New absorption peaks!

SP?

Waveguide?

estimating the position of a plasmon peak

normal incidence

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

ESTIMATING THE POSITION OF A PLASMON PEAK

APF03:PCBM 1:4-Al dispersion relation

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

numerical results green s function method

TE (P)-polarized light

Ey

Hz

Ex

NUMERICAL RESULTS (Green’s function method)

~120nm

Flat surface…

APFO3:PCBM

1:4

Al

Air

Air

theoretical results ideal sinosoidal surface

~120nm

TE (P)-polarized light

Ey

277nm

Hz

APFO3:PCBM

1:4

Ex

Air

Al

Air

46nm

THEORETICAL RESULTS (Ideal sinosoidal surface)
realistic surface

Roughness ~ 6x4nm

~120nm

Smooth surface variation

TE (P)-polarized light

Ey

277nm

Hz

APFO3:PCBM

1:4

Ex

Air

Al

Air

46nm

Realistic surface
conclusions
CONCLUSIONS
  • 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
Acknowledgements
  • Nils-Christer Persson for optical characterization of the materials
  • Chalmers for materials
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