Taming light with plasmons –theory and experiments

1. Taming light with plasmons –theory and experiments Aliaksandr Rahachou, ITN, LiU Kristofer Tvingstedt, IFM, LiU 2006.10.19, Hjo

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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)

8. 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

9. 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:

10. 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

11. 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

12. surface plasmon length scales: metall e2 decay into metal propagation length decay intodielectric dielectric e1 z

13. 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?

14. prism prism Grating q1 q1 coupling gap metal metal Kretschmann-Raether geometry Otto geometry METHODS OF PLASMON EXCITATION

15. Observation of plasmon enhanced absorbtion in Apfo3/PCBM

16. 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

17. 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

18. Grating Manufacturing Grating is metallized by thermal evaporation of ~90 nm Al

19. Grating Characterization Period: 277 nm Depth: ~48 nm Rougness ~5 nm

20. 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)

21. Grating mirror reflectance Different orientation/polarization shows very different reflectance in the UV region. *Polarized reflection *Air metal SP

22. Sample reflectance New absorption peaks! SP? Waveguide?

23. Initial results: Photocurrent from inverted cells

24. Al-air plasmonic peak CLEAN GRATING MIRROR

25. 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

26. TE (P)-polarized light Ey Hz Ex NUMERICAL RESULTS (Green’s function method) ~120nm Flat surface… APFO3:PCBM 1:4 Al Air Air

27. Flat surface and experiment once again...

28. ~120nm TE (P)-polarized light Ey 277nm Hz APFO3:PCBM 1:4 Ex Air Al Air 46nm THEORETICAL RESULTS (Ideal sinosoidal surface)

29. THEORETICAL RESULTS (Sinusoidal surface)

30. Roughness ~ 6x4nm ~120nm Smooth surface variation TE (P)-polarized light Ey 277nm Hz APFO3:PCBM 1:4 Ex Air Al Air 46nm Realistic surface

31. 25nm Realistic surface

32. ~250 nm thick polymer Absoptance peaks ?

33. 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!

34. Acknowledgements • Nils-Christer Persson for optical characterization of the materials • Chalmers for materials