Plasmonic Photovoltaics Harry A. Atwater California Institute of Technology

Transcription

1 Plasmonic Photovoltaics Harry A. Atwater California Institute of Technology Surface plasmon polaritons and localized surface plasmons Plasmon propagation and absorption at metal-semiconductor interfaces Coupling of sunlight to surface plasmons Coherent sunlight!

2 Plasmonic Photovoltaics Team: Harry Atwater Albert Polman Mark Brongersma Domenico Pacifici Kylie Catchpole Luke Sweatlock Ewold Verhagen Vivian Ferry

3 Lycurgus Cup, Roman, 4 th Century AD La Farge s Opalescent Window 19 th Century

4 Plasmons at planar metal surfaces and interfaces surface plasmons are longitudinal charge density fluctuations on the surface of a conductor Can form an optical signal with these spatial frequencies on the Ag surface (Light line) dielectric metal see also: Scientific American April 2007 Surface Plasmon dispersion relation for Ag/SiO 2 SPP dispersion relation: k x ω = c ε1ε 2 ε + ε 1 2

5 dielectric metal Surface Plasmon Polariton (SPP) dielectric dielectric metal metal dielectric Asymmetric Coupled Film SPP dielectric Symmetric Coupled Film SPP

6 Thin Metal Film SPP Waveguides Insulator Metal Decreasing Thickness, L+ Insulator Geometry: Planar structures support coupled and decoupled SPP modes, depending upon transverse dimensions Field: Depending on charge symmetry, the E-field can be either symmetric or antisymmetric Spatial Decay: Long-ranging SPPs can extend up to 20um in the dielectric, potentially limiting their mode confinement in the absorber

7 Plasmons in thin metal films dielectric metal dielectric Symmetric Coupled Film SPP dielectric metal dielectric Asymmetric Coupled Film SPP

8 Metal-Insulator-Metal Structures SPP field confinement in unbounded metal/dielectric structure MIM schematic: SPP analog of dielectric slot waveguides Metal Insulator Metal L+: L-: k z1d ε1 k z2 + ε 2k z1 tanh( ) = 2i k z1d ε1 k z2 + ε 2k z1 coth( ) = 2i 0 0

9 MIM Plasmon Slot Waveguide Modes Antisymmetric Symmetric Dionne, et al., Phys. Rev. B (2006)

10 MIM Plasmon Slot Waveguide Propagation Symmetric Modes Propagation Antisymmetric Modes Propagation Skin Depth Skin Depth Symmetric Mode: Thinner films support evanescent modes at longer wavelengths, while thicker films (d>100nm) support propagating modes over an increasing bandwidth Asymmetric Mode: Propagation increases to >30μm with increasing thickness, with fields localized to within 20nm of the structure Zia et.al, JOSA B, (2005); Dionne, et al., MRS Bulletin May (2005)

11 Omnidirectional Surface Plasmon Absorption in MIM Structures MIM SPP analog of dielectric slot waveguides Metal Insulator Metal L+: L-: k z1d ε1 k z2 + ε 2k z1 tanh( ) = 2i k z1d ε1 k z2 + ε 2k z1 coth( ) = 2i 0 0

12 Materials Selection 0.8 Ag ( 41 nm) in solution 20 Au ( 30 nm) in solution extinction coefficient (a.u.) extinction coefficient (a.u.) wavelength (nm) wavelength (nm) λ res =410 nm τ relax 10 fs λ res =524 nm τ relax 4 fs silver: strong resonance but environmentally sensitive gold: interband absorption but environmentally robust

13 Resonances in Small Metal Particles Absorption efficiency for a small metal particle: ε + iε ε ε ε Qabs = 4xIm = 12x ε ε ε ε ε ε 1 2 m m i m ( 1+ 2 m ) + 2 resonant enhancement at the plasmon frequency w p where ε = ε 1 2 m Excitation of electric dipole mode Resonance of polarizability (mode of uniform polarization) Absorption efficiency at plasmon frequency: Want this resonance to occur in the visible and small damping: Noble metals Au or Ag are the metals of choice! Q abs ( ω ) (wf p ) 12xε = m ε 2 ((w ωf p )

14 Carrier Collection vs. Diffusion Length L L d Optically Thick, but Collection-Limited L d << L 1/α<< L Collection across base, but Optically Thin L d >> L 1/α>> L

15 Surface Plasmon Excitation in Thin Films Surface plasmon polaritons (SPPs): 1. guided EM waves at a metal dielectric interface 2. high field confinement at the interface. k i k SPP dielectric metal 6 5 Energy (ev) Δk light cone SPP k i Δk=G=2π/p p k SPP k ' x (nm-1 )

16 Plasmonic Solar Cells Incident Light SPP Low f Contact QW Dot Active Layer Ag SPP Guiding Layer Ag SPP Guiding Layer QW Active Region n-type QW Cladding p-type QW Cladding p+ contact Incident Light p contact Ag nanoparticle plasmon resonant scattering layer n contact semiconductor absorber p-region n-region Glass Glass

17 Intensity decay depth L z,d =1/(2k z,d ) Lossy SP waves Si/Ag GaAs/Ag Si/Al GaAs/Al Si Ag z E 2 λ=785 nm Most energy is in a < 50 nm thick layer L z,d

18 Efficiency : P abs,semiconductor /P abs,total Ewold Verhagen Si/Ag GaAs/Ag Si/Al GaAs/Al η = d d dz Im[ ε ] E( z) + dz Im[ ε ] E( z) d dz Im[ ε ] E( z) d m m To do: integrate efficiency over solar spectrum

19 Surface Plasmon Enhanced Absorption in Ultrathin Layers of CdSe Quantum Dots I 0 Domenico Pacifici thick Ag film CdSe quantum dots I T (P,λ)

20 Plasmonic Modulator 514.5nm Without CdSe QDs With CdSe QDs Normalized Transmission w/out CdSe QDs n surf =1.08 w/ CdSe QDs λ 0 =514.5nm, 0.57 W/cm 2 n surf =1.14 L=25μm L=1.2μm Slit-Groove Distance (nm) Plasmon Extinction via Excitonic Absorption above Quantum Dot Optical Gap

21 SPP-Induced Quantum Dot Excitonic 514.5nm 15 2 σ = cm -1 ρ cm -3 α = σ ρ 10 4 cm L 1 = α 10 4 cm = 1 μ m CdSe QD Layer 20 nm Normalized Transmission e -Dα/2 Fit L=α -1 =1.2μm Slit-Groove Distance (nm) Pacifici, et.al., Nature Photonics July 2007

22 Surface plasmon coupling in hole arrays: Role of symmetry and periodicity A B Domenico Pacifici C D a k i Dk=G=2p/p k SPP a=[200,800nm] p

23 Transmission spectra w/ lamp a=400nm 10 8 A Square Triang. B Normalized Transmission C Penrose D Dodeca Wavelength (nm) Wavelength (nm)

24 SPP at a Ag/SiO 2 interface n SPP 1.7 Normalized Transmission nm 450 nm 500 nm 550 nm 600 nm λ (nm) Square Array - 150nm Ag - glass, glass Wavelength (nm)

25 Quantum Dot Layer Increases Plasmonic Mode Refractive index

26 Universal Hole Array spectra Normalized Transmission k SPP =G pq =2π(p 2 +q 2 ) 1/2 /a k SPP =G pq =4π(p 2 +q 2 +pq) 1/2 /(a 3 1/2 ) Square k=g 12 k=g 02 k=g 11 Penrose Triangular k=g 10 k=g k=g k=g Dodecagonal a(nm)= λ/(n SPP a)

27 Comparison between Coherent and Incoherent illumination 514.5nm Normalized Transmission Square array Incoherent Coherent a (nm) 514.5nm Normalized Transmission Triangular array Incoherent Coherent a (nm) 514.5nm Normalized Transmission Penrose array Incoherent Coherent a (nm) 514.5nm Normalized Transmission Dodeca array Incoherent Coherent a (nm)

28 Coherent! The Sun: an Incoherent Illumination Source... Coherence Length R c

29 Surface Plasmon Incoupling at Sub-λ (100 nm)groove Si k i Vivian Ferry, Luke Sweatlock Ag k Si i Ag

30 Coupling to Plasmonic and Photonic Modes in Si Films λ = 1 μm Plasmonic Photonic

31 Summary Plasmonic modes strongly confined < 100 nm in semiconductors 100x Light Absorption w.r.t. in plasmonic modes in CdSe w.r.t. bulk CdSe Incoupling to waveguide modes (SPP and dielectric) requires momentum matching structure short range order Incoupling structures with angular and spectral insensitivity under investigation Surface plasmon polariton modes well-suited to direct bandgap semiconductors