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Surface Enhanced Fluorescence Ellane J. Park Turro Group Meeting July 15, 2008 1 Surface Enhanced Fluorescence (SEF) • Motivation: biological systems – Enhancement of Fluorescence Intensity – Adjust fluorescence lifetime • Techniques to couple SPP and photons SEFS Applications Metals Aluminum Silver Gold Copper Nanoholes Nanoparticles Thin Films – High index materials and specific optical geometry --> Increase momentum of photon • Example: metal on glass (e.g. Kretschmann configuation) – Break up translational motion with corrugated or roughened metallic surfaces. J. Phys. D: Appl. Phys. (2008) 41, 013001 2 J. R. Lackowicz et. al. Proc. Of SPIE, 6099, 609909-1 (2006) SEF Distance and Orientation Dependence • What is the “enhancement factor”? – Magnitude of increase in fluorescence intensity of fluorophore in presence of metal, compared to that of fluorophore in the absence of metal • Distance – Less than 10 nm: Non-radiative energy transfer can occur (i.e. quenching) – Greater than 10 nm: Fluorescence Intensity can be enhanced by up to 100-fold • Dipole Orientation – Dipoles that oscillate perpendicular to surface couple efficiently with surface plasmon modes Calculated Fluorescence intensity of free fluorophore as a function of distance to Au NP J. Phys. D: Appl. Phys. (2008) 41, 013001 3 Fluorescence Intensity of Au Nanorods y x AR= x y CSR. 35, 209 (2006) 4 Fluorescence Enhancement in Nanoholes (a) Fluorescence decay curves of Rhodamine 6G in open solution and into a d=150 nm hole (b) 3-D finite-element time-domain simulation of intensity distribution for a d=50 nm nanohole • • Lifetime reduction from 3.87 ns in open solution to 0.9 ns above nanohole Effects of nanohole diameters and periodicity 5 J. Phys. D: Appl. Phys. (2008) 41, 013001 Theory: SEF in Ag Nanoshells • Quantum Dots are highly photostable. • Light absorbed by metal --> fluorophore • Potential Effects of fluorophore in Nanoshell: – Photostability (Protection from oxygen) – Higher radiative decay rates and higher quantum yields – Emission is 2-fold narrower – Shorter lifetime --> less time for photochemistry in excited state J. R. Lackowicz et. al. Proc. Of SPIE, 6099, 609909-1 (2006) 6 Thank You 7 Surface Plasmons +: lower e- density -: higher e- density : angular frequency c: Speed of light : Incident angle with the normal to surface in dielectric medium kspp: In-plane wavevector of SPP modes Kspp (greater momentum) is greater than k0 (free space photon of same pulsation) • Note: Energy flow from fluorophore to SPP modes can be detrimental to SEF process 8 J. Phys. D: Appl. Phys. (2008) 41, 013001 9