Localized surface plasmon resonance of optically coupled metal particles

Localized surface plasmon resonance of optically coupled metal particles Takumi Sannomiya*, Christian Hafner**, Janos Vörös* * Laboratory of Biosensors and Bioelectronics, IBT ** Computational Optics Group, IFH ETH Zürich

Outline BackgroundLSPR of colloid particles, motivations Method- MMP Calculation - Experimental setup Results- inter-particle coupling- selective excitation of coupling mode- possibility of more sensitive adsorption detection Summary & Outlook

Resonance shift upon molecular binding Colloidal particle light Extinction l lr lr´ Molecule-adsorbed Localized Surface Plasmon Resonance(LSPR) for Sensors Adsorption Sensor Functionalize for certain molecules motivation: investigate adsorption of single colloid particle single molecule detection ?

Different colors illumination particle <<1/l Scattered light Objective Lens spectrometer How to take a spectrum of a single colloidal particle Dark field microscopy + spectrometer  scattering spectrum Dark field image of 50nm Au colloids

Calculated Spectra Second peak appears Same as single particle Non-coupled mode Optically coupled gold colloid particles Electric field one mode for single particle coupled mode

Purpose To investigate optical property of coupled particles with the aid of numerical calculation • Dependence on Inter particle distance • Selective excitation of coupling mode • Possibility for better adsorption sensing?

Method y less order less parameters more order more parameters x z MMP Calculation (modeling) MMP: Multiple Multipole Program (MaX-1, Prof Ch.Hafner) Semi-analytical calculation  proper location and order of multipoles Model medium boundary gold 3D multipole E S Symmetry plane y Calculated scattering intensity≡ Sz(0,0,500nm) 50nm z x εAu : P.B.Johnson et al; Phys.Rev. B v6.n12(1972)

Method Light source Dark Field Microscope polarizer CCD Camera sample Spectrometer Experimental setup - Spectrophotometer - HAL - Colloid sample - 50nm Au colloid particles were deposited on a glass slide by incubating the glass slide in colloid solution, rinsed and dried with nitrogen gas.

Result d =0.5nm =500nm =700nm =540nm =580nm =620nm Calculation: coupled mode

Result Calculation: non-coupled mode d =0.5nm =460nm =700nm =500nm =520nm =600nm

Result d E Separation Dependence(calculation) d = inter-particle separation - Coupled mode - Poynting Vector Map ( d = 0.5nm, l = 620nm ) • The second peak shifts to red as the distance decreases • Two particles behave independently when separated enough

Result d E Separation Dependence(calculation) - Non-coupled mode - Poynting Vector Map ( d = 1.0nm, l = 520nm ) No separation dependence Only the intensity is twice as high as single…

Result Experimental spectra of different particles Dark Field Image „Particle“ A „Particle“ B „Particle“ C by comparing with calculation

Result Coupling direction 90° 45° 135° polarization 0° 180° Experiment (selective excitation)

Result Experiment ( selective excitation) Intensity change by polarization 700nm> Longpass filtered Color: intensity polarization 2mm

Result Calculation (medium dependence) single particle coupled particle peak shift peak shift solution solution water water air air nwater = 1.333, nsolution = 1.370 separation : 0.75nm Peak of coupled mode is more sensitive to the surrounding refractive index sensitive to dimension  sensitive to refractive index  More sensitive adsorption detection !?

Result Experiment (medium dependence) air buffer solution single particle coupled particle Problem in air : water layer most sensitive part

Summary coupled mode…. • can be selectively excited by polarized light • is sensitive to the dimension and surrounding medium • can be a more sensitive adsorption detector Combination of MMP Calculation and experiment helps for better understanding and designing LSPR sensor. Outlook • Detect molecule adsorptions by coupled mode • Produce coupled particles by use of bio-molecules • Design new structures

Acknowledgements • Prof. Christian Hafner • Prof. Janos Vörös • LBB members Funding : ETH Zürich Thank you for the attention…

Result d =0.75nm d =0.75nm =500nm Calculation: three particles(symmetric separation) =900nm =580nm =620nm =760nm

Result Calculation: three particles(non-symmetric separation) d =1.5nm d =0.75nm =500nm =840nm =600nm =660nm =720nm

Result Calculation: three particles(symmetric separation) d =0.77nm d =0.75nm =900nm =500nm =560nm =700nm =740nm

Localized surface plasmon resonance of optically coupled metal particles

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