1 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

2 Outline Background LSPR 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

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

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

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

6 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?

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

8 Experimental setup sample Light source Dark Field Microscope Spectrometer CCD Camera polarizer - 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. Method

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

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

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

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

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

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

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

16 Experiment ( selective excitation) Intensity change by polarization polarization 2m2m 700nm> Longpass filtered Color: intensity Result

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

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

19 Summary can be selectively excited by polarized light is sensitive to the dimension and surrounding medium can be a more sensitive adsorption detector coupled mode…. 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

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

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

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

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