Plasmon Assisted Nanotrapping E. P. Furlani, A. Baev and P. N. Prasad The Institute for Lasers, Photonics and Biophotonics University at Buffalo, SUNY

Overview  Introduction  Applications  Experimental Results  Modeling Nanotrapping Systems  Summary

Optical Trapping – Laser Tweezers D. G. Grier Nature Powerful tool for remote manipulation of microscopic biomaterial. Strongly focused laser beam creates gradient optical force that traps particles. Not ideal for nanoscale trapping (diffraction limitation, heating). Not well suited for integration with Lab- on-Chip systems (opto- fluidics).

Plasmonic-based Optical Nano-trapping Locally enhanced field near illuminated metallic nanostructures creates gradient optical force that traps nanoparticles. Well suited for trapping sub-wavelength metallic or dielectric particles. Potential for integration with Lab-on-Chip systems (opto-fluidics). Gold Nanocones Dielectric Nanoparticle E inc (t) pp + -

Surface Plasmon Resonance (SPR) and Localized SPR (LSPR) in Metallic Nanostructures Plasmon: Quantized charge density wave in free electron gas. LSPR: Resonant scattering modes in sub-wavelength metallic nanoparticles SPR: Surface plasmons confined to metal/dielectric interface. Wave vectors E(t)  mm dd E H. Strong Local Field

Motivation for LSPR Nanotrapping  Higher Resolution: optical nano-manipulation of sub- wavelength particles (d << ) (overcome diffraction limit).  Reduced Power: optical intensity an order of magnitude lower then conventional optical tweezers  Multiplexed Nano-trapping: multiplexed parallel manipulation of particles using arrays of metallic nanopaticles  Microsystem Integration: integrated optical particle manipulation/separation for BioMEMS, Lab-on-a-Chip systems.

E(t) Local Field Enhancement Metallic Nanoparticles Optical Absorption - Scattering Local Field Enhancement Absorption frequency/bandwidth depend on particle size, shape, composition and surrounding media etc. P(t) =  E(t)  mp dd

Analytical Dielectric Function for Au Nanostructures Experimental and analytical dielectric values vs. Analytical Dielectric Function for Au used in Analysis* *P. G. Etchegoin et al. J. Chem. Phys. 125, (2006) E inc + -

Optical Trapping of Sub-Wavelength Neutral Particles Dielectric Nanoparticle Metallic Nanostructures Force on Dielectric Nanoparticle caused by Local Field Gradient produced by Illuminated Metallic Nanoparticles

J. Aizpurua et al., PRL T. Atay et al., Nano Letters Nano-cone Array Nano-Pillar Array Nano-Ring Array Fabricated Metallic Nanostructures

Experimental Results Optical trapping of nanoparticles using tapered metallic nanopillars Collaboration with A. N. Grigorenko et. al, Nanometric optical tweezers based on nanostructured substrates, U. Manchester UK 1  m 120 nm 90 nm

Optical Trapping of Microbubbles on Nanostructured Substrate A.R. Sidorov et al. Optics Communications 278 (2007) 120 nm 90 nm

Enhanced Optical Trapping Au Nanoparticle Array Array of Au Nanostructures Trapped Dielectric SphereMoving Dielectric Sphere X. Maio and L. Y. Lin, Opt. Letters , also unpublished work 2008 Size of Trapped Particle D = 6.8  mD = 1  mD = 0.8  m Optical Trapping Intensity (  W/  m 2 ) with Au NP Array Optical Trapping Intensity (  W/  m 2 ) with Glass Slide

Plasmonic Trapping of Cells Single Yeast Cell Trapped in Square (other cells moving at constant speed) Trapped Cell Moving Cells X. Maio and L. Y. Lin, IEEE J. Sel. Topics Quant. Elec Optical intensity required for stable trapping of single yeast cell is 78.8  W/  m 2 Array of Au Nanostructures

Modeling Optical Nanotrapping  Dielectric Model for Metallic Nanoparticles.  Predict EM Field (Full-Wave Time-Harmonic Analysis)  Compute Time-averaged Optical Force  F opt  on Dielectric Nanoparticles (Dipolar Force)  Identify Regions of Trapping  Use  F opt  to Predict Particle Motion.

Optical Force on a Dielectric Nanoparticle Time-averaged Optical Dipolar Force  F opt  is a function of several variables:,  p,  mp (  ),  d, and the geometry, composition and coupling of metallic nanostructures. dd E inc (t) pp + -  mp (  ) pp

Trapping and Scattering Force Components Trapping Potential V trap :

2  m k Symmetry Boundary Conditions: PEC, PMC Full-Wave Time-Harmonic Analysis (Array of Nanopillars: Glass Substrate covered with H2O) Computational Domain 3.4  m 2  m PML y x k PEC PMC Glass H2O y x z pp

Symmetry Boundary Conditions: PEC, PMC Computational Model Computational Domain Surface current J x BC chosen to produce plane wave: Ex = 2  10 6 V/m. FEA Model: 43,904 cubic vector elements with 838,485 degrees of freedom. 3.4  m 2  m PML y x k PEC PMC Glass H2O y x z JxJx Incident Intensity 5.3 mW/  m 2 CPU Platform Dual Processor ( 3 GHz) Quad Core Windows XP 64 Bit 32 GB Ram Time: 15 min per given pp

Axial Optical Force vs. Field Polarization pp E inc k TE TM F z along this line Glass H2O k Trap k Trap TM polarization

Optical Force Analysis Force Vectors in x-y Plane Glass H2O Trapping Potential - J/m 3 Glass H2O k Trap Rp = 50 nm = 1000 nm TE Analysis TM Analysis - J/m 3 TE Analysis E inc k TE TM pp

Trapping Force Analysis k k Force vs. Particle Size Force vs. Cone Separation No Trapping for Large Particles Scattering Force Dominates E inc k TE TM d pp TE Polarization = 635 nm Rp = 50 nm

Axial Optical Force vs. Field Polarization pp E inc k TE TM F z along this line Glass H2O k Trap k Trap

Induced Electromagnetic Modes Top View – Induced E z Side View E(t) -z x Induced E z Top View = 635 nm = 1000 nm

pp E inc k TE TM F z along this line Glass H2O 2D Array of Pillars TE Trapping vs. k R p = 100 nm TE Analysis k R p = 100 nm Trap Glass/Air Glass/H2O TE Analysis Trap

pp E inc k TE TM F z along this line Glass H2O 2D Array of Rings TE Trapping vs. 600 nm 200 nm 300 nm k Air Only Trap R p = 100 nm k Glass/H2O Trap

 Optical trapping of neutral sub-wavelength particles can be achieved using local field enhancement near illuminated metallic nanostuctures.  Nano-trapping can be achieved with plane wave illumination.  Trapping force depends on particle size,, polarization and background permittivity.  Integration in Lab-on-Chip applications: Opto-fluidics Summary and Conclusions