Nano Optics Based on Coupled Metal Nanoparticles Shangjr Gwo ( 果尚志 ) Department of Physics National Tsing-Hua University, Hsinchu 30013, Taiwan E-mail: gwo@phys.nthu.edu.tw NDHU-Phys (2010/03/01)
Background Nanoelectronics is currently available. Present limit: ~50 nm (<100 nm) 3D Visualization Nanophotonics is still work to do! Present limit: ~1000 nm Airy Disk for Circular Aperture Barely Resolved No Longer Resolved
Explosive Development in Metal Nanophotonics (Plasmonics) in Recent Years Key Word: Surface Plasmon Source: M.L. Brongersma & P. G. Kik, Ed., Surface PlasmonNanophotonics (Springer, 2007)
Historic review of gold nanoparticles Recent Trend: Light absorption, scattering, extinction, emission can be enhanced, manipulated, and guided on the nanometer scale by noble metal (Au, Ag) nanostructures.
Historic review of gold nanoparticles a Byzantine Empire (4th century A.D.) b transmission nanoparticle Lycurgus cup fabricated by dispersing colloidal gold into glass Is it possible to assemble nanoparticle plasmonic crystals with tunable properties? transmission reflection reflection 5
Spherical noble metal nanoparticle of radius a << λ Depending on particle size, shape, material composition, and local dielectric environment SA Maier & HA Atwater, JAP 98, 011101 (2005) For a spherical metal nanoparticle of radius a embedded in a nonabsorbing surrounding medium of dielectric constant ε m, the quasistatic analysis gives the following expression for the particle polarizability α: α = 4πa 3 ε εm ε + 2ε with the complex ε = ε(ω) describing the dispersive dielectric response of the metal. m
Outline: Introduction & Motivation -Brief literature review Plasmon Hybridization in Individual Gold Nanocrystal Dimers: Direct Observation of Bonding and Antibonding Modes (0-D System) S.-C. Yang ( 楊舒淳 ) - In collaboration with Prof. T. Teranishi (University of Tsukuba) Plasmonic Properties of Near-Field-Coupled One-dimensional (1-D) Gold Nanoparticle Chains C.-L. He ( 何介倫 ) Plasmonic Properties of Near-Field-Coupled Two-dimensional (2-D) Gold Nanoparticle Arrays C.-F. Chen ( 陳季汎 )
H k E Important Applications of Plasmonic Dimer Structures: Large enhancement of local near fields (nano-antenna effects) Amplification of florescence and nonlinear optical processes Surface-enhanced spectroscopies (SERS) Tunable plasmon resonances (strong gap dependence) And many others Gold Bowtie Nanoantenna
anti-phase ( dark ) l =1 in-phase ( bright ) l =1 anti-phase ( dark ) Plasmon energies of a nanosphere dimer as a function of interparticle separation Formation of bright and dark plasmons P. Nordlander et al. Nano Letters, 4, 899-903 (2004)
Sample Preparation Octahedral Au nanocrystal adsorption Importance of monodispersed SEM nanocrytsl: Plasmon resonance energy depends on size, shape, composition, and dielectric environment. Plasmonic building block (LEGO material): Octahedral gold nanocrystal Prof. Toshiharu Teranishi, University of Tsukuba
Polarization-Selective Total Internal Reflection Microscopy and Spectroscopy Average peak position : 633 nm ± 2 nm (measured from 10 nanoparticles) Scattering Intensity (a.u.) 200 400 600 800 1000 Wavelength (nm) Scattering Intensity (a.u.) 200 400 600 800 1000 Wavelength (nm)
Polarization-Selective Total Internal Reflection Microscopy and Spectroscopy Scattering Intensity (a.u.) TM polarization TE polarization 200 400 600 800 1000 Wavelength (nm) The scattering spectrum acquired with polarization parallel to nanocrystal long axis corresponds to the scattering signal from TMpolarized illumination.
Dimer Preparation SEM Manipulation Octahedral gold nanocrystals (supplied by Prof. Toshiharu Teranishi) Manipulate by C.-L. He ( 何介倫 )
SEM Images of Dimers with Varying Nanocrystal Separation (Edge to Edge) Separation: 125 nm Separation: 105 nm Separation: 85 nm Separation: 65 nm Separation: 45 nm Separation: 25 nm
Experimental Setup SEM image Octahedral gold monomer and dimer are placed in an evanescent field produced by total internal reflection of white light in a glass prism. The scattering image of monomer and dimer was recorded using a digital camera and was spectrally analyzed by polarization-selective spectroscopy.
Scattering Spectra from Au Nanocrystal Dimers s = 125 nm s = 125 nm scattering intensity ( a.u.) s = 105 nm s = 85 nm s = 65 nm s = 45 nm s = 25 nm scattering intensity ( a.u.) s = 105 nm s = 85 nm s = 65 nm s = 45 nm s = 25 nm 400 500 600 700 800 wavelength ( nm ) 400 500 600 700 800 wavelength ( nm )
Using TM-polarized incident light, the evanescent field is elliptically polarized in the x-z plane, and produces z- and x-polarized evanescent intensities in a 9:1 intensity ratio. Evanescent wave Dimer separation (edge to edge) =25 nm to 125 nm Projected length of octahedral nanoparticle = 200 nm Dimer distance (center to center) =225 nm to 325 nm 0.5λ Daniel Axelrod et al., Ann. Rev. Biophys. Bioeng, 13, 247-268 (1984)
Results and Discussion Plasmon Energy v.s. Interparticle Separation plasmon resonance energy ( ev ) 2.00 1.98 1.96 1.94 1.92 1.90 1.88 1.86 + _ + _ 20 40 60 80 100 120 140 dimer separation (nm) + + l =1 We measure the m=1 ( l =1) in-phase ( bright ) and anti-phase ( dark ) coupled plasmon modes by polarization-selective total internal reflection microscopy and spectroscopy. P. Nordlander et al. Nano Letters, 4, 899-903 (2004)
Large-Scale Self-Assembly of 2-D Au Nanoparticle Superlattices ¾Au@C18 closely packed nanoparticle superlattice on quartz ¾Copper grid 1. precisely locating for optical measurement 2. preventing sample charging during SEM observation ¾Au@C18 superlattice 1. highly ordered 2. spatial extension exceeding 1 1 mm2. Optical image 22
2-D Plasmon Coupling C.-F. Chen et al. J. Am. Chem. Soc. 130, 824 826 (2008) Evidence Theoretical for near field calculations coupling! Near field coupling needs higher spatial resolution and is difficult to be achieved. near-field effect Far field Experiment couplingin far field coupling (gap distance/ diameter) LinLin Zhao, K. Lance Kelly, and George C. Schatz, J. Phys. Chem. B 2003, 107, 7343-7350 Far field coupling Christy L. Haynes, Adam D. McFarland, Lin-Lin Zhao, Richard P. Van Duyne, and George C. Schatz, J. Phys. Chem. B 107, 7337-7342 (2003) 23
Summary The dimer separation can be precisely controlled by using a nanomanipulator. The plasmon resonance energies of single near-fieldcoupled nanocrystal dimers can be tuned with nm spectral resolution. We can measure both the in-phase ( bright ) and antiphase ( dark ) coupled plasmon modes by polarizationselective total internal reflection microscopy and spectroscopy.