plasmonics - Plasmonics using Metal Nanoparticles Tammy K....

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Unformatted text preview: Plasmonics using Metal Nanoparticles Tammy K. Lee and Parama Pal ECE 580 Nano-Electro-Opto-Bio April 1, 2007 Motivation Why study plasmonics? Miniaturization of optics and photonics to subwavelength scales Applications: fully integrated electro-opto circuits, high resolution microscopy, effective biosensors What is plasmonics? Exploitation of the optical properties of surface plasmons (SP) in metals for local field enhancements and radiation confinement Metal nanoparticles to create simple and novel structures (thin films, colloids, wires, shells, stars etc.) Surface Plasmons (SP) Observed since the late 17th century (e.g. Lycurgus cup) Addition of gold powder to glass to color it red Scattering looks green Absorption transmitted light looks red What is a SP? EM wave propagates along surface of metaldielectric interface coupled to collection of oscillating free conduction electrons Optical Properties of Metals To model the complex dielectric function of metals, need to consider: (i) motion of free conduction electrons (ii) interband transitions of bound electrons to conduction band given a excitation photon with sufficient energy Calculated Dielectric Functions DrudeSommerfeld Model Theoretical vs. Experimental Results Interband Transitions SP at Metal-Dielectric Interface ε1(ω) complex dielectric funtion (e.g. metal), ε2 real dielectric funtion (e.g. air) p-polarized wave satisfies wave equation: Solving wave equation with boundary conditions, get: Assuming no sources: Solving these equation yields: Normal component: Dispersion relation: METALS!!!!! Properties of SPP SPP Wavevector: Wavelength of SP: Propagation Length: Sample numbers for λ=630 nm, ε2=1 SP Excitation Dispersion relation for SP Momentum of SP larger than free space photon Need dielectric material with n<1 to tilt light line Kretschmann configuration Glass prism Optimal angle of incidence, θK Reflectivity (θ, film thickness) Near-Field Microscopy detector Photon Scanning Tunneling Microscopy (PSTM) Kretschmann configuration Evanescent field couples into propagating mode of fiber Topography & field intensity measurements Ag film z x piezo-electric scanner y tuning fork fiber d index matching fluid θk prism Typical results (a) SPP standing wave (b) tuned away from θK, (c) s-polarized light Sensors Surface Plasmon Resonance (SPR) sensor for gas detection and biosensors Components: optical system (light excitation, metal structure) transducing material (whose properties being sensed) detection system Sensor performance: sensitivity, resolution, and operation range Types of Sensors Types of sensors: prism-based grating-based Waveguide-based Materials & Fabrication: prism-based: glass or plastic grating-based: holograhic technique in plastic waveguide-based: CVD for semiconductor, ion exchange for glasses Sensor Examples To measure chemisorption of 1 monolayer of hexadecanethiol using Ag nanoparticles Methods Top Row: Nanosphere lithography Bottom row: single Ag particles Results: Both: ~40 nm shift in SPR from 1 monolayer Biophotonics Nanoshell with dielectric cores for imaging and therapy silica cores with colloidal gold nanoparticles formed as shells tuned to NIR for biological tissue to either scatter light for imaging or absorb light for therapy Results from Halas group: nanoshell particles tuned to both scatter and treat cancer cells by photothermally Nanoshells were antibody conjugated to target HER2 which are overexpressed by the type of cancer cells used SERS SERS enhancement of Raman signals which are typically very small Attachment of molecule to nanoparticles results in high scattering cross sections Surface Plasmon Subwavelength Optics Issues: Scaling of interconnects Component dimensions How to beat the diffraction limit ? Solution Surface Plasmons Recap: SPs are light waves that propagate along metal/dielectric interfaces on interaction with surface electrons Key aspect: different relative permittivities k SP = k 0 Light Conversion by circuit ε dε m εd +εm Logical processing Light Surface Plasmons Propagation distances: 10-100 µm Surface Plasmon Waveguides Ordered arrays Stripe Waveguides Nanowires (Barnes et al, Nature 2003) Topography Optical near field intensity (Krenn et al, Europhys. Letts. 2002) Surface Plasmon Photonic Bandgaps (SPPBG) Regions of periodic index modulations that form a stop band (Barnes et al, Nature 200...