using Plasmonics 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
http://www.thebritishmuseum.ac.uk/science/lycurguscup/sr-lycurgus-p2b.html
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
1() complex dielectric funtion (e.g. metal), 2 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 =630 nm, 2=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, K
Reflectivity (, 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
k
prism
Typical results
(a) SPP standing wave (b) tuned away from K, (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 grating-based: plastic 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
d m d +m
Logical processing Light
Surface Plasmons
Propagation distances: 10-100 m
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, Na...