From plasmonic principles to plasmonic applications Bruno Soares b'f'soaresphy'cam'ac'uk

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From plasmonic principles to plasmonic
applicationsBruno Soares b.f.soares_at_phy.cam.ac.
uk
EPSRC NanoPhotonics Portfolio Centre
np.cam.phy.ac.uk
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The Vision of NanoPhotonics
NanoPhotonics Force light to interact with
nano-structures New material properties and
effects
Optical functionality on the smallest size
scale on the lowest energy level on the
shortest timescale
Applications
IT, telecoms, energy, defence, security, medicine
and biotechnologies.
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Why plasmonics?
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In words

• Plasmon gt collective oscillation of a metals
  free electrons

• Surface plasmon gt collective electron
  oscillations near a metal surface

• Polariton gt quasiparticle resulting from the
  coupling of EM waves
• with excitations in a material

• Surface plasmon polariton gt combined excitation
  consisting of a surface
• plasmon and a photon
• or in other words
• light waves trapped on the surface
  of a conductor

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In pictures
that move
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Searching for Surface Plasmons
Wave equation
z
e2
Dielectric
Harmonic time dependence
x
e1
Metal
Wave equation reduces to
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Searching for Surface Plasmons
Wave equation reduces to
z
e2
Dielectric
Spatial dependence
x
Wave equation becomes
e1
Metal
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Searching for Surface Plasmons
Wave equation
z
e2
Dielectric
Take each component E and H
Two type of solutions Transverse Magnetic
(TM) Transverse Electric (TE)
x
e1
Metal
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Searching for Surface Plasmons
Transverse Magnetic (TM)
For each medium gt
Waves propagating along x Confined in z
z
e2
Dielectric
x
e1
Metal
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Searching for Surface Plasmons
Transverse Magnetic (TM)
Boundary conditions
z
e2
Dielectric
x
e1
Metal
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Searching for Surface Plasmons
Transverse Magnetic (TM)
z
e2
Dielectric
Dispersion relation
x
e1
Metal
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What about TE modes?
Boundary conditions
z
e2
Dielectric
No non-zero solutions!
x
Surface plasmons only exist for TM polarization!
e1
Metal
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Understanding the dispersion relation
Light
SPP
Frequency w
Wave vector k
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Understanding the dispersion relation
Not confined
Light
SPP
Frequency w
Wave vector k
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Understanding the dispersion relation
Light
Frequency w
Real SPP
Wave vector k
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SPP length scales
z
?SPP
Dielectric
?SPP - Wavelength
dSPP - Propagation length
dd
x
---


dm
dd - Penetration depth into dielectric
Metal
dm - Penetration depth into metal
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SPP wavelength, ?SPP
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SPP propagation length, dSPP

• For large dSPP one needs large
• (negative) em and small em.
• dSPP gt upper limit on size for plasmonic circuits

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SPP penetration depths, dd and dm

• Dielectric

• Metal

Metal Ag, ed 1
dSPP ?0
Barnes, JOptA 8 (2006)

• Distance over which SPP is sensitive to
  refractive index changes.
• Surface feature sizes for SPP control.

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Problem momentum mismatch
air
metal/air
Frequency w

• SPP momentum always greater than photon momentum
  in dielectric
• Need to compensate for the mismatch somehow.

Wave vector k
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Solution prism coupling
Attenuated Total Reflection ATR methods
Almost 100 coupling
Zayats, et al., Phys. Rep. 408 (2005)
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Solution prism coupling
prism
air
metal/air
metal/prism
Frequency w
Wave vector k
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Other optical excitation methods

• Diffraction grating
• Grating vector kG 2p/?, ? period, a
  incident angle
• Can couple efficiently to either side of film
  with correct design.

Zayats, et al., Phys. Rep. 408 (2005)

• SNOM probe
• Re-positionable but low efficiency point
  source.
• Can be seen as diffraction or tunnelling
  mechanism.

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How do I know I got one?
Its all about balance
D
Reflectivity
Detector
Angle of incidence
Near-field imaging
Allows for detailed propagation and mode study
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Localised plasmons
Non-propagating excitations of the electrons
coupled to the electromagnetic field
Curved surface gt restoring force Resonance Field
amplification
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Summary of things so far
Surface plasmon on plane surfaces
Localised plasmon on nanostructures
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Nanovoids our favourite structures
EPSRC NanoPhotonics Portfolio Centre
np.cam.phy.ac.uk
Robin Cole Jeremy J. Baumberg
S. Mahajan P. N. Bartlett C.Milhano
Xiaoli Li C. H. de Groot
Javier Garcia de Abajo
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Nanovoid Fabrication
Control Plasmons
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Self-assembled colloidal templates
Evaporation leaves self-organised structure
Process governed by capillary forces
Can control ordering and number of layers Can
use monodisperse spheres 50nm-20mm
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Sample Measurement
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Sample Measurement
KSP K0sin? G
Dispersion relation
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Localised and delocalised modes
S. Coyle et al., Phys. Rev. Lett. 87, 176801
(2001) T. A. Kelf et al, Phys. Rev. Lett. 95,
116802 (2005)
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Propagating plasmons
Energy (eV)
Incident angle (deg)
t/d 0.3
thin region
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Localised plasmons
(1,0)
(2,1)
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Localised Plasmons
Mie plasmons - photonic atoms (l,m)
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Localised Plasmons
boundary integral 3D calculations
R500nm
0F
1F
1D
Energy (eV)
L3
3F
0D
2F
L2
2D
1P-
0P
L1
1P
Normalised thickness
Nano. Lett. 7, 2094-2100 (2007)
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From thin to thick
increasing thickness
(b)
(c)
(a)
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Identifying molecules
What molecule have I got?
- Many techniques when moles of molecules - Very
few methods for sparse molecules -Important
question e.g. contamination, biohazards,
health screening, security, research
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Raman Scattering
hwRaman hwin hwvib
Raman Spectrum
wvib2
Incident light
hwin
wvib1
Raman Intensity
wvib3
Frequency
(or Raman shift)
molecule
(vibration wvib )
Identify molecules and their structure
vibrating bonds absorb quanta of energy
re-emitted light has different colour
Problem!
Very very small scattering rate
Only about 1 in every 1012 photons undergo Raman
scattering
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Surface Enhanced Raman Scattering (SERS)

• Discovered 30 years ago at Univ. of Southampton
• Corrugated metal surface in proximity to molecule
• Signal enhancements of 102 to 1010
• Not fully understood
• charge-transfer vs. electromagnetic

Chem. Phys. Lett. 26 163 (1974)
Surface acts like molecular antenna
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Problem!
Not very reproducible! Not good for practical
applications!
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Previous attempts
electrochemically roughened Au, Ag
colloidal Au, Ag (eg. Natan)
array of gold nanoparticles (eg. van Dyne)
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Nanovoid structures for SERS
Surface acts like molecular antenna
Pump
1
Metal surface
3

• Compared to other attempts
• Stronger optical fields
• Better coupling
• Reproducible geometry

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Reproducible SERS
S
benzenethiol
SERS intensity
Calibrated gain 106-108
3.106
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Beaming SERS
Scan laser/detection angles
hwRaman hwin - hwvib
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In-situ SERS

• Watch molecular binding

BTh
Ag nv

• Watch molecule orientation vs. V

1 hour
SERS intensity
500
1500
1000
Raman shift (cm-1)

• Watch attachment and ordering in real time

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Passive SPP manipulation Bragg mirror
SPP beam generated by scattering 750 nm light
from a nanowire (160 nm wide) on an Ag surface.
SPP visualization via excitation of fluorophores
in a 30 nm polymer layer. Ditlbacher et al., APL
81 (2002)
SPP Bragg reflector consisting of 140 nm
diameter, 70 nm high Ag particles prepared by
e-beam lithography.
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Passive SPP manipulation beamsplitters and
interferometers
SPP beamsplitter consisting of a single row of
nanoparticles. Ditlbacher et al., APL 81 (2002)
SPP interferometer output beam direction
changed by changing the position of the
right-hand mirror to change path length by
?SPP/2. Krenn et al., J. Micros. 209 (2002)
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Passive SPP manipulation ring resonator
(wavelength filtering)
Left SEM, topographical and SNOM images of the
waveguide ring resonator schematically shown in
the inset to a. Above Transmission spectra for
two WR resonators and a curve corresponding to
the calculated response function. Bozhevolnyi et
al., Nature 440 (2006)
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Reflection and refraction at material interfaces
Total external reflection of an SPP wave from a
20-nm thin dielectric layer on metal. Stockman,
Nano Lett. 6 (2006)
Negative refraction of SPP waves using a
metal-dielectric-metal structure. Shin Fan,
PRL 96 (2006)
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Active SPP manipulation variable relative
permittivity
Control beam
Krasavin Zheludev, APL 84, (2004)
Silica
SPP
Light
SPP
m-Ga
Gold
Input coupling grating

• Liquid (m) gallium
• Almost ideally metallic.
• eL/e? 7 at 1.55 µm.

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Active SPP manipulation thermo-plasmonic
modulators and switches
(Left) Interferometer and (Right) directional
coupler response as a function of applied
electrical power (SPP propagation constant
changed in the electrically heated arm)
Schematic layout of long-range SPP-based (a) MZ
interferometer modulator and (b) Directional
coupler switch. The metal stripe waveguides (in
polymer) are also used to carry electrical
signals for heating (c) Optical microscope image
of MZIM. (d) Close-up showing electrical
isolation break (small enough not disrupt SPP
propagation). ?0 1.55 µm Nikolajsen et al.,
APL 85 (2004)
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Ideas to take home today
Surface plasmons collective excitation of free
electrons coupled with photons Two types of
surface plasmons propagating and localised
Range of applications possible
Further Reading

• Maier, Plasmonics Fundamentals and
  Applications (2007) in the library
• Shalaev Kawata (eds.) Nanophotonics with
  Surface Plasmons (2007)
• Brongersma Kik (eds.) Surface Plasmon
  Nanophotonics (2007)

b.f.soares_at_phy.cam.ac.uk