Download Graphene Controls Colour of Plasmonic Nanoantennas What is plasmonics? Nanoparticle on mirror

Graphene Controls Colour of Plasmonic Nanoantennas
J. Mertens1, A. L. Eiden2, C. Tserkezis3, J. Aizpurua3, A. C. Ferrari2, J. J. Baumberg1
1 NanoPhotonics
Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, UK
2 Cambridge Graphene Centre, University of Cambridge, UK
3 Materials Physics Centre, Donostia-San Sebastián, Spain
Why nanoantennas?
• APPLICATIONS: Nanoantennas concentrate light in small gaps. Trapped light in the antenna provides a
nanometre-scale sensor that changes colour depending on the environment
• FUNDAMENTAL: We can observe quantum processes for gaps in the subnanometre regime using light
• light causes the electrons in
metallic nanoparticles (NPs)
to oscillate back and forth
Nanoparticle on mirror
light field
What is plasmonics?
time
oscillating
electrons
• dimer geometry is useful for surface enhanced
Raman scattering to study substances in gap
• problem: Dimers are hard to fabricate / handle
• solution: Equivalent geometry – NP on a mirror
 the particle shape and size
define the frequency of oscillation, i.e. the colour
 light is strongly enhanced in a gap
when NPs are close together (dimer geometry)
NP and its
reflection
one atomic
thin gap
• graphene: thinnest possible spacer
robust, stable, and controlled subnanometre gap
Graphene sets the colour
We observe coupling between a NP and its reflection using a microscope:
bulk graphite
white
light
dark field
microscopy
few layer
graphene
1
Scattering [normalised]
dark field scattering
spectra of single NPs on different graphene layers
0
1
monolayer
graphene
𝑺𝑷
𝑸
5 layer graphene
𝑷
𝒄𝒐𝒖𝒑𝒍𝒆𝒅
𝑺𝑷
0
500
600
700
Wavelength [nm]
800
The coupled resonance shifts with the number of graphene layers: we observe different plasmon modes
SP: single particle resonance – visible without a mirror
coupled: plasmon-image coupling – depends on junction between NP and mirror
• splitting of coupled resonance only for single graphene layer (1)
Interpretation: Plasmonic dipole coupling
The spectroscopy indicates a mixing of plasmonic dipoles:
global dipole: electron tunnelling through graphene layer (charge transfer)
gap dipole: highly localised charges at gap
• parallel and antiparallel alignment leads to different energies
• quantum mechanical effect sets splitting due to gap of 0.34 nm
P -resonance
Q -resonance
low energy
high energy
What can it be used for?
• tuning plasmon resonances: optically with semiconductor spacers (2) or electronically via gating
• non invasive optical probing of electronic and optical properties of the spacer material
(1) Mertens et al., Nano Lett., 2013, 13 (11)
(2) Sigle & Mertens et al., to be published, 2014
www.np.phy.cam.ac.uk