Nanoscale control of plasmon dipolar and quadrupolar resonances in Au
triangles using polarization- and energy-dependent light excitation
Traian Popescu1, Chawki Awada1, Ludovic Douillard1, Fabrice Charra1, Antoine Perron2, Hélène Yockell-Lelièvre2,
Anne-Laure Baudrion2, Pierre-Michel Adam2 and Renaud Bachelot2
1
CEA Saclay, IRAMIS, Service de Physique et Chimie des Surfaces et Interfaces, F-91191 Gif sur Yvette, France
, traian.popescu@cea.fr; 2LNIO, ICD UMR CNRS 6279, Université de Technologie de Troyes, 7 rue Marie-Curie,
BP 2060, F-10010 Troyes, France
Localized surface plasmon resonances (LSPR) are
coherent charge oscillations within metallic particles
offering a way to concentrate and manipulate light on
the nanoscale. Aside from size, shape and dielectric
environment, the effect of directionality upon the lightplasmon coupling is crucial for applications, including
directional nano-antennas, selective control of single
molecule fluorescence [1] and surface-enhanced Raman
spectroscopy (SERS) [2]. We study this directional
aspect here by obtaining qualitative and quantitative
measurements of the plasmon resonances in dipolar and
quadrupolar modes as a function of the polarization and
wavelength of the incident light [3].
We are able to lift the degeneracy between orthogonal
plasmonic resonances by varying the linear polarization
of the incident light. The experimental results are
obtained using photoemission electron microscopy
(PEEM), an optical excitation scheme (photon-in,
electron out) that allows control over the polarization
and energy of the exciting field. We obtain full field
images of the plasmon resonance “hot spots” via
photoelectrons collected using traditional electron
optics. Nanometer scale spatial resolution (20 nm lateral
resolution) and a spectral resolution of < 5 meV at 800
nm are achievable.
In the present study, we analyze regular gold triangles
with in-plane altitudes spanning the range [100, 300]
nm and a height of 50 nm, fabricated by electron beam
lithography (EBL) and deposited on a 5 nm titanium
oxide (TiO2) adhesion layer and 5 nm of ITO.
According to group theory calculations [4], an isolated
triangular prism of group D3h has three dipolar modes
that can couple to the linearly-polarized incident field:
one inaccessible high-energy out of plane mode, and
two degenerate in-plane modes. By varying the
polarization between orthogonal p- and s-modes, we are
able to excite these two degenerate modes
independently. A similar analysis yields the quadrupolar
modes (Fig. 2a), which we observe experimentally (Fig.
2c). Finally, PEEM results correspond well to
simulations using finite difference time domain (FDTD)
simulations (Fig. 2b) and the wavelength dependence of
the dipolar and quadrupolar modes can be correlated to
measurements obtained using extinction spectrometry
(ES).
Fig. 1: PEEM sequence of the polarization
dependence of the dipolar LSPR of a 200 nm
in-plane altitude equilateral triangle. Top row
shows incident field polarized along triangle’s
edges; Bottom row corresponds to incident
field polarized along triangle’s altitude. Photon
wavelength λ = 800 nm, scale bar = 200 nm.
Fig. 2: Plasmonic mapping of the quadrupolar mode
of a 300-nm in-plane regular triangle. a)
Quadrupolar eigenmode in p-polarisation b)
FDTD simulation c) PEEM experimental result
at λ = 730 nm; p-pol.
References:
[1] T. Kosako, Y. Kadoya, H.F. Hofmann, Nat.
Photonics 4, 312−315 (2010)
[2] T. Ming, L. Zhao, H. Chen, K. Choi Woo, H.Q. Lin,
Nano Lett. 11, 2296−2303 (2011)
[3] C. Awada, T. Popescu, L. Douillard et. al., J. Phys.
Chem. C 27, 14591–14598 (2012)
[4] A. Gelessus, W. Thiel, W. Weber, J. Chem. Educ.
72, 505−508 (1995)