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Surface plasmon resonance

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Chapter 12

Surface plasmons
12.1

Introduction

The interaction of metals with electromagnetic radiation is largely dictated by the
free conduction electrons in the metal. According to the simple Drude model, the
free electrons oscillate 180◦ out of phase relative to the driving electric field. As a
consequence, most metals possess a negative dielectric constant at optical frequencies
which causes e.g. a very high reflectivity. Furthermore, at optical frequencies the
metal’s free electron gas can sustain surface and volume charge density oscillations,
called plasmon polaritons or plasmons with distinct resonance frequencies. The existence of plasmons is characteristic for the interaction of metal nanostructures with
light. Similar behavior cannot be simply reproduced in other spectral ranges using the scale invariance of Maxwell’s equations since the material parameters change
considerably with frequency. Specifically, this means that model experiments with
e.g. microwaves and correspondingly larger metal structures cannot replace experiments with metal nanostructures at optical frequencies.The surface charge density
oscillations associated with surface plasmons at the interface between a metal and
a dielectric can give rise to strongly enhanced optical near-fields which are spatially
confined to the interface. Similarly, if the electron gas is confined in three dimensions,
as in the case of a small subwavelength particle, the overall displacement of the electrons with respect to the positively charged lattice leads to a restoring force which
in turn gives rise to specific particle plasmon resonances depending on the geometry
of the particle. In particles of suitable (usually pointed) shape, extreme local charge
accumulations can occur that are accompanied by strongly enhanced optical fields.
The study of optical phenomena related to the electromagnetic response of metals
has been recently termed as plasmonics or nanoplasmonics. This rapidly growing
field of nanoscience is mostly concerned with the control of optical radiation on the
subwavelength scale. Many innovative concepts and applications of metal optics have
407

408

CHAPTER 12. SURFACE PLASMONS

been developed over the past few years and in this chapter we will discuss a few examples. We will first review the optical properties of noble metal structures of various
shapes, ranging from two-dimensional thin films to one and zero dimensional wires
and dots, respectively. The analysis will be based on Maxwell...
Chapter 12
Surface plasmons
12.1 Introduction
The interaction of metals with electromagnetic radiation is largely dictated by the
free conduction electrons in the metal. According to the simple Drude model, the
free electrons oscillate 180
out of phase relative to the driving electric field. As a
consequence, most metals possess a negative dielectric constant at optical frequencies
which causes e.g. a very high reflectivity. Furthermore, at optical frequencies the
metal’s free electron gas can sustain surface and volume charge density oscillations,
called plasmon polaritons or plasmons with distinct resonance frequencies. The ex-
istence of plasmons is characteristic for the interaction of metal nanostructures with
light. Similar behavior cannot be simply reproduced in other spectral ranges us-
ing the scale invariance of Maxwell’s equations since the material parameters change
considerably with frequency. Specifically, this means that model experiments with
e.g. microwaves and correspondingly larger metal structures cannot replace experi-
ments with metal nanostructures at optical frequencies.The surface charge density
oscillations associated with surface plasmons at the interface between a metal and
a dielectric can give rise to strongly enhanced optical near-fields which are spatially
confined to the interface. Similarly, if the electron gas is confined in three dimensions,
as in the case of a small subwavelength particle, the overall displacement of the elec-
trons with respect to the positively charged lattice leads to a restoring force which
in turn gives rise to specific particle plasmon resonances depending on the geometry
of the particle. In particles of suitable (usually pointed) shape, extreme local charge
accumulations can occur that are accompanied by strongly enhanced optical fields.
The study of optical phenomena related to the electromagnetic response of metals
has been recently termed as plasmonics or nanoplasmonics. This rapidly growing
field of nanoscience is mostly concerned with the control of optical radiation on the
subwavelength scale. Many innovative concepts and applications of metal optics have
407
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