Integral Equation Formalism for Electromagnetic Scattering from Small Particles
1. Integral Equation Formulation for Electromagnetic
Scattering from Small Particles
積分方程方法在小粒子的電磁波散射的應用
TAM, Ho Yin
譚浩賢
A Thesis Submitted in Partial Fulfilment
of the Requirements for the Degree of
Master of Philosophy
in
Physics
The Chinese University of Hong Kong
October 2014
2. Thesis Assessment Committee
Professor CHING, Shuk Chi Emily (Chair)
Professor YOUNG, Kenneth (Thesis Supervisor)
Professor LEUNG, Pui Tang (Thesis Co-supervisor)
Professor WANG, Jianfang (Committee Member)
Professor POON, Wing On Andrew (External Examiner)
3. Abstract
Applications in nanophotonics rely on three important properties when op-
tical light is incident on a metallic nanoparticle (size < 100 nm): (a) it is
possible to have a greatly enhanced intensity inside and just outside the par-
ticle; (b) the intensity-enhanced regions have much smaller dimensions (tens
of nm) than the wavelength (about 400 to 700 nm); and (c) the enhance-
ment is resonant (i.e., strongly wavelength-dependent), with the resonance
sensitive to the size and shape of the particle. The last property allows the
resonance to be tuned according to need.
In fact, the dielectric constant (complex and strongly frequency-dependent)
has a complicated interplay with the geometry of the particle(s), making
it difficult to obtain a good understanding of the problem; many results
are obtained only by brute-force numerical calculations, validated against
measurements typically made only on the far field. The present work exam-
ines different theoretical methodologies, especially those based on an integral
equations, establishes a numerical method of choice, and develops a quasi-
analytic approximation that provides physical insight.
The T-Matrix method relies on an integral equation that enjoys the “leap-
frog” property: the internal field is related directly to the far field, bypassing
the external near field. This property is exhibited in a novel derivation, and
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4. is also used to obtain accurate results for small to medium nanoparticles
under moderate truncation of the infinite set of linear equations. Because
computation is not intensive, convergence is easily achieved, and the results
can serve as benchmarks.
For the Finite Difference Time Domain (FDTD) method, commercial
packages are readily available. Though supposedly exact for sufficiently small
grid size, FDTD suffers from several possible problems: (a) it is computation-
ally intensive, even with several tricks to restrict the computational domain;
and (b) consequently it is not possible in practice to use a sufficiently small
grid size, leading to significant numerical errors in certain regions, some of
which may not be widely appreciated.
Both the T-Matrix method and FDTD, being brute-force numerical al-
gorithms, are also difficult to understand in a physical and qualitative way.
The integral equation method, in particular an analytic approximation
based thereon, allows these problems to be assessed and in some cases over-
come. When the size of particles and the spatial region of interest are much
smaller than the wavelength, and the skin depth is also much longer than
the wavelength, the situation can be treated in a quasi-static approximation.
In this case, the integral equation formalism yields a simple formula for the
internal field, with the dependence on wavelength, size and shape separately
exhibited.
The analytic approximation is evaluated against accurate numerical re-
sults, and then employed to understand the optical response of gold nanopar-
ticles, especially the resonance peak of different shapes: spheroidal and cylin-
drical, leading to an improved physical picture.
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7. Acknowledgement
I would like to express my greatest thanks to my supervisors, Professor Ken-
neth Young and Professor P. T. Leung for their patient guidance on my re-
search. Their insight on physics and philosophy on research have broadened
my horizon.
I also thank Professor J. F. Wang and his research group, especially Dr.
H. Chen, L. Shao, and Q. Ruan, for their kindly cooperation on FDTD
computation in this thesis.
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