qpms/lepaper/intro.lyx

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#LyX 2.4 created this file. For more info see https://www.lyx.org/
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\pdf_author "Marek Nečada"
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\begin_layout Section
Introduction
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LatexCommand label
name "sec:Introduction"
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\begin_layout Standard
The problem of electromagnetic response of a system consisting of many relativel
y small, compact scatterers in various geometries, and its numerical solution,
is relevant to many branches of nanophotonics (TODO refs).
The most commonly used general approaches used in computational electrodynamics
are often unsuitable for simulating systems with larger number of scatterers
due to their computational complexity: differential methods such as the
finite difference time domain (FDTD) method or the finite element method
(FEM) include the field degrees of freedom (DoF) of the background medium
(which can have very large volumes), whereas integral approaches such as
the boundary element method (BEM) need much less DoF but require working
with dense matrices containing couplings between each pair of DoF.
Therefore, a common (frequency-domain) approach to get an approximate solution
of the scattering problem for many small particles has been the coupled
dipole approximation (CDA) where drastic reduction of the number of DoF
is achieved by approximating individual scatterers to electric dipoles
(characterised by a polarisability tensor) coupled to each other through
Green's functions.
\end_layout
\begin_layout Standard
CDA is easy to implement and demands relatively little computational resources
but suffers from at least two fundamental drawbacks.
The obvious one is that the dipole approximation is too rough for particles
with diameter larger than a small fraction of the wavelength, which results
to quantitative errors.
The other one, more subtle, manifests itself in photonic crystal-like structure
s used in nanophotonics: there are modes in which the particles' electric
dipole moments completely vanish due to symmetry, regardless of how small
the particles are, and the excitations have quadrupolar or higher-degree
multipolar character.
These modes typically appear at the band edges where interesting phenomena
such as lasing or Bose-Einstein condensation have been observed and CDA
by definition fails to capture such modes.
\end_layout
\begin_layout Standard
The natural way to overcome both limitations of CDA mentioned above is to
include higher multipoles into account.
Instead of polarisability tensor, the scattering properties of an individual
particle are then described a more general
\begin_inset Formula $T$
\end_inset
-matrix, and different particles' multipole excitations are coupled together
via translation operators, a generalisation of the Green's functions in
CDA.
This is the idea behind the
\emph on
multiple-scattering
\begin_inset Formula $T$
\end_inset
-matrix method
\emph default
(MSTMM), a.k.a.
\emph on
superposition
\begin_inset Formula $T$
\end_inset
-matrix method
\emph default
(TODO a.k.a something; refs??), and it has been implemented previously for
a limited subset of problems (TODO refs and list the limitations of the
available).
\begin_inset Note Note
status open
\begin_layout Plain Layout
TODO přestože blablaba, moc se to nepoužívalo, protože je težké udělat to
správně.
\end_layout
\end_inset
Due to the limitations of the existing available codes, we have been developing
our own implementation of MSTMM, which we have used in several previous
works studying various physical phenomena in plasmonic nanoarrays (TODO
examples with refs).
\end_layout
\begin_layout Standard
Hereby we release our MSTMM implementation, the
\emph on
QPMS Photonic Multiple Scattering
\emph default
suite, as an open source software under the GNU General Public License
version 3.
(TODO refs to the code repositories.) QPMS allows for linear optics simulations
of arbitrary sets of compact scatterers in isotropic media.
The features include computations of electromagnetic response to external
driving, the related cross sections, and finding resonances of finite structure
s.
Moreover, in QPMS we extensively employ group theory to exploit the physical
symmetries of the system to further reduce the demands on computational
resources, enabling to simulate even larger systems.
\begin_inset Note Note
status open
\begin_layout Plain Layout
(TODO put a specific example here of how large system we are able to simulate?)
\end_layout
\end_inset
Although systems of large
\emph on
finite
\emph default
number of scatterers are the area where MSTMM excels the most—simply because
other methods fail due to their computational complexity—we also extended
the method onto infinite periodic systems (photonic crystals); this can
be used for quickly evaluating dispersions of such structures and also
their topological invariants (TODO).
The QPMS suite contains a core C library, Python bindings and several utilities
for routine computations, such as TODO.
It includes extensive Doxygen documentation, together with description
of the API, making extending and customising the code easy.
\end_layout
\begin_layout Standard
The current paper is organised as follows: Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Finite"
\end_inset
is devoted to MSTMM theory for finite systems, in Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Infinite"
\end_inset
we develop the theory for infinite periodic structures.
Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Applications"
\end_inset
demonstrates some basic practical results that can be obtained using QPMS.
Finally, in Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Comparison"
\end_inset
we comment on the computational complexity of MSTMM in comparison to other
methods.
\end_layout
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