Gyroid Optical Metamaterials: Solvent Vapour Annealing, Confined Crystallisation, and Optical Anisotropy

146 Pages·2018·8.08 MB·English

by  James A. Dolan

Springer Theses
Recognizing Outstanding Ph.D. Research
James A. Dolan

Gyroid Optical
Metamaterials
Solvent Vapour Annealing,
Confined Crystallisation,
and Optical Anisotropy
Springer Theses
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James A. Dolan
Gyroid Optical Metamaterials
fi
Solvent Vapour Annealing, Con ned
Crystallisation, and Optical Anisotropy
Doctoral Thesis accepted by
the University of Cambridge, Cambridge, UK
123
Author Supervisors
Dr. James A.Dolan Prof. Ullrich Steiner
University of Cambridge SoftMatterPhysicsGroup,AdolpheMerkle
Cambridge, UK Institute
University of Fribourg
Fribourg, Switzerland
Prof. Jeremy J.Baumberg
NanoPhotonics Centre, Cavendish
Laboratory
University of Cambridge
Cambridge, UK
Prof. Timothy D.Wilkinson
CentreofMolecularMaterialsforPhotonics
andElectronics,Department of
Engineering
University of Cambridge
Cambridge, UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-03010-0 ISBN978-3-030-03011-7 (eBook)
https://doi.org/10.1007/978-3-030-03011-7
LibraryofCongressControlNumber:2018958933
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Everything stinks till it’s finished.
—Dr. Seuss
Dedicated to
Douglas William Richard Tate
1934–2005
Engineer, teacher, friend.
’
Supervisor s Foreword
The concept of a “left-handed” or negative-index material was first proposed by
Victor Veselago in 1968 through his theoretical predictions of the electrodynamic
behaviour of materials that simultaneously exhibit negative values of dielectric
permittivity and magnetic permeability. Thirty years later, John B. Pendry
demonstrated that artificial electromagnetic materials—“metamaterials”—consist-
ing,inthiscase,ofinterpenetratedorderedarraysofwiresandsplit-ringresonators
could also exhibit similar phenomena. Initially demonstrated for microwave radi-
ation, metamaterials have borne out Pendry’s predictions, and the quest since then
hasbeentheirimplementationinthevisiblerangeoftheelectromagneticspectrum.
Whilst the physical principles are scale-invariant, practical realisation of the
sub-wavelength structural elements that constitute an optical metamaterial is far
fromtrivial,asitrequiresperiodicarraysofwell-definedstructuralfeaturesinthree
dimensions on the 10–50 nm length scale. Nearly all approaches so far employ
some type of lithographic method, which is not well suited for the manufacture of
3D architectures on such small length scales.
James Dolan’s thesis makes an important contribution to the quest for optical
metamaterials.Asopposedto“top-down”lithographicmethods,histhesisexplores
“bottom-up” polymer self-assembly methods for the manufacture of 3D metama-
terials in the visible range of the spectrum. This methodology, however, presents
the researcher with a new set of challenges. First, macromolecular self-assembly
mustbecontrolledwithahithertounprecedentedlevelofprecision.Thisisdifficult
not only from a thermodynamic point of view, but also because the outcome of
self-assembly is “invisible”. Since molecular self-assembly takes place on the
sub-100 nm length scale, the morphologies attained cannot be probed by optical
microscopy. WhilstX-ray scattering and electron and atomic force microscopy are
all possible, the lack of optical methods is cumbersome for the experimenter. The
first part of the thesis addresses these two challenges: the control of polymer
self-assembly in thin films and the optical visualisation of its structural perfection.
Chapter 4 explores in detail the structural evolution of block copolymer films that
are exposed to a solvent atmosphere in the so-called solvent vapour annealing
experiment. The results of this chapter provide the thermodynamic basis for the
ix
x Supervisor’sForeword
control over polymer self-assembly that is required to fabricate optical metamate-
rials.Thefollowingchapter,Chap. 5,tacklesthe“invisibility”issue.Byexploiting
the crystallisation of one of the copolymer blocks, the lateral organisation of the
self-assembled morphology gives rise to birefringence. This chapter carefully
investigatesthiseffectanddevelopsamodelthatdirectlylinksthebirefringenceto
thestructuralorganisation,therebyofferingavaluablenewtooltooptimisepolymer
self-assembly for the manufacture of optical metamaterials. This chapter also, for
the first time, demonstrates 100 lm large domains of the self-assembled gyroid
morphology, from which some of the most interesting metamaterial properties are
predicted to arise.
The second part of the thesis focuses on the optical properties of metamaterials
created by polymer self-assembly. To this end, the copolymer morphologies were
partially voided and then replicated into a plasmonic metal (gold). Chapter 6
explores the question of whether long-range order in this optical metamaterial is
required. By combining experimental studies, simulations, and analytical calcula-
tions, this chapter explores the optical response of locally-ordered gyroid mor-
phologies that are disordered on the micrometre-length scale. The finding that the
optical spectra of these disordered gyroids are well described by an effective
mediummodelillustrates,ontheonehand,thatorderisrequiredtoaccessthemost
interesting aspects of optical metamaterials, but also, on the other hand, that these
materialshaveinterestingpropertiesintheirownright(e.g.achangeofsigninthe
permittivity as afunctionof wavelength). The finalexperimental chapter, Chap. 7,
solves a long-standing puzzle in the field of self-assembled optical metamaterials,
therebyprovidinganimportantsteppingstoneinmetamaterialresearch.Thepuzzle
in question is the strong linear dichroism of gyroid metamaterials, which is unex-
pected given the cubic nature of the gyroid unit cell. By combining experimental
observations with numerical simulations, this chapter demonstrates that gyroid
samples can be considered as a bulk metamaterial terminated by a distinct meta-
surface—the 2D equivalent of a metamaterial. It is the symmetry-breaking of the
metasurface that induces the strong, experimentally observed linear dichroism.
Importantly,thischapteruncoversthatthein-couplingandout-couplingoflightto
and from the metamaterial depend extremely sensitively on the specific surface
termination of the metamaterial. The control of such terminations will require a
manufacturing precision ofaround 1nm,therebysetting ahigh barfor thereliable
manufacture of optical metamaterials for visible wavelengths.
The scalable manufacture of optical metamaterials with a negative refractive
index has the potential for transformative changes in the application of optical
materials, utilising, for example, their super-focusing and “cloaking” properties.
This thesis demonstrates, on the one hand, that the manufacture of these materials
by self-assembly isa promising way forward.On theother hand,the results ofthe
Supervisor’sForeword xi
four main thesis chapters elucidate the significant challenges in mastering this
approach,therebysettingfutureresearchersasetofchallengingtasksthattheywill
havetoovercomeonthepathtothedemonstrationanduseofopticalmetamaterial
effects visible to the naked eye.
Fribourg, Switzerland Prof. Ullrich Steiner
October 2018 Soft Matter Physics Group
Adolphe Merkle Institute
University of Fribourg