6 research outputs found
Symmetry Reduction and Shape Effects in Concave Chiral Plasmonic Structures
Chiral
metamaterials have shown a number of interesting properties
which result from the interaction of the chiral near-field they produce
with light and matter. We investigate the influence of structural
imperfections on the plasmonic properties of a chiral gold “gammadion”,
using electron energy loss spectroscopy to directly inform simulations
of realistic, imperfect structures. Unlike structures of simple convex
geometry, the lowest energy modes of the ideal concave gammadion have
a quadrupole and dipole character, with the mode energies determined
by the nature of electrostatic coupling between the gammadion arms.
These modes are strongly affected by structural imperfections that
are inherent to the material properties and lithographic patterning.
Even subwavelength-scale imperfections reduce the symmetry, lift mode
degeneracies convert dark modes into bright ones and significantly
alter the mode energy, its near-field strength, and chirality. Such
effects will be common to a number of multitipped concave structures
currently being investigated for the chiral fields they support
Symmetry Reduction and Shape Effects in Concave Chiral Plasmonic Structures
Chiral
metamaterials have shown a number of interesting properties
which result from the interaction of the chiral near-field they produce
with light and matter. We investigate the influence of structural
imperfections on the plasmonic properties of a chiral gold “gammadion”,
using electron energy loss spectroscopy to directly inform simulations
of realistic, imperfect structures. Unlike structures of simple convex
geometry, the lowest energy modes of the ideal concave gammadion have
a quadrupole and dipole character, with the mode energies determined
by the nature of electrostatic coupling between the gammadion arms.
These modes are strongly affected by structural imperfections that
are inherent to the material properties and lithographic patterning.
Even subwavelength-scale imperfections reduce the symmetry, lift mode
degeneracies convert dark modes into bright ones and significantly
alter the mode energy, its near-field strength, and chirality. Such
effects will be common to a number of multitipped concave structures
currently being investigated for the chiral fields they support
Controlling Metamaterial Transparency with Superchiral Fields
The
advent of metamaterials has heralded a period of unprecedented control
of light. The optical responses of metamaterials are determined by
the properties of constituent nanostructures. The current design philosophy
for tailoring metamaterial functionality is to use geometry to control
the nearfield coupling of the elements of the nanostructures. A drawback
of this geometry-focused strategy is that the functionality of a metamaterial
is predetermined and cannot be manipulated easily postfabrication.
Here we present a new design paradigm for metamaterials, in which
the coupling between chiral elements of a nanostructure is controlled
by the chiral asymmetries of the nearfield, which can be externally
manipulated. We call this mechanism dichroic coupling. This phenomenon
is used to control the electromagnetic induced transparency displayed
by a chiral metamaterial by tuning the chirality of the near fields.
This “non-geometric” paradigm for controlling optical
properties offers the opportunity to optimally design chiral metamaterials
for applications in the polarization state control and for ultrasensitive
analysis of biomaterials and soft matter
Induced Chirality through Electromagnetic Coupling between Chiral Molecular Layers and Plasmonic Nanostructures
We report a new approach for creating chiral plasmonic
nanomaterials.
A previously unconsidered, far-field mechanism is utilized which enables
chirality to be conveyed from a surrounding chiral molecular material
to a plasmonic resonance of an achiral metallic nanostructure. Our
observations break a currently held preconception that optical properties
of plasmonic particles can most effectively be manipulated by molecular
materials through near-field effects. We show that far-field electromagnetic
coupling between a localized plasmon of a nonchiral nanostructure
and a surrounding chiral molecular layer can induce plasmonic chirality
much more effectively (by a factor of 10<sup>3</sup>) than previously
reported near-field phenomena. We gain insight into the mechanism
by comparing our experimental results to a simple electromagnetic
model which incorporates a plasmonic object coupled with a chiral
molecular medium. Our work offers a new direction for the creation
of hybrid molecular plasmonic nanomaterials that display significant
chiroptical properties in the visible spectral region
Chiral Plasmonic Fields Probe Structural Order of Biointerfaces
The
structural order of biopolymers, such as proteins, at interfaces
defines the physical and chemical interactions of biological systems
with their surroundings and is hence a critical parameter in a range
of biological problems. Known spectroscopic methods for routine rapid
monitoring of structural order in biolayers are generally only applied
to model single-component systems that possess a spectral fingerprint
which is highly sensitive to orientation. This spectroscopic behavior
is not a generic property and may require the addition of a label.
Importantly, such techniques cannot readily be applied to real multicomponent
biolayers, have ill-defined or unknown compositions, and have complex
spectroscopic signatures with many overlapping bands. Here, we demonstrate
the sensitivity of plasmonic fields with enhanced chirality, a property
referred to as superchirality, to global orientational order within
both simple model and “real” complex protein layers.
The sensitivity to structural order is derived from the capability
of superchiral fields to detect the anisotropic nature of electric
dipole–magnetic dipole response of the layer; this is validated
by numerical simulations. As a model study, the evolution of orientational
order with increasing surface density in layers of the antibody immunoglobulin
G was monitored. As an exemplar of greater complexity, superchiral
fields are demonstrated, without knowledge of exact composition, to
be able to monitor how qualitative changes in composition alter the
structural order of protein layers formed from blood serum, thereby
establishing the efficacy of the phenomenon as a tool for studying
complex biological interfaces
Superchiral Plasmonic Phase Sensitivity for Fingerprinting of Protein Interface Structure
The
structure adopted by biomaterials, such as proteins, at interfaces
is a crucial parameter in a range of important biological problems.
It is a critical property in defining the functionality of cell/bacterial
membranes and biofilms (<i>i.e.</i>, in antibiotic-resistant
infections) and the exploitation of immobilized enzymes in biocatalysis.
The intrinsically small quantities of materials at interfaces precludes
the application of conventional spectroscopic phenomena routinely
used for (bio)Âstructural analysis due to a lack of sensitivity. We
show that the interaction of proteins with superchiral fields induces
asymmetric changes in retardation phase effects of excited bright
and dark modes of a chiral plasmonic nanostructure. Phase retardations
are obtained by a simple procedure, which involves fitting the line
shape of resonances in the reflectance spectra. These interference
effects provide fingerprints that are an incisive probe of the structure
of interfacial biomolecules. Using these fingerprints, layers composed
of structurally related proteins with differing geometries can be
discriminated. Thus, we demonstrate a powerful tool for the bioanalytical
toolbox