6 research outputs found
Photothermal Circular Dichroism Induced by Plasmon Resonances in Chiral Metamaterial Absorbers and Bolometers
Chiral photochemistry remains a challenge
because of the very small asymmetry in the chiro-optical absorption of
molecular species. However, we think that the rapidly developing fields of
plasmonic chirality and plasmon-induced circular dichroism demonstrate very
strong chiro-optical effects and have the potential to facilitate the
development of chiral photochemistry and other related applications such as
chiral separation and sensing. In this study, we propose a new type of chiral
spectroscopy – photothermal circular dichroism. It is already known that the
planar plasmonic superabsorbers can be designed to exhibit giant circular
dichroism signals in the reflection. Therefore, upon illumination with chiral
light, such planar metastructures should be able to generate a strong asymmetry
in their local temperatures. Indeed, we demonstrate this chiral photothermal effect
using a chiral plasmonic absorber. Calculated temperature maps show very strong
photothermal circular dichroism. One of the structures computed in this paper could
serve as a chiral bolometer sensitive to circularly polarized light. Overall, this
chiro-optical effect in plasmonic metamaterials is much greater than the
equivalent effect in any chiral molecular system or plasmonic bio-assembly. Potential
applications of this effect are in polarization-sensitive surface photochemistry and chiral bolometers
Localization of Excess Temperature Using Plasmonic Hot Spots in Metal Nanostructures: Combining Nano-Optical Antennas with the Fano Effect
It
is challenging to strongly localize temperature in small volumes
because heat transfer is a diffusive process. Here we show how to
overcome this limitation using electrodynamic hot spots and interference
effects in the regime of continuous-wave (CW) excitation. We introduce
a set of figures of merit for the localization of excess temperature
and for the efficiency of the plasmonic photothermal effect. Our calculations
show that the local temperature distribution in a trimer nanoparticle
assembly is a complex function of the geometry and sizes. Large nanoparticles
in the trimer play the role of the nano-optical antenna, whereas the
small nanoparticle in the plasmonic hot spot acts as a nanoheater.
Under the specific conditions, the temperature increase inside a nanoparticle
trimer can be localized in a hot spot region at the small heater nanoparticle
and, in this way, a thermal hot spot can be realized. However, the
overall power efficiency of local heating in this trimer is much smaller
than that of a single nanoparticle. We can overcome the latter disadvantage
by using a trimer with a nanorod. In the trimer assembly composed
of a nanorod and two spherical nanoparticles, we observe a strong
plasmonic Fano effect that leads to the concentration of optical energy
in the small heater nanorod. Therefore, the power efficiency of generation
of local excess temperature in the nanorod-based assembly greatly
increases due to the strong plasmonic Fano effect. The Fano heater
incorporating a small nanorod in the hot spot has obviously the best
performance compared to both single nanocrystals and a nanoparticle
trimer. The principles of heat localization described here can be
potentially used for thermal photocatalysis, energy conversion and
biorelated applications
DNA-Assembled Nanoparticle Rings Exhibit Electric and Magnetic Resonances at Visible Frequencies
Metallic nanostructures can be used
to manipulate light on the subwavelength scale to create tailored
optical material properties. Next to electric responses, artificial
optical magnetism is of particular interest but difficult to achieve
at visible wavelengths. DNA-self-assembly has proved to serve as a
viable method to template plasmonic materials with nanometer precision
and to produce large quantities of metallic objects with high yields.
We present here the fabrication of self-assembled ring-shaped plasmonic
metamolecules that are composed of four to eight single metal nanoparticles
with full stoichiometric and geometric control. Scattering spectra
of single rings as well as absorption spectra of solutions containing
the metamolecules are used to examine the unique plasmonic features,
which are compared to computational simulations. We demonstrate that
the electric and magnetic plasmon resonance modes strongly correlate
with the exact shape of the structures. In particular, our computations
reveal the magnetic plasmons only for particle rings of broken symmetries,
which is consistent with our experimental data. We stress the feasibility
of DNA self-assembly as a method to create bulk plasmonic materials
and metamolecules that may be applied as building blocks in plasmonic
devices
DNA Scaffolds for the Dictated Assembly of Left-/Right-Handed Plasmonic Au NP Helices with Programmed Chiro-Optical Properties
Within
the broad interest of assembling chiral left- and right-handed
helices of plasmonic nanoparticles (NPs), we introduce the DNA-guided
organization of left- or right-handed plasmonic Au NPs on DNA scaffolds.
The method involves the self-assembly of stacked 12 DNA quasi-rings
interlinked by 30 staple-strands. By the functionalization of one
group of staple units with programmed tether-nucleic acid strands
and additional staple elements with long nucleic acid chains, acting
as promoter strands, the promoter-guided assembly of barrels modified
with 12 left- or right-handed tethers is achieved. The subsequent
hybridization of Au NPs functionalized with single nucleic acid tethers
yields left- or right-handed structures of plasmonic NPs. The plasmonic
NP structures reveal CD spectra at the plasmon absorbance, and the
NPs are imaged by HR-TEM. Using geometrical considerations corresponding
to the left- and right-handed helices of the Au NPs, the experimental
CD spectra of the plasmonic Au NPs are modeled by theoretical calculations
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
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