6 research outputs found

    Photothermal Circular Dichroism Induced by Plasmon Resonances in Chiral Metamaterial Absorbers and Bolometers

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    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

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    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

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    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

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    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

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    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

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    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)­structural 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
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