The Coulombic interaction between a chiral molecule and a nonchiral metal nanocrystal creates new plasmonic lines in a circular dichroism (CD) spectrum. In many cases, optical lines in CD spectra of ...chiral molecules and plasmon absorption bands in metal nanocrystals are not in resonance. Typically, CD lines of chiral molecules are in the UV spectral range, whereas plasmon bands of metal nanocrystals are in the visible range. Nevertheless, plasmon excitations can strongly interact with dipoles of chiral molecules and become chiral. We demonstrate this effect theoretically and derive a general equation for the CD spectrum of a complex composed of metal nanocrystals and a single chiral molecule. In our theory, new plasmon peaks in the CD spectra come from an effect of interference between external and induced fields inside a hybrid complex. Our models involve single spherical nanoparticles, nanoparticle pairs, and nanoshells. The results obtained here can be used to design new optical materials and hybrid nanosystems with sensor properties.
We investigate theoretically photogeneration of excited carriers in plasmonic nanocrystals. The theory is based on the solution of the quantum equation of motion for the density matrix. Efficient ...photogeneration of plasmonic electrons and holes in small nanocrystals becomes possible due to the nonconservation of the electron momentum. The confinement and reflection of electrons in small nanocrystals allowed photon-assisted electron transitions with high excitation energies and therefore lead to a large number of energetic carriers. This process is a surface-scattering effect and efficient only for nanostructures with small sizes. Other important factors for the photogeneration effect are the field enhancement and the inhomogeneity of electromagnetic fields inside a plasmonic nanostructure. The plasmonic field effects strongly depend on the shape of the nanocrystal. For example, a plasmonic nanocube is more efficient for the electron photogeneration than a nanosphere and a nanosphere generates more energetic carriers compared to a plasmonic slab. The results obtained here can be used for designing plasmonic nanostructures for solar and photocatalytic applications.
We describe from the theoretical point of view a plasmonic mechanism of optical activity in chiral complexes composed of metal nanoparticles (NPs). In our model, the circular dichroism (CD) signal ...comes from the Coulomb interaction between NPs. We show that the CD spectrum is very sensitive to the geometry and composition of a chiral complex and also has typically both positive and negative bands. In our calculations, the strongest CD signals were found for the helix geometry resembling helical structures of many biomolecules. For chiral tetramers and pyramids, the symmetry of a frame of a complex is very important for the formation of a strong CD response. Chiral natural molecules (peptides, DNA, etc.) often have strong CD signals in the UV range and typically show weak CD responses in the visible range of photon energies. In contrast to the natural molecules, the described mechanism of plasmonic CD is able to create strong CD signals in the visible wavelength range. This plasmonic mechanism offers a unique possibility to design colloidal and other nanostructures with strong optical chirality.
A reconfigurable plasmonic nanosystem combines an active plasmonic structure with a regulated physical or chemical control input. There have been considerable efforts on integration of plasmonic ...nanostructures with active platforms using top-down techniques. The active media include phase-transition materials, graphene, liquid crystals and carrier-modulated semiconductors, which can respond to thermal, electrical and optical stimuli. However, these plasmonic nanostructures are often restricted to two-dimensional substrates, showing desired optical response only along specific excitation directions. Alternatively, bottom-up techniques offer a new pathway to impart reconfigurability and functionality to passive systems. In particular, DNA has proven to be one of the most versatile and robust building blocks for construction of complex three-dimensional architectures with high fidelity. Here we show the creation of reconfigurable three-dimensional plasmonic metamolecules, which execute DNA-regulated conformational changes at the nanoscale. DNA serves as both a construction material to organize plasmonic nanoparticles in three dimensions, as well as fuel for driving the metamolecules to distinct conformational states. Simultaneously, the three-dimensional plasmonic metamolecules can work as optical reporters, which transduce their conformational changes in situ into circular dichroism changes in the visible wavelength range.
The life is inherently chiral. Consequently, chirality plays a pivotal role in biochemistry and the evolution of life itself. Optical manifestation of chirality of biomolecules, so-called circular ...dichroism, is a remarkable but relatively weak effect appearing typically in the UV. In contrast to the biomolecules, plasmonic nanocrystals offer an interesting opportunity to create strong circular dichroism (CD) in the visible spectral range. Here we describe plasmonic properties of single chiral nanocrystals and focus on a new mechanism of optical chirality originating from a chiral shape of a nanocrystal. After careful examination, we found that this CD mechanism is induced by the mixing between different plasmon harmonics and is qualitatively different from the previously described dipolar CD effect in chiral assemblies of spherical nanoparticles. Chiral plasmonic nanocrystals studied here offer a new approach for the creation of nanomaterials with strong chiroptical responses in the visible spectral interval.
The ability to dynamically tune the self-assembled structures of nanoparticles is of significant interest in the fields of chemistry and material studies. However, it continues to be challenging to ...dynamically tune the chiral superstructures of nanoparticles and actively switch the chiral optical properties thereof. Here, we dynamically controlled a gold nanorod 3D chiral plasmonic superstructure (a stair helix with a pinwheel end view) templated by a DNA origami supramolecular polymer, using DNA-toehold-mediated conformational change in the DNA template. The gold nanorod chiral plasmonic helix was controllably reconfigured between a tightly folded state (with a small inter-rod angle) and an extended state (with a wide inter-rod angle) of the same handedness, or between two mirror-image-like structures of opposite handedness. As a result, the chiral plasmonic properties of the gold nanorod helix superstructures, in terms of the circular dichroism amplitude, peak response frequency, and signature of chirality, were actively switched upon the DNA-guided structural reconfiguration. We envision that the strategy demonstrated here will boost the advancement of reconfigurable chiral materials with increased complexity for active light control applications through rational molecular design and predictable self-assembly procedures.
Nature has developed striking light-powered proteins such as bacteriorhodopsin, which can convert light energy into conformational changes for biological functions. Such natural machines are a great ...source of inspiration for creation of their synthetic analogues. However, synthetic molecular machines typically operate at the nanometre scale or below. Translating controlled operation of individual molecular machines to a larger dimension, for example, to 10-100 nm, which features many practical applications, is highly important but remains challenging. Here we demonstrate a light-driven plasmonic nanosystem that can amplify the molecular motion of azobenzene through the host nanostructure and consequently translate it into reversible chiroptical function with large amplitude modulation. Light is exploited as both energy source and information probe. Our plasmonic nanosystem bears unique features of optical addressability, reversibility and modulability, which are crucial for developing all-optical molecular devices with desired functionalities.
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 prominent 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 Letter 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 bioassembly. Potential applications of this effect are in polarization-sensitive surface photochemistry and chiral bolometers.
Plasmonic nanocrystals strongly interact with chiral molecular shells through electric and magnetic fields and in this way acquire new chiro‐optical properties. Transfer of chirality from ...biomolecules to the plasmonic resonances is a collective phenomenon and strongly depends on the geometry of nanostructure. Collective effects in a molecular chiral shell may suppress or enhance plasmonic circular dichroism (CD) depending on the geometry of hybrid nanocrystal. In large chiral plasmonic structures, we identify a new electrodynamic mechanism of plasmonic CD that is qualitatively different to the near‐field, dipolar mechanism of the plasmonic chirality described by us previously. Our models also show that anisotropic nanocrystals, such as nanorods or oriented molecular shells, have strongly enhanced CD at the plasmonic frequency. A family of chiral plasmonic nanostructures proposed and modeled here can be used for designing new optical media and chiral sensors.
Chirality transfer: In large chiral plasmonic structures, a new electrodynamic mechanism of plasmonic CD that is qualitatively different to near‐field, dipolar mechanism of plasmonic chirality is described. The models presented also show that anisotropic nanocrystals have strongly enhanced CD at the plasmonic frequency.
The recent rise of metamaterials opens new opportunities for absorbers due to their designed electrodynamic properties and effects, allowing the creation of materials with effective values of ...permittivity and permeability that are not available in naturally occurring materials. Since their first experimental demonstration in 2008, recent literature has offered great advances in metamaterial perfect absorbers (MMPAs) operating at frequencies from radio to optical. Broadband absorbers are indispensable in thermophotovoltaics, photodetection, bolometry, and manipulation of mechanical resonances. Although it is easy to obtain MMPAs with single band or multiband, achieving broadband MMPA (BMMPA) remains a challenge due to the intrinsically narrow bandwidth of surface plasmon polaritons, localized surface plasmon resonances generated on metallic surfaces at nanoscale or high Q‐factor in GHz region. To guide future development of BMMPA, recent progress is reviewed here: the methods to create broadband absorption and their potential applications. The four mainstream methods to achieve BMMPAs are introduced, including planar and vertical element arrangements, their welding with lumped elements and the use of plasmonic nanocomposites, accompanied by the description of other, less common approaches. Following this, applications of BMMPA in solar photovoltaics, photodetection, bolometry, and manipulation of mechanical resonances are reviewed. Finally, challenges and prospects are discussed.
Broadband metamaterial perfect absorbers are reviewed, including design methods and applications. First, four mainstream methods are presented—using planar and vertical resonators, lumped elements, and plasmonic nanocomposites, followed by unconventional methods such as using complementary metamaterial structures, space‐filling, dielectric tailoring, and resonators tailoring. Finally, applications such as photovoltaic energy harvesting, photodetection, bolometers, and mechanical resonance manipulation are discussed.
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