Chiral Plasmonic Research Articles

Plasmonic chirality of L-shaped nanostructure composed of two slices with different thickness.

A concise method is proposed to fabricate L-shaped Ag nanostructures (LSANs) for generating chirality. Prepared by glancing angle deposition, the LSAN composed of two slices with different thickness is stacked on self-assembled monolayer polystyrene nanosphere arrays by controlling substrate azimuth and deposition time. The strong optical chirality of LSANs is achieved in visible and near-IR regions by measurement. For the circular dichroism spectrum of LSANs, the intensity is enlarged, and its peaks red-shift with increasing thickness difference. When LSANs are stacked on polystyrene spheres of different diameters, enlargement and red-shift are also observed in their circular dichroism spectra with increasing thickness difference. The numerical calculations of finite element method show that the two slices composing LSAN provide cross-electric dipoles and their thickness difference provides phase difference for generating optical chirality. This study not only provides a concise and scalable method for fabricating chiral plasmonic nanostructures but also contributes to understand the knowledge of the mechanism of circular dichroism.

Chiral plasmon in gapped Dirac systems

We study the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. We discuss the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays.

Hydrogen-Regulated Chiral Nanoplasmonics.

Chirality is a highly important topic in modern chemistry, given the dramatically different pharmacological effects that enantiomers can have on the body. Chirality of natural molecules can be controlled by reconfiguration of molecular structures through external stimuli. Despite the rapid progress in plasmonics, active regulation of plasmonic chirality, particularly in the visible spectral range, still faces significant challenges. In this Letter, we demonstrate a new class of hybrid plasmonic metamolecules composed of magnesium and gold nanoparticles. The plasmonic chirality from such plasmonic metamolecules can be dynamically controlled by hydrogen in real time without introducing macroscopic structural reconfiguration. We experimentally investigate the switching dynamics of the hydrogen-regulated chiroptical response in the visible spectral range using circular dichroism spectroscopy. In addition, energy dispersive X-ray spectroscopy is used to examine the morphology changes of the magnesium particles through hydrogenation and dehydrogenation processes. Our study can enable plasmonic chiral platforms for a variety of gas detection schemes by exploiting the high sensitivity of circular dichroism spectroscopy.

Nanoscale chirality in metal and semiconductor nanoparticles.

The field of chirality has recently seen a rejuvenation due to the observation of chirality in inorganic nanomaterials. The advancements in understanding the origin of nanoscale chirality and the potential applications of chiroptical nanomaterials in the areas of optics, catalysis and biosensing, among others, have opened up new avenues toward new concepts and design of novel materials. In this article, we review the concept of nanoscale chirality in metal nanoclusters and semiconductor quantum dots, then focus on recent experimental and theoretical advances in chiral metal nanoparticles and plasmonic chirality. Selected examples of potential applications and an outlook on the research on chiral nanomaterials are additionally provided.

Quantitatively analyzing the mechanism of giant circular dichroism in extrinsic plasmonic chiral nanostructures by tracking the interplay of electric and magnetic dipoles.

Plasmonic chirality has drawn much attention because of tunable circular dichroism (CD) and the enhancement for chiral molecule signals. Although various mechanisms have been proposed to explain the plasmonic CD, a quantitative explanation like the ab initio mechanism for chiral molecules, is still unavailable. In this study, a mechanism similar to the mechanisms associated with chiral molecules was analyzed. The giant extrinsic circular dichroism of a plasmonic splitting rectangle ring was quantitatively investigated from a theoretical standpoint. The interplay of the electric and magnetic modes of the meta-structure is proposed to explain the giant CD. We analyzed the interplay using both an analytical coupled electric-magnetic dipole model and a finite element method model. The surface charge distributions showed that the circular current yielded by the splitting rectangle ring causes the ring to behave like a magneton at some resonant modes, which then interact with the electric modes, resulting in a mixing of the two types of modes. The strong interplay of the two mode types is primarily responsible for the giant CD. The analysis of the chiral near-field of the structure shows potential applications for chiral molecule sensing.

Free-Standing Optically Switchable Chiral Plasmonic Photonic Crystal Based on Self-Assembled Cellulose Nanorods and Gold Nanoparticles.

Photonic crystals incorporating with plasmonic nanoparticles have recently attracted considerable attention due to their novel optical properties and potential applications in future subwavelength optics, biosensing and data storage device. Here we demonstrate a free-standing chiral plasmonic film composed of entropy-driven self-co-assembly of gold nanoparticles (GNPs) and rod-like cellulose nanocrystals (CNCs). The CNCs-GNPs composite films not only preserve the photonic ordering of the CNCs matrix but also retain the plasmonic resonance of GNPs, leading to a distinct plasmon-induced chiroptical activity and a strong resonant plasmonic-photonic coupling that is confirmed by the stationary and ultrafast transient optical response. Switchable optical activity can be obtained by either varying the incidence angle of lights, or by taking advantage of the responsive feature of the CNCs matrix. Notably, an angle-dependent plasmon resonance in chiral nematic hybrid film has been observed for the first time, which differs drastically from that of the GNPs embed in three-dimensional photonic crystals, revealing a close relation with the structure of the host matrix. The current approach for fabricating device-scale, macroscopic chiral plasmonic materials from abundant CNCs with robust chiral nematic matrix may enable the mass production of functional optical metamaterials.

Ultraviolet Plasmonic Chirality from Colloidal Aluminum Nanoparticles Exhibiting Charge-Selective Protein Detection.

Chiral aluminum nanoparticles, dispersed in water, are prepared, which provide strong ultraviolet plasmonic circular dichroism, high-energy superchiral near-fields, and charge-selective protein detection.

Circular Dichroism of Chiral Nematic Films of Cellulose Nanocrystals Loaded with Plasmonic Nanoparticles.

In the search for induced chiral plasmonic activity, cholesteric films formed by cellulose nanocrystals have attracted great interest as potential hosts for plasmonic nanoparticles. Circular dichroism (CD) spectra of the composite films exhibit two peaks, one of which is ascribed to the cholesteric host and the other one to plasmonic chiroptical activity of the plasmonic nanoparticles. Here we report the results of comprehensive studies of extinction and CD properties of composite films formed by different types of cellulose nanocrystals and different types of plasmonic nanoparticles. We show that the second peak in the CD spectra acquired using CD spectrometers appears as the result of the local reduction of the CD signal of the host material, due to excessive absorption by the nanoparticles, and thus it cannot be interpreted as induced plasmonic chiroptical activity. Instead, we propose an alternative way to measure CD spectra of plasmonic cholesteric films by using Mueller matrix transmission ellipsometry. The results of this study are important for ongoing research in the field of chiral plasmonics and for the optical characterization of a broad range of chiral nematic nanostructured materials.

Optical Rotation Reversal in the Optical Response of Chiral Plasmonic Nanosystems: The Role of Plasmon Hybridization

Chirality is an important molecular property for structural analysis. Similarly, it has been shown that plasmonic chiral systems exhibit strong circular dichroism (CD) responses that can be used to determine the relative positions of their constituent plasmonic elements. Here we show that the sign of the circular dichroism spectrum in a plasmonic system can be controllably changed through small geometric perturbations that change the energetic ordering of the hybridized modes. This mechanism is distinct from geometrical changes that explicitly change the handedness of the system. In a simple system composed of two stacked L-shaped resonators we observe a reversal of the optical rotation spectral signature for small relative shifts, and we show through electromagnetic modeling and experiments on lithographically patterned samples that this is due to a rearrangement of the relative energies between modes. The plasmonic system allows for geometric perturbation along controlled directions and therefore offers...

"Superchiral" Spectroscopy: Detection of Protein Higher Order Hierarchical Structure with Chiral Plasmonic Nanostructures.

Optical spectroscopic methods do not routinely provide information on higher order hierarchical structure (tertiary/quaternary) of biological macromolecules and assemblies. This necessitates the use of time-consuming and material intensive techniques, such as protein crystallography, NMR, and electron microscopy. Here we demonstrate a spectroscopic phenomenon, superchiral polarimetry, which can rapidly characterize ligand-induced changes in protein higher order (tertiary/quaternary) structure at the picogram level, which is undetectable using conventional CD spectroscopy. This is achieved by utilizing the enhanced sensitivity of superchiral evanescent fields to mesoscale chiral structure.

Frequency-dependent circular-polarization of terahertz chiral spoof surface plasmon polariton on helically grooved metallic wire

Chiral spoof surface plasmon polaritons in the terahertz region, resulting from a coherent superposition of TM and HE modes, can be generated by normally exciting helically corrugated metal wire with a linearly polarized Gaussian light. The handedness of the chiral SPP depends on the handedness of the helical groove. The chirality of the surface wave causes a spiral spatial extent of average power whose period is determined by the difference in the wave vector between two modes. Simultaneously, highly circular polarization of a confined surface wave emerges from the chirality of the geometry. Moreover, the polarization state is heavily dependent on working frequency relative to the characteristic frequency of each mode. This type of chiral SPP, originating from the helical groove, makes helically corrugated metal wire useful for some special applications, such as circularly polarized sources for terahertz near-field microscopy or terahertz sensors for chiral biomolecules.

Ag@helical chiral TiO2 nanofibers for visible light photocatalytic degradation of 17α-ethinylestradiol.

Ag-modified helical chiral TiO2 NFs (Ag@chiral TiO2 NFs) were fabricated and characterized by ultraviolet-visible absorption spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. This novel material exhibited efficient photocatalytic activity for the degradation of 17α-ethinylestradiol (EE2) in water under visible light irradiation with an optimum size of deposited silver nanoparticles (Ag NPs) in the range of 12 ∼ 14 nm. The pseudo-first-order rate constant (k obs) for EE2 photodegradation by Ag@chiral TiO2 NFs increased by up to a factor of 20.1 when compared with that of pure chiral TiO2 NFs. The high photocatalytic activity can be attributed to the interactions between helical chiral TiO2 NFs and surface plasmon resonance effect of Ag NPs. The new catalyst retains its photocatalytic activity at least up to five consecutive cycles. The results clearly demonstrate the feasibility of using Ag@chiral TiO2 NFs for the photocatalytic removal of EE2 and other endocrine-disrupting chemicals from water.

Strong Chiroptical Activities in Gold Nanorod Dimers Assembled Using DNA Origami Templates

Asymmetric three-dimensional (3D) nanoarchitectures that cannot coincide with their mirrored-symmetric counterparts are known as chiral objects. Numerous studies have focused on chiral plasmonic nanoarchitectures created intentionally with 3D asymmetric configurations, whose plasmonic chirality is promising for various nanoplasmonic and nanophotonic applications. Here, we show that gold nanorod (AuNR) plasmonic nanoarchitectures assembled on a soft 2D DNA origami template, which was often simplified to be a rigid rectangle, can exhibit strong chiroptical activities. The slight flexibility of the origami templates was found to play a critical role in inducing the plasmonic chirality of the assembled nanoarchitectures. Our study set a new example of reflecting the native conformation of nanostructures using chiral spectroscopy and can inspire the exploration of the softness of DNA templates for the future design of assembled chiral nanoarchitectures.

Local optical responses of plasmon resonances visualised by near-field optical imaging.

The unique optical characteristics of noble metal nanostructures have their origin principally in surface plasmon resonances. To exploit and design the unique characteristics arising from plasmons, an investigation of optical field structures adjacent to the nanostructure is of fundamental importance. As the spatial scale of the optical field structures is essentially smaller than the radiation wavelength in resonance with the plasmon, optical imaging methods that achieve spatial resolution beyond the diffraction limit of light are necessary to visualise the fields. In this article, we review the studies of direct experimental visualisation of plasmon resonances using near-field optical microscopy. We briefly describe the method of near-field optical microscopy used to study noble metal nanoparticles and show with several typical single gold nanoparticles that the spatial features of plasmon resonances, in particular the standing wave functions of the plasmons, can be directly visualised by near-field imaging. We then describe our recent efforts to visualise ultrafast dynamics in metal nanostructures following plasmonic excitation, which are based on near-field ultrafast imaging measurements. Another notable aspect of metal nanostructures that has attracted attention recently is the chirality of plasmons. Here, we describe a method and examples of near-field optical imaging and analyses of chiral plasmons excited on metal nanostructures.

Nanoscale 3D Chiral Plasmonic Helices with Circular Dichroism at Visible Frequencies

The nanoscaling of metamaterial structures represents a technological challenge toward their application in the optical frequency range. In this work we demonstrate tailored chiro-optical effects in plasmonic nanohelices, by a fabrication process providing a nanometer scale control on geometrical features, that leads to a fine tuning of operation band even in the visible range. Helicoidal 3D nanostructures have been prototyped by a bottom-up approach based on focused ion and electron beam induced deposition, investigating resolution limits, growth control and 3D proximity effects as a function of the interactions between writing beam and deposition environment. The fabricated arrays show chiro-optical properties at the optical frequencies and extremely high operation bandwidth tailoring dependent on the dimensional features of these 3D nanostructures: with the focused ion beam we obtained a broadband polarization selection of about 600 nm and maximum dissymmetry factor up to 40% in the near-infrared regio...

Optical rotation by plasmonic circular dichroism of isolated gold nanorod aggregates

We show that plasmonic chirality in single gold nanorod (GNR) aggregates leads to the rotation of polarization of the scattered light. 3D glasses in conjunction with linearly polarized dark field scattering microspectroscopy were used to study the chirality of single GNR aggregates. Using this hetero-polarizer setup, we not only detect but also quantify their chirality. A polar mapping strategy was used for providing direct evidence for the emergence of light of different polarization angles when chiral GNR aggregates were excited with circularly polarized light of different handedness. Further, we have developed a methodology to eliminate fluctuations in the scattering intensity by averaging and normalizing the data. This allows calculation of plasmonic circular dichroism scattering spectra with high accuracy.

Plasmon-induced Lorentz forces of nanowire chiral hybrid modes

The mechanical forces associated with surface currents are widely overlooked and point to a new family of plasmonically-driven processes. Here, we investigate the Lorentz forces acting on a free electron gas that is bound to the surface of a nanowire. We demonstrate that appreciable mechanical forces are produced by longer illumination wavelengths between longitudinal and transverse absorption resonances via the excitation of chiral hybrid plasmon modes. We are the first to associate plasmonic activity as the underlying mechanism for nanowire rotation, which explains prior experimental results. The presence of chiral hybrid plasmon modes yields the greatest net translation and torque forces. The asymmetric plasmon behavior subsequently affects the complex nonlinear dynamics of plasmonic nonspherical nanoparticles in fluids.

Enantioselective control of lattice and shape chirality in inorganic nanostructures using chiral biomolecules.

A large number of inorganic materials form crystals with chiral symmetry groups. Enantioselectively synthesizing nanostructures of such materials should lead to interesting optical activity effects. Here we report the synthesis of colloidal tellurium and selenium nanostructures using thiolated chiral biomolecules. The synthesis conditions are tuned to obtain tellurium nanostructures with chiral shapes and large optical activity. These nanostructures exhibit visible optical and chiroptical responses that shift with size and are successfully simulated by an electromagnetic model. The model shows that they behave as chiral optical resonators. The chiral tellurium nanostructures are transformed into chiral gold and silver telluride nanostructures with very large chiroptical activity, demonstrating a simple colloidal chemistry path to chiral plasmonic and semiconductor metamaterials. These materials are natural candidates for studies related to interactions of chiral (bio)molecules with chiral inorganic surfaces, with relevance to asymmetric catalysis, chiral crystallization and the evolution of homochirality in biomolecules.

Preparation, Characterization and Application of Noble-Metal Colloids Stabilized By Chiral Molecules

Gold or platinum colloids stabilized with chiral compounds having a hydroxyl or amino group and multiple carboxylic acids (CHMCA) were prepared by a modified citric-acid method in which L-, D- or DL-tartaric acid, L-, D- or DL-malic acid were used instead of citric acid. Controlling the molar ratio to metal and concentration of CHMCAs and pH of the reaction mixture during the colloid preparation process, CHMCA-stabilized gold or platinum colloids, without giving precipitates even when stored for long time, was successfully prepared. The CHMCA-stabilized gold colloids showed a photoabsorption peak at arou nd 520 nm, corresponding to the particle size of a few tens nanometer, and assigned to the surface plasmon resonance (SPR) and a peak in the circular dichromic spectra in the wavelength region of the SPR absorption though CHMCAs have no absorption there (Fig. 1). These results suggest that CHMCA-modified metal nanoparticles, i.e., "chiral surface plasmon resonance (cSPR)" depending on adsorption rotation direction of circularly polarized light. Application of the gold and platinum colloids loaded onto e.g., titania particles, to catalysis and photocatalysis using cSPR is discussed.

Characteristics of surface plasmon polaritons at a chiral–metal interface

The characteristics of surface plasmon polaritons at a chiral-metal interface are analyzed in detail. Compared to conventional surface plasmon waves at a dielectric-metal interface, it is shown that chiral surface plasmon waves have distinguishing features such as the presence of an s-wave at the metal surface, the existence of a cutoff frequency and chirality value, and the dependence of the propagation length on the chiral parameter. These properties of chiral surface plasmon waves can be exploited for on-chip chiral sensing and enantiometric detection applications.