Chiral Metamolecules Research Articles

Observation of Enantiomeric Switching of Individual Plasmonic Metamolecules.

Active plasmonic metamolecules under microscopic observation are promising for optical reporters in single molecule sensing applications. While self-assembled reconfigurable chiral plasmonic metamolecules can be conveniently engineered with sensing functionalities, their observation is usually based on ensemble measurements, where the chiroptical response of enantiomers tend to cancel each other in ensemble circular dichroism. Herein, we demonstrate microscopic observation of enantiomeric switching of individual active DNA origami-assembled plasmonic metamolecules. The metamolecules are immobilized on a glass substrate in a microfluidic chamber, in which the plasmonic metamolecule can maintain their activities upon certain local stimuli as in solution. In circular differential scattering, two enantiomeric states controlled by the strand-displacement reaction display opposite spectral signals to each other, representing successful enantiomeric switching of the chirality. Moreover, in a close-to-racemic mixture of chiral metamolecules controlled by pH-sensitive strands, the coexistence of enantiomeric individuals, which is concealed in ensemble measurements, is clearly identified.

Multipole Analysis of the Extinction Cross Section and Circular Dichroism of Chiral Metamolecules with Optical Theorem

AbstractPlasmonic chirality of metamolecules attracts increasing attention because of the tunable circular dichroism (CD) and near‐field enhancement for chiral molecule detection. Although various mechanisms have been proposed to explain the plasmonic CD, the quantitative analysis of particles with sub‐wavelength dimensions remains to be refined. Unlike biomolecules and small particles for which only absorption effects are considered, both multipolar scattering and absorption of sub‐wavelength metamolecules need to be analyzed. Here, the extinction and CD of chiral metamolecules are experimentally measured, and the contribution of excited multipole moments is theoretically investigated via combining multipole scattering and optical theorem. It is found that for the low‐order resonance mode, the absorption dominates the CD signal. While for high‐order resonance modes, both scattering and absorption significantly influence the CD, and the high‐order electric quadrupole moment that is commonly ignored plays a crucial role. Moreover, the intrinsic mechanism of the chiral response is also intuitively explained with linear polarization analysis. The mechanism proposed here is not only applicable for biomolecules and tiny particles with low‐order resonance modes but also works well for sub‐wavelength particles with high‐order modes, where conventional mixed electric and magnetic polarizability mechanisms usually do not work very well.

Perovskite metasurfaces with large superstructural chirality

Recent attempts to synthesize hybrid perovskites with large chirality have been hampered by large size mismatch and weak interaction between their structure and the wavelength of light. Here we adopt a planar nanostructure design to overcome these limitations and realize all-dielectric perovskite metasurfaces with giant superstructural chirality. We identify a direct spectral correspondence between the near- and the far- field chirality, and tune the electric and magnetic multipole moments of the resonant chiral metamolecules to obtain large anisotropy factor of 0.49 and circular dichroism of 6350 mdeg. Simulations show that larger area metasurfaces could yield even higher optical activity, approaching the theoretical limits. Our results clearly demonstrate the advantages of nanostructrure engineering for the implementation of perovskite chiral photonic, optoelectronic, and spintronic devices.

A Knotted Metamolecule with Axisymmetric Strong Optical Activity

AbstractOptical activity is the ability of chiral materials to rotate linearly polarized electromagnetic waves. A knotted chiral metamolecule is introduced here that exhibits strong optical activity corresponding to a 90° polarization rotation of the incident waves. More importantly, the torus knot structure is intrinsically chiral and multifold axisymmetric. Consequently, the observed polarization rotation behavior is found to be independent of how the incident wave is polarized. The metamolecule is fabricated through selective laser melting and experimentally validated in the microwave spectrum. This work represents the first ever metamolecule to be reported that is intrinsically axisymmetric and capable of simultaneously exhibiting strong optical activity.

Lattice Collective Interaction Engineered Optical Activity in Metamaterials

AbstractThe metamaterials with chiral metamolecular structure designs would show optical activity. In the past, researchers devoted themselves to proper designs of the individual metamolecules to improve the optical activity performance, however the roles of the lattice collective modes in the metamaterial array to the final optical activity have received much less attention. Here, how collective resonances can be utilized to engineer the optical activity of the metamaterials is systematically studied. Metamaterial arrays consisting of various numbers of “L”‐shaped chiral metamolecules are fabricated. It is illustrated that the intermolecule collective interactions are capable to dramatically improve the quality factor of the resonances of metamolecule ensembles. Also, it is shown that with the aid of lattice collective resonances, the spectral linewidth of the optical rotation and ellipticity angle modification to the light collapse efficiently and the magnitudes of the optical activity effects can be dramatically boosted. Both the spectral line collapses and the optical activity magnitude becomes saturated as the size of metamaterial array becomes larger than certain sizes, which implies the maximum coupling length between the metamolecules. The results provide a new freedom in optimizing the optical activity of the chiral metamaterials.

Chiral Metamolecules with Active Plasmonic Transition.

Energy-dissipating self-assembly is at the basis of many important cellular processes, such as cell organization, proliferation, and morphogenesis. Beyond equilibrium self-assembled molecular systems and materials, it is increasingly recognized that the control of assembly kinetics provides great opportunity for the next generation of molecular materials with intelligent behavior including programmed spatiotemporal organization. Here we show the transient self-assembly of active chiral plasmonic metamolecules (CPMs), which is controlled by the proton flux generated from a positive-feedback chemical reaction network. The fuel-conversion kinetics allows for temporal control and adaptive tuning of multiple structures of plasmonic metamolecules (PMs). This approach enables autonomous tuning of chiroptical properties of metamolecules with dynamic behavior. Moreover, we show that 11 types of spatial configurations of PMs are assembled, and 9 types of temporal configurations of CPMs are differentiated.

All-optical reconfigurable chiral meta-molecules.

Chirality is a ubiquitous phenomenon in the natural world. Many biomolecules without inversion symmetry such as amino acids and sugars are chiral molecules. Measuring and controlling molecular chirality at a high precision down to the atomic scale are highly desired in physics, chemistry, biology, and medicine, however, have remained challenging. Herein, we achieve all-optical reconfigurable chiral meta-molecules experimentally using metallic and dielectric colloidal particles as artificial atoms or building blocks to serve at least two purposes. One is that the on-demand meta-molecules with strongly enhanced optical chirality are well-suited as substrates for surface-enhanced chiroptical spectroscopy of chiral molecules and as active components in optofluidic and nanophotonic devices. The other is that the bottom-up-assembled colloidal meta-molecules provide microscopic models to better understand the origin of chirality in the actual atomic and molecular systems.

Chiral plasmonic nanostructures via DNA self-assembly

Controlling molecular chirality is of great importance in nanotechnology. Many biologically active molecules are chiral, including the naturally occurring amino acids, nuclear acids and sugars. In biological systems, most of these compounds are of the same chirality and the circular dichroism (CD) response of natural molecules is very weak. On the other hand, when metallic nanostructures, especially noble metal, illuminated by light with proper energy and momentum, surface plasmons can be excited, which have been used to enhance the electric field and excite higher electric and magnetic modes, leading to a series of fantastic optical phenomena and applications. Chirality of natural molecules can be manipulated by reconfiguring molecular structures through light, electric field, and thermal stimuli. While, the fabrication of complex metal structures is limited by the condition of current technology, especially for the precise fabrication and manipulation molecules at the nanoscale. Moreover, how to achieve chiroptical response in the visible range needs more efforts. In recent years, DNA nanotechnology, using DNA as building blocks of self-assembly, could be finely engineered into desired nanoarchitectures with high complexity and precision. It provides an effective way to easily control and tailor the arrangement of nanoparticles, and to form chiral metamolecules with complicated geometry. Among a variety of functionalized particles, metal nanoparticles such as gold nanoparticles feature an important pathway to endow DNA origami assembled nanostructures with tailored optical functionalities. Such DNA nanostructures were used for building versatile chiral plasmonic nanostructures from static to dynamic. Taking advantages of the spherical metal nanomaterials own isotropy and the programable of DNA nanostructures, the chiral configuration of self-assembled plasmonic nanostructures mainly consider the overall geometry of chiral space, which is easy to expand to more chiral and complex structure. Researchers can arrange achiral metal nanoparticles including gold nanoparticles, silver nanoparticles and quantum dots to fabricate chiral plasmonic nanostructures by analyzing and simulating the optically active molecular analogs. In addition, the interest in self-assembly of chiral plasmonic nanostructures, such as gold nanorods, as anisotropic building blocks is growing quickly. Researchers have developed a variety of complex superstructures such as chiral tetrahedral nanoparticles, pyramid nanoparticles, helical structures and three-dimensional plasmonic nanostructures. DNA nanotechnology provides one of the few ways to form designed, complex structures with precise control over nanoscale features. As a result, plasmonic chiral nanostructures assembled by DNA allow for dynamic manipulation of chirality and reversible switching of strong CD responses, hold great promise for applications in adaptable nanophotonic circuitry, artificial nanomachinery, as well as optical sensing of molecular binding and interaction activities. This article briefly reviews the developments and achievements of chiral plasmonic nanostructures enabled by DNA nanotechnology. Firstly, we show chiral plasmonic nanostructures based on spherical AuNPs, including plasmonic helices, tetramers, and chiral geometric conformations. Then, to challenge the complex configurations and enhance the CD responses, anisotropic gold nanorods with larger extinction coefficients are utilized to fabricate chiral plasmonic nanostructures including dimers, tripod and superhelix. Finally, we introduce dynamic manipulation based on DNA nanostructures with the fast development of this interdisciplinary field. We envision that the combination of DNA nanotechnology and plasmonics will open an avenue toward a new generation of functional plasmonic systems with tailored optical properties and useful applications, including polarization conversion devices, biomolecular sensing, surface-enhanced Raman and fluorescence spectroscopy, and diffraction-limited optics.

Modular Assembly of Plasmonic Nanoparticles Assisted by DNA Origami.

Arraying noble metal nanoparticles with nanoscale features is an important way to develop plasmonic devices with novel optical properties such as plasmonic chiral metamolecules, optical waveguides, and so forth. Along with top-down methods of fabricating plasmonic nanostructures, solution-based self-assembly provides an alternative approach. There are mainly two routes to organizing metal nanoparticles via self-assembly. One is directly linking nanoparticles through linker molecules, and the other is using nanoparticles to decorate a preformed template. We combine these two routes and herein report a strategy for the DNA origami-assisted modular assembly of gold nanoparticles into homogeneous and heterogeneous plasmonic nanostructures. For each module, we designed W-shaped DNA origami with two troughs as two domains. One domain is used to host a gold nanoparticle, and the other domain is designed to capture another gold nanoparticle hosted on a different module. By simply tuning the sequences of capture DNA strands on each module, gold nanoparticles including spherical and rod-shaped gold nanoparticles (denoted as AuNPs and AuNRs) could be well organized in a predefined manner to form versatile plasmonic nanostructures. Since the interparticle distances could be precisely controlled at the nanoscale, we also studied the plasma coupling among the assembled plasmonic nanostructures. This modular assembly strategy represents a simple yet general and effective design principle for DNA-assembled plasmonic nanostructures.

Microwave Spectroscopy of a Single Permalloy Chiral Metamolecule on a Coplanar Waveguide

A metamaterial's properties primarily originate from those of its component metamolecules, which makes single-metamolecule spectroscopy important for input in designing a metamaterial. However, detailed spectroscopic studies have been out of reach, due to the difficulties in manipulating individual micrometer-scale metamolecules. This work utilizes a coplanar waveguide to study the microwave transmission spectrum of a single magnetic chiral metamolecule, revealing its magnetic resonance and nonreciprocity signals. This approach could have real impact on high-throughput metamaterial engineering.

Circular Dichroism Control of Tungsten Diselenide (WSe2) Atomic Layers with Plasmonic Metamolecules.

Controlling circularly polarized (CP) states of light is critical to the development of functional devices for key and emerging applications such as display technology and quantum communication, and the compact circular polarization-tunable photon source is one critical element to realize the applications in the chip-scale integrated system. The atomic layers of transition metal dichalcogenides (TMDCs) exhibit intrinsic CP emissions and are potential chiroptical materials for ultrathin CP photon sources. In this work, we demonstrated CP photon sources of TMDCs with device thicknesses approximately 50 nm. CP photoluminescence from the atomic layers of tungsten diselenide (WSe2) was precisely controlled with chiral metamolecules (MMs), and the optical chirality of WSe2 was enhanced more than 4 times by integrating with the MMs. Both the enhanced and reversed circular dichroisms had been achieved. Through integrations of the novel gain material and plasmonic structure which are both low-dimensional, a compact device capable of efficiently manipulating emissions of CP photon was realized. These ultrathin devices are suitable for important applications such as the optical information technology and chip-scale biosensing.

Enhanced Circular Dichroism via Symmetry Breaking in a Chiral Plasmonic Nanoparticle Oligomer

A chiral plasmonic nanoparticle oligomer, consisting of four symmetrically arranged nanodisks of different heights and having different optical absorption responses to left and right-handed circularly polarized light illumination, has been experimentally reported in the literature. The resulting circular dichroism (CD) signal was detectable with state of the art CD spectrometers but was much weaker than those of existing chiral nanostructures, i.e., three-dimensional (3-D) chiral metamaterials. In this letter, via symmetry breaking in such an oligomer, the author demonstrates that the CD can be enhanced up to six times compared to that of a symmetric oligomer, and is in the range of a relevant 3-D chiral metamolecule. Through investigation of geometrical parameters including particle size, asymmetric and symmetric gaps, the CD evolution was reported, which provides a useful guideline for design of two-dimensional chiral oligomers adopted as efficient probes for CD spectroscopic applications.

Fano-Induced Circular Dichroism in Three-Dimensional Plasmonic Chiral Metamolecules

This paper introduces a three-dimensional (3-D) artificial chiral nanostructure, which has variant total optical loss spectroscopies under left- and right-handed circularly polarized (LCP and RCP) incident light. The resulting circular dichroism is induced by Fano resonance generated by the engineered chiral metamolecule, which consists of asymmetrically arranged gold (Au) nanoparticles in three dimensions. The Fano resonance generation is a consequence of modal interference between bright and dark plasmonic modes of asymmetric constituent dimers of the metamolecule.

Fabrication and ferromagnetic resonance of cobalt chiral meta-molecule arrays

We have succeeded in fabricating arrays of micrometer-sized, three-dimensional chiral meta-molecule (CMM) of cobalt (Co) by using a strain-driven self-coiling technique. In the arrays, the Co CMMs were fixed on a silicon substrate and aligned in the same direction. We investigated the ferromagnetic resonance of the arrays with varying angles of an external dc magnetic field. The Co CMM arrays show isotropic resonance signals, whose resonance fields are independent of the dc field orientation with respect to the helical axis.

Optical activity enhanced by strong inter-molecular coupling in planar chiral metamaterials.

The polarization of light can be rotated in materials with an absence of molecular or structural mirror symmetry. While this rotating ability is normally rather weak in naturally occurring chiral materials, artificial chiral metamaterials have demonstrated extraordinary rotational ability by engineering intra-molecular couplings. However, while in general, chiral metamaterials can exhibit strong rotatory power at or around resonances, they convert linearly polarized waves into elliptically polarized ones. Here, we demonstrate that strong inter-molecular coupling through a small gap between adjacent chiral metamolecules can lead to a broadband enhanced rotating ability with pure rotation of linearly polarized electromagnetic waves. Strong inter-molecular coupling leads to nearly identical behaviour in magnitude, but engenders substantial difference in phase between transmitted left and right-handed waves.

Interference-induced asymmetric transmission through a monolayer of anisotropic chiral metamolecules

We show that asymmetric transmission for linear polarizations can be easily achieved by a monolayer of anisotropic chiral metamolecules through the constructive and destructive interferences between the contributions from anisotropy and chirality. Our analysis is based on the interaction of electromagnetic waves with the constituent electric and magnetic dipoles of the metamaterials, and an effective medium formulation. In addition, asymmetric transmission in amplitude can be effectively controlled by the interference between spectrally detuned resonances. Our findings shed light on the design of metamaterials for achieving strong asymmetric transmission.

Chiral meta-molecules consisting of gold nanoparticles and genetically engineered tobacco mosaic virus

We demonstrate a chiral meta-molecule in the ultraviolet (UV) and visible (VIS) regions using a complex of Au nanoparticles (NPs) and rod-shaped tobacco mosaic virus (TMV). Au NPs five nm in diameter are uniformly formed on peptide-modified TMV. The peptide-modified TMV with uniform-sized Au NPs has improved dispersion in solution. A negative circular dichroism (CD) peak is produced around 540 nm, at plasmonic resonance wavelength of Au NPs. Additionally, modification of a CD peak in the UV region is observed. Attaching NPs to a virus causes the enhancement and modification of CD peaks in both the UV and VIS regions. Our results open a new avenue for the preparation of three dimensional chiral metamaterials at optical frequencies.

Photoinduced handedness switching in terahertz chiral metamolecules

Switching the handedness, or the chirality, of a molecule is of great importance in chemistry and biology, as molecules of different handedness exhibit dramatically different physiological properties and pharmacological effects. Here we experimentally demonstrate handedness switching in metamaterials, a new class of custom-designed composites with deep subwavelength building blocks, in response to external optical stimuli. The metamolecule monolayer flips the ellipticity and rotates the polarization angle of light in excess of 10° under optical excitation, a much stronger electromagnetic effect than that of naturally available molecules. Furthermore, the experimentally demonstrated optical switching effect does not require a structural reconfiguration, which is typically involved in molecular chirality switching and is inherently slow. The handedness switching in chiral metamolecules allows electromagnetic control of the polarization of light and will find important applications in manipulation of terahertz waves, such as dynamically tunable terahertz circular polarizers and polarization modulators for terahertz radiations.

Electromagnetic wave analogue of an electronic diode

An electronic diode is a nonlinear semiconductor circuit component that allows conduction of electrical current in one direction only. A component with similar functionality for electromagnetic waves, an electromagnetic isolator, is based on the Faraday effect of rotation of the polarization state and is also a key component in optical and microwave systems. Here we demonstrate a chiral electromagnetic diode, which is a direct analogue of an electronic diode: its functionality is underpinned by an extraordinarily strong nonlinear wave propagation effect in the same way as the electronic diode function is provided by the nonlinear current characteristic of a semiconductor junction. The effect exploited in this new electromagnetic diode is an intensity-dependent polarization change in an artificial chiral metamolecule. This microwave effect exceeds a similar optical effect previously observed in natural crystals by more than 12 orders of magnitude and a direction-dependent transmission that differs by a factor of 65.