Chirality Enhancement Research Articles

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces* *Project supported by the National Natural Science Foundation of China (Grant No. 11804035) and Science and Technology Research Program of Chongqing Municipal Education Commission, China (Grant No. KJ1706153).

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.

Polymerizable AEE-active Dye with Optical Activity for Fluorescent Nanoparticles Based on Phenothiazine: Synthesis, Self-assembly and Biological Imaging

In consideration of various advantages such as less harmful, higher sensitivity, and deeper imaging depth, etc, AIE materials with long-wave emission are attracting extensive attention in many fields of vascular visualization, organelle imaging, cells tracker, forensic detection, bioprobe and chemosensor, etc. In this paper, a novel fluorescent (R)-PVHMA monomer with chirality and aggregation-induced emission enhancement (AEE) characteristics was acquired through enzymatic transesterification reaction basing on phenothiazine, and its〖 [α]〗_D^(25 ℃) value was about -6.39° with 3.08eV energy level of HOMO and LUMO situations by the quantum calculations. Afterwards, a series of PEG-PVH1 and PEG-PVH2 copolymers with chirality feature were achieved through RAFT polymerization of the obtained (R)-PVHMA and PEGMA with various feed ratios. When the feed molar ratio of (R)-PVHMA increased to 29.6% from 21.5%, its actual molar fractions in the PEG-PVH1 and PEG-PVH2 copolymers accordingly increased to 25.7% from 18.1%. The molecular weight of PEG-PVH1 was about 2.2×104 with narrow PDI, and their kinetics estimation showed a first-order quasilinear procedure. In aqueous solution, the amphiphilic copolymers PEG-PVH could self-assembles into about 100 nm nano-particles. In a 90% water solution of H2O and THF mixture, the fluorescence intensity had the maximum value, and the emission wavelength presented at 580 and 630 nm. The investigation of cytotoxicity and cells uptake showed that PEG-PVH FONs had outstanding biocompatibility and excellent cells absorption effects, which have great potential in bioimaging application.

The mechanism of optical chirality enhancement in vertical-cut slant-gap antenna

The mechanism of optical chirality enhancements (OCE) in slant-gap antenna with vertical-cut configuration by normal and oblique incident light with linear polarization is demonstrated. The main contribution of OCE is originated from the inner product of the incident H field and the localized surface plasmon resonant (LSPR) evanescent E field. The pure LSPR near-field term E·H gives a net zero contribution. Based on these findings, we can accurately count for the asymmetric OCE distribution due to the substrate effect. The detailed analysis of OCE contribution might provide a route for designing a more efficient chiral molecule sensor.

Dual Nanoresonators for Ultrasensitive Chiral Detection.

The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal–dielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and π/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities.

Bright and Robust Phosphorescence Achieved by Non-Covalent Clipping.

Phosphorescent materials with bright emission in versatile media are important for their practical applications, which require to lower the susceptibility of triplet excitons to surroundings. Herein a non-covalent clipping strategy has been developed to attain this objective, by designing a tweezer receptor to encapsulate PtII -based triplet emitters through two-fold π-stacking interactions. The PtII emitters display robust phosphorescence by virtue of synergistic rigidifying and shielding effects, which are hardly influenced by emitter concentration, oxygen content, and solvent polarity changes. The phosphorescent colors are elaborately modulated by varying ligand substitutes on PtII emitters. Circularly polarized phosphorescence is further amplified for chiral PtII emitters, by taking advantage of dual phosphorescence and chirality enhancement upon non-covalent tweezer complexation. Overall, the clipping approach paves the way for the development of high-performance phosphorescent materials with bright emission, environmental robustness, and facile color tunability.

Bright and Robust Phosphorescence Achieved by Non‐Covalent Clipping

AbstractPhosphorescent materials with bright emission in versatile media are important for their practical applications, which require to lower the susceptibility of triplet excitons to surroundings. Herein a non‐covalent clipping strategy has been developed to attain this objective, by designing a tweezer receptor to encapsulate PtII‐based triplet emitters through two‐fold π‐stacking interactions. The PtII emitters display robust phosphorescence by virtue of synergistic rigidifying and shielding effects, which are hardly influenced by emitter concentration, oxygen content, and solvent polarity changes. The phosphorescent colors are elaborately modulated by varying ligand substitutes on PtII emitters. Circularly polarized phosphorescence is further amplified for chiral PtII emitters, by taking advantage of dual phosphorescence and chirality enhancement upon non‐covalent tweezer complexation. Overall, the clipping approach paves the way for the development of high‐performance phosphorescent materials with bright emission, environmental robustness, and facile color tunability.

Magnetically active terahertz wavefront control and superchiral field in a magneto-optical Pancharatnam-Berry metasurface.

Nowadays, the manipulation of the chiral light field is highly desired to characterize chiral substances more effectively, since the chiral responses of most molecules are generally weak. Terahertz (THz) waves are related to the vibration-rotational energy levels of chiral molecules, so it is significant to actively control and enhance the chirality of THz field. Here, we propose a metal/magneto-optical (MO) hybrid Pancharatnam-Berry (PB) phase structure, which can serve as tunable broadband half-wave plate and control the conversion of THz chiral states with the highest efficiency of over 80%. Based on this active PB element, MO PB metasurfaces are proposed to manipulate THz chiral states as different behaviors: beam deflector and scanning, Bessel beam, and vortex beam. Due to the magnetic-tunablibity, these proposed MO PB metasurfaces can be turned from an "OFF" to "ON" state by changing the external magnetic field. We further investigate the near-field optical chirality and the chirality enhancement factors in far field of the chiral Bessel beam and vortex beam, achieving the superchiral field with the highest chiral enhancement factor of 40 for 0th Bessel beam. These active, high efficiency and broadband chiral PB metasurfaces have promising applications for manipulation the THz chiral light and chiroptical spectroscopic techniques.

Terahertz chiral sensing and magneto-optical enhancement for ferromagnetic nanofluids in the chiral metasurface.

The highly sensitive detection and magnetic field sensing of magnetic nanomaterials have received extensive attention, and its weak magneto-optical effect in the terahertz (THz) band limits its application. In this study, we investigated a chiral metasurface sensor filled with ferromagnetic nanofluids. Based on its artificial chiral resonance, the nanoparticle concentration and magneto-optical chiral response of the ferromagnetic nanofluids have both been detected using the THz time-domain polarization spectroscopy. The results show that the detection sensitivity of the concentration of the magnetic nanoparticles can reach 5.5 GHz %−1 by chiral sensing, needing only a trace amount of the nanofluid. More importantly, in this hybrid device, the magneto-optical chiral response of the ferromagnetic nanoparticles can be greatly enhanced by the chiral metasurface, which results in higher sensitivity to the external magnetic field. The Verdet constant of the ferromagnetic nanofluid in the metasurface is 15 times stronger than that without the chiral microstructure. This THz chiral sensing for nanoparticles and the chirality enhancement mechanism will promote a new sensing method and chiral manipulation device, especially for the highly sensitive magneto-optical device in the THz band.

Tunable Chiral Optics in All-Solid-Phase Reconfigurable Dielectric Nanostructures.

Subwavelength nanostructures with tunable compositions and geometries show favorable optical functionalities for the implementation of nanophotonic systems. Precise and versatile control of structural configurations on solid substrates is essential for their applications in on-chip devices. Here, we report all-solid-phase reconfigurable chiral nanostructures with silicon nanoparticles and nanowires as the building blocks in which the configuration and chiroptical response can be tailored on-demand by dynamic manipulation of the silicon nanoparticle. We reveal that the optical chirality originates from the handedness-dependent coupling between optical resonances of the silicon nanoparticle and the silicon nanowire via numerical simulations and coupled-mode theory analysis. Furthermore, the coexisting electric and magnetic resonances support strong enhancement of optical near-field chirality, which enables label-free enantiodiscrimination of biomolecules in single nanostructures. Our results not only provide insight into the design of functional high-index materials but also bring new strategies to develop adaptive devices for photonic and electronic applications.

Optical Chirality Enhancement in Hollow Silicon Disk by Dipolar Interference

AbstractOptical chirality enhancement is highly demanded for enantioselective interaction of circularly polarized light with chiral molecules. The chirality enhancement in the coaxial air hole of a hollow silicon disk depends on three aspects, namely, the enhancements of electric and magnetic fields and a factor determined by the phases of their field components. In the spectral regime of dipole resonances, maximum chirality enhancement with sign consistency and uniform spatial distribution in the air hole can be obtained in association with both magnetic dipole resonance and anapole. Due to dipolar interference, the chirality is nulled at their coincidence, around which the sign of chirality is reversed. Maximum chirality with both positive and negative signs can be found between magnetic dipole resonance and anapole in the vicinity of their coincidence. This situation is maintained under size scaling so that the operation wavelength can be broadly tuned. The optical chirality can be further improved by merely adjusting the hole radius, by which the optimal spatially averaged optical chirality enhancement factor can reach 39 and −23. The simple strategy for optimizing Mie resonators presented in this work may benefit the design of Mie resonator‐based achiral metasurfaces for chirality detection application.

Broadband asymmetric transmission via angle-induced chirality enhancement in split ring resonators

Asymmetric Transmission (AT) allows direction-sensitive control of electromagnetic waves that is highly desirable in optical and microwave communication applications. In this paper, an angle-induced chirality enhancement approach is proposed through the introduction of angled splits in a split ring resonator based unit cell. This design approach, which enhances electromagnetic cross coupling, has been analytically and experimentally validated, yielding efficient and broadband AT operation with 27.1% fractional bandwidth from 14.8 GHz to 19.4 GHz. The AT magnitude of greater than 80% and transmission efficiency of 90% are achieved across the 4.6 GHz bandwidth. These results when compared with recently published designs depict that the presented metasurface exhibits the most broadband AT in a bi-layered configuration with the thinnest reported form factor of 0.78 mm in the microwave regime. The underlying physics of AT operation has been illustrated through analysis of surface currents and electric field distributions. More importantly, new insights into the effects of optical activity on the AT magnitude are presented. The proposed ultra-thin metasurface, operating in K and Ku bands, finds its application as a polarization-control device for modern radar and satellite communication systems.

Phenethylamine@Pillar[5]arene Biointerface for Highly Enantioselective Adsorption of Protein.

In the life system, the biointerface plays an important role in cell adsorption, platelet adsorption and activation. Therefore, the study of protein adsorption on the biointerface is of great significance for understanding life phenomena and treatment in vitro. In this paper, a chiral biointerface was constructed by the virtue of host-guest interaction between a water-soluble pillar[5]arene (WP5) and phenethylamine (PEA) over a gold surface for adsorption of lysozyme proteins. From the experimental results it was identified that the host-guest biointerface has a high adsorption capacity and strong chiral selectivity. Furthermotre, it was identified that the host-guest interaction plays the decisive role in the enhancement of chirality of the interface, which was much beneficial for increasing protein adsorption and amplifying the capacity of chiral discrimination. Therefore, this work provides a new idea for the construction of biointerface materials with high protein adsorption capacity and high chiral selectivity through supramolecular interaction, which will have potential applications in the fields of biosensors, biocatalysts, biomaterials.

Generation of optical chirality patterns with plane waves, evanescent waves and surface plasmon waves.

We systematically investigate the generation of optical chirality patterns by applying the superposition of two waves in three scenarios, namely free-space plane waves, evanescent waves of totally reflected light at dielectric interface and propagating surface plasmon waves on a metallic surface. In each scenario, the general analytical solution of the optical chirality pattern is derived for different polarization states and propagating directions of the two waves. The analytical solutions are verified by numerical simulations. Spatially structured optical chirality patterns can be generated in all scenarios if the incident polarization states and propagation directions are correctly chosen. Optical chirality enhancement can be obtained from the constructive interference of free-space circularly polarized light or enhanced evanescent waves of totally reflected light. Surface plasmon waves do not provide enhanced optical chirality unless the near-field intensity enhancement is sufficiently high. The structured optical chirality patterns may find applications in chirality sorting, chiral imaging and circular dichroism spectroscopy.

Extreme enhancement of secondary chirality through coordination-driven steric changes of terpyridyl ligand in glutamide-based molecular gels.

Aggregation-induced chirality is potentially useful in sensor technology applications. Herein we show extreme enhancement of secondary chirality through coordination-driven steric changes of terpyridyl ligand in molecular gels. The secondary chirality reflecting on enhancement of chiral signals (i.e., circular dichroism (CD) and circularly polarised luminescence (CPL)) of the molecular gels formed from glutamide-attached terpyridine (G-tpy) is extremely enhanced by the coordination of its terpyridyl groups to metal ions such as Cu2+, Zn2+ and Ru2+, which is due to dramatic changes in the stacked structure of the chromophore groups through the formation of metal ion complex. Metal-free terpyridine exists in a non-planar geometry, which suppress π–π stacking interactions among aggregates. The planarity of the terpyridyl group is improved through metal-ion complexation, which induces the metal-ion-coordinated terpyridyl groups to stack. The thermal stabilities of the CD signals are strongly affected by the metal-ion species. CPL signal is generated in the molecular gel formed from G-tpy–Zn2+ complex accompanied by chelation-enhanced fluorescence. It is expected that large and sensitive coordination-driven secondary chirality signals (CD and CPL) are useful for sensing guest molecules and the surrounding environment.

Time-controllable roll-up onset of polythiophene sheets into nanotubes that exhibit circularly polarized luminescence.

Self-assembly of a polythiophene-conjugated glycolipid exclusively produced square sheets a few micrometers on each side. Seventeen hours after the sheets were dispersed in ethanol at 25 °C, they suddenly started to roll up, and eventually they were completely transformed into nanotubes. The onset timing of the roll-up was temperature-dependent. The roll-up involved rearrangement of the molecular packing within the bilayer membranes, which was accompanied by strengthening of the intermolecular hydrogen bonds, alteration of the polythiophene aggregation mode and enhancement of supramolecular chirality due to chiral packing. The nanotubes exhibited not only strong fluorescence derived from J-type aggregation of the polythiophene aromatic moiety but also circularly polarized luminescence (CPL) originating from the left-handed helicity of the polythiophene main chain backbone. Because the CPL onset was concurrent with the sheet roll-up, the CPL onset was also able to be controlled by varying the temperature. Such delayed CPL onset has never been reported in chiral supramolecular structures, in which CPL onset and helicity inversion usually begin immediately upon application of a stimulus and then progress either quickly or gradually. Our findings can be expected to facilitate the development of new stimulus-responsive supramolecular structures that can be used for delayed-action capsules or optical switching devices.

Chirality reversal, enhancement and transfer by pH-adjusted surfactant assembly.

The controllable chirality reversal and enhancement at a supramolecular level is crucial for the synthesis and applications of circularly active materials, which has been achieved by a pH-adjusted amphiphilic chiral surfactant assembly approach, and reveals the relationship between the chirality behavior and its assembly morphology in a non-covalent interaction regime and its ability to transfer chirality from chiral molecules to achiral ones under appropriate conditions.

Chemical Mechanisms of Prebiotic Chirality Amplification.

This review article surveys the recent experimental findings that suggest alternative chemical models of directed chirality amplification at the early, prebiotic Earth. It is believed that the chirality amplification step followed the initial emergence of small enantiomeric imbalance and preceded (as a necessary condition) the occurrence of homochiral biopolymers, assembled from enantiomerically pure building blocks. This work focuses on the chemical nature of possible mechanisms of primordial chirality enhancement, without going into detail of the preceding and subsequent phases of origination of biological homochirality and life on Earth. These mechanisms are discussed through the prism of integrity of biological natural selection and chemical kinetic selection.

Concurrent helix extension and chirality enhancement for an artificial helical foldamer complexed with sterically hindered chiral molecules

Constructing and regulating the helical structures of artificial foldamers have been constantly pursued because of their wide applications in chiral recognition, asymmetric catalysis, circularly polarized luminescence, etc. On the one hand, stabilizing the helical structures of foldamers can usually enhance the chirality by decreasing the pitches. On the other hand, the extended helix can be obtained when the π-π stacking interaction in the main chain is weakened. In general, the former aspect is inconsistent with the later. Herein, chiral 1-phenylethylamines, 2-phenylglycinols and leucinols are used as the guests to modulate the folded helix of an artificial poly(phenylenediethynylene)-based foldamer. With the aid of chiral center, the steric hindrance of the chiral guests can increase the distances between the pitches. Meanwhile, a more ordered chain arrangement is induced by such steric effect. As a result, we successfully endow the foldamers with an extended helix and enhanced chirality at the same time.

Dynamic manipulation of optical chirality for gammadion nanostructures

We present a method to achieve the dynamic manipulation of optical chirality for a gammadion nanostructure. The gammadion nanostructure is illuminated by two counter-propagating linearly polarized lights (LPLs) with orthogonal polarization directions. By adjusting the phase difference between the two LPLs, the optical chirality enhancement can be adjusted periodically as a function of the phase difference and the optical chirality manipulation range lies between those obtained by single left- and right-handed circularly polarized light excitation. This work provides a convenient and feasible method to achieve the dynamic manipulation of optical chirality for chiral plasmonic nanostructures.

A \u03bc-\u03c4-philic scalar doublet under Zn flavor symmetry

We propose a minimal model which accommodates the long-standing anomaly of muon magnetic moment based on abelian discrete flavor symmetries. The standard model is extended by scalar doublets charged under a Zn lepton flavor symmetry. In these models, a large contribution to the muon magnetic moment can be obtained by the chirality enhancement from new scalar mediated diagrams without conflicting with the flavor symmetry. Thanks to the lepton flavor symmetry, these models automatically forbid lepton flavor violation. The minimal model is based on Z4 symmetry with only one extra scalar doublet. In this model, we show that the parameter space favored by the muon g − 2 can easily be consistent with experimental constraints and theoretical bounds such as the electroweak precision tests, lepton universality, potential stability condition and triviality bound as well as the LHC direct search mass bound. The new contributions to the muon electric dipole moment and the Higgs decay into γγ can be indirect signals of the model.