Chiral Response Research Articles

Experimental realization of chiral Landau levels in two-dimensional Dirac cone systems with inhomogeneous effective mass

Chiral zeroth Landau levels are topologically protected bulk states. In particle physics and condensed matter physics, the chiral zeroth Landau level plays a significant role in breaking chiral symmetry and gives rise to the chiral anomaly. Previous experimental works on such chiral Landau levels are mainly based on three-dimensional Weyl degeneracies coupled with axial magnetic fields. Their realizations using two-dimensional Dirac point systems, being more promising for future applications, were never experimentally realized before. Here we propose an experimental scheme for realizing chiral Landau levels in a two-dimensional photonic system. By introducing an inhomogeneous effective mass through breaking local parity-inversion symmetries, a synthetic in-plane magnetic field is generated and coupled with the Dirac quasi-particles. Consequently, the zeroth-order chiral Landau levels can be induced, and the one-way propagation characteristics are experimentally observed. In addition, the robust transport of the chiral zeroth mode against defects in the system is also experimentally tested. Our system provides a new pathway for the realization of chiral Landau levels in two-dimensional Dirac cone systems, and may potentially be applied in device designs utilizing the chiral response and transport robustness.

Chiral electroluminescence from thin-film perovskite metacavities.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media used for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP) based on perovskite materials, being critical for the development of practical devices. Here, we propose a concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with a peak DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with a close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.

Broadband plasmonic chiral meta-mirrors.

Chiral meta-mirrors provide a unique opportunity for achieving handedness-selective strong light-matter interaction at the nanometer scale. Importantly, the chiral resonances observed in chiral meta-mirrors arise from the spin-dependent resonant cavity which, however, is generally narrowband. In this paper, by exploiting a genetic algorithm (GA) based optimization method, we numerically validate a chiral meta-mirror with octave bandwidth. In particular, in the wavelength range from 1000 to 2000 nm, the proposed chiral meta-mirror strongly absorbs circularly polarized light of one handedness while highly reflecting the other. A field analysis indicates that the observed broadband chiroptical response can be attributed to the multiple chiral resonances supported by the optimized meta-mirror across the band of interest. The observed broadband chiral response confirms the potential of advanced inverse-design approaches for the creation of chiral metadevices with sophisticated functionalities. Based on the Lorentz reciprocity theorem, we show that the proposed meta-mirror can enable chiral-selective broadband second harmonic generation (SHG). Our study indicates that the application of advanced inverse-design approaches can greatly facilitate the development of metadevices with strong chiral response in both the linear and nonlinear regimes.

What Contributes to the Measured Chiral Optical Response of the Glutathione-Protected Au25 Nanocluster?

The water-soluble glutathione-protected [Au25(GSH)18]-1 nanocluster was investigated by integrating several methodologies such as molecular dynamics simulations, essential dynamics analysis, and state-of-the-art time-dependent density functional theory calculations. Fundamental aspects such as conformational, weak interactions and solvent effects, especially hydrogen-bonds, were included and found to play a fundamental role in assessing the optical response of this system. Our analysis demonstrated not only that the electronic circular dichroism is extremely sensitive to the solvent presence but also that the solvent itself plays an active role in the optical activity of such system, forming a chiral solvation shell around the cluster. Our work demonstrates a successful strategy to investigate in detail chiral interfaces between metal nanoclusters and their environments, applicable, e.g., to chiral electronic interactions between clusters and biomolecules.

Strong and Broadband Pure Optical Activity in 3D Printed THz Chiral Metamaterials

AbstractOptical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization‐related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, a chiral metamaterial (CMM) structure composed of pairs of vertical U‐shape resonators of “twisted” arms is studied, and it is revealed that it demonstrates large pure optical activity (i.e., optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25° for an unmatched bandwidth of 1 THz (relative bandwidth 80%), from a 130 µm‐thickness structure, while theoretical optimizations show that the rotation can reach 45°. The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which also provides simple optimization rules for the enhancement of its chiral response. The proposed chiral design allows for easy fabrication via direct laser writing (DLW) and electroless metal plating, making the associated structures suitable candidates for polarization control applications.

Enhanced chirality induced in a composite structure consisting of α-MoO3 film and a silver metasurface.

Chiral structures have been widely used in many fields, such as biosensing and analytical chemistry. In this paper, the chiral response of a composite structure consisting of α-M o O 3 film and a silver (Ag) metasurface is studied. First, the effect of the thickness of α-M o O 3 film on the circular dichroism (CD) is discussed, and it is found that CD can reach 0.93 at a wavelength of 9.6µm when the thickness of α-M o O 3 film is 6.075µm. To better understand the physical mechanism, we analyze the transverse electric and transverse magnetic wave components in the transmitted wave for the whole structure and each layer. One can see that the strong chirality of the structure is attributed to the polarization conversion of α-M o O 3 film and the selective transmissivity of Ag ribbons. In addition, the influence of the filling factor of the Ag ribbons on chirality is also studied. This work combines hyperbolic material α-M o O 3 with Ag ribbons to enhance CD. Also, it provides greater freedom in the tuning of chirality. We believe that this work not only deepens the understanding of the chiral response of anisotropic materials, but also gives promise for its applications in the fields of polarization optics and biosensing.

Chiral Lattice Resonances in 2.5-Dimensional Periodic Arrays with Achiral Unit Cells.

Lattice resonances are collective electromagnetic modes supported by periodic arrays of metallic nanostructures. These excitations arise from the coherent multiple scattering between the elements of the array and, thanks to their collective origin, produce very strong and spectrally narrow optical responses. In recent years, there has been significant effort dedicated to characterizing the lattice resonances supported by arrays built from complex unit cells containing multiple nanostructures. Simultaneously, periodic arrays with chiral unit cells, made of either an individual nanostructure with a chiral morphology or a group of nanostructures placed in a chiral arrangement, have been shown to exhibit lattice resonances with different responses to right- and left-handed circularly polarized light. Motivated by this, here, we investigate the lattice resonances supported by square bipartite arrays in which the relative positions of the nanostructures can vary in all three spatial dimensions, effectively functioning as 2.5-dimensional arrays. We find that these systems can support lattice resonances with almost perfect chiral responses and very large quality factors, despite the achirality of the unit cell. Furthermore, we show that the chiral response of the lattice resonances originates from the constructive and destructive interference between the electric and magnetic dipoles induced in the two nanostructures of the unit cell. Our results serve to establish a theoretical framework to describe the optical response of 2.5-dimensional arrays and provide an approach to obtain chiral lattice resonances in periodic arrays with achiral unit cells.

Molecular-Induced Chirality Transfer to Plasmonic Lattice Modes.

Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances of gold nanoparticles. Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. We use colloidal self-assembly to fabricate non-close packed, periodic arrays of achiral gold nanoparticles, which are embedded in a polymer film containing chiral molecules. In the presence of the chiral molecules, the surface lattice resonances (SLRs) become optically active, i.e., showing handedness-dependent excitation. Numerical simulations with varying lattice parameters show circular dichroism peaks shifting along with the spectral positions of the lattice modes, corroborating the chirality transfer to these collective modes. A semi-analytical model based on the coupling of single-molecular and plasmonic resonances rationalizes this chirality transfer.

Colloidal Assemblies of Chiral Plasmonic Nanoparticles Induce Tunable Circular Dichroism Response

AbstractPlasmonic nanoparticles with strong chiral optical activity are promising systems for a wide range of applications. Self‐assembly of achiral metallic nanoparticles thanks to chiral molecules or templates is a well‐known strategy to prepare chiral plasmonic nanomaterials. In this work, the assembly of chiral plasmonic particles without using a chiral templating agent is investigated. For that purpose, previously developed chiral core‐shell Au@Ag and Au@Au particles are assembled in a controlled way by lowering the surfactant concentration in solution. The assembly leads to pseudo‐linear structures that are revealed by electron microscopy, and the plasmonic responses are characterized by circular dichroism (CD) and UV‐Vis‐NIR spectroscopies. The chiral optical response strongly depends on the system and assembly conditions. For Au@Ag, the CD signal is tuned with contributions appearing in the red region of the spectrum, while for Au@Au, the CD response is unexpectedly lost. The origin of this difference remains undetermined, it is hypothesized that the coupling of circular dichroism modes or the generation of chiral superstructures may influence the chiroptical properties. These experimental results highlight the non‐obvious and complex nature of the chirality generation mechanisms in such plasmonic systems.

Hybrid bilayer plasmonic metasurfaces with intrinsic chiral optical responses

Plasmonic metasurfaces with giant chiral optical responses are good candidates for chirality study and application, which are usually composed of few-layer or three-dimensional nanostructures. Here, we demonstrate that hybrid bilayer plasmonic metasurfaces simply fabricated via a facile one-step nanofabrication process are good candidates for the realization of intrinsic chiral optical responses and spin-selective transmission. We prove that the intrinsic chiral optical response of the proposed metasurface, which can be easily manipulated, is attributed to the spin-selective collective interference of locally enhanced electric fields caused by the strong interlayer coupling. We further experimentally prove the advantages of fabricated hybrid bilayer plasmonic metasurfaces for optical encryption. With the advantages of compact design, chiral optical response easy to control and lower fabrication demand, the proposed metasurfaces can further expand the applications of chiral plasmonic nanostructures in the area of spin nanophotonics, nonlinear optics, and optical sensing.

Nanophotonic Chirality Transfer to Dielectric Mie Resonators.

Nanophotonics can boost the weak circular dichroism of chiral molecules. One mechanism for enhanced chiral sensing relies on using a resonator to create fields with high optical chirality at the molecular position. Here, we elucidate how the reverse interaction between molecules and the resonator, called chirality transfer, can produce stronger circular dichroism. The chiral analyte modifies the electric and magnetic dipole moments of the resonator, imprinting a chiral response on an otherwise achiral resonance. We demonstrate that silicon nanoparticles and metasurfaces tailored for chirality transfer generate chiroptical signals orders of magnitude higher than the contribution from optical chirality alone. We derive closed-form equations for the dependence of chirality transfer on molecular chirality, molecule-resonator distance, and Mie coefficients. We propose a dielectric metasurface for a 900-fold circular dichroism enhancement on the basis of these principles. Finally, we identify a fundamental limit to chirality transfer. Our findings thus establish key concepts for nanophotonic chiral sensing.

Multipole resonances-mediated dual-band tunable circular dichroism via vanadium dioxide chiral metamaterials

Chiral response based on phase transition metamaterial of vanadium dioxide (VO2) is a promising candidate for chirality-related applications such as biochemical sensing, spectral detection, and advanced imaging. However, chiroptical peaks of VO2 chiral metamaterials are usually vanished after VO2 undergoes the phase transition from metallic state to dielectric state, which limits the dynamic control of their chiral properties. Herein, we propose a complementary z-shaped VO2 chiral metamaterial to achieve dual-band circular dichroism (CD) responses, and the two resonant CD peaks can be maintained in both metallic and dielectric states of VO2. The mechanisms of the novel robust CD responses are associated with the excitations of hybrid multipole resonances according to the far-field multipole decomposition and the near-field distribution of the device, and their peak values as well as their resonance locations can be dynamically tuned in the mid-infrared region by changing the volume fraction. In addition, the variations of the length and the width of the proposed device slightly alter the CD responses particular the resonance location, but the CD responses are almost immune to the variation of its height. Highlights The concept of multipole resonances-mediated tunable CD is proposed via the VO2-based chiral metamaterials. Robust dual-band CD responses associated with the excitation of hybrid multipole resonances are demonstrated. The peak value, frequency and bandwidth of CD response can be dynamically tuned by changing volume fraction. Good CD performances can be maintained even if the geometrical parameters are significantly altered.

Neutral Magic‐Angle Bilayer Graphene: Condon Instability and Chiral Resonances

The full optical response of twisted bilayer graphene at the neutrality point close to the magic angle within the continuum model (CM) is discussed. First, three different channels consistent with the underlying symmetry are identified, yielding the total, magnetic, and chiral response. Second, the full optical response in the immediate vicinity of the magic angle is numerically calculated, which provides a direct mapping of the CM onto an effective two‐band model. It is, further, shown that the ground state of the CM in the immediate vicinity of is unstable toward transverse current fluctuations, a so‐called Condon instability. Third, due to the large counterflow, the acoustic plasmonic excitations with typical wave numbers have larger energies than the optical ones and their energy density may be largely enhanced at certain frequencies which are denominated as chiral resonances. Finally, symmetry relations for the optical response and their consequences for the chiral response are discussed.

Plasmonic Polarization Rotation in SERS Spectroscopy.

Surface-enhanced Raman optical activity (SEROA) has been extensively investigated due to its ability to directly probe stereochemistry and molecular structure. However, most works have focused on the Raman optical activity (ROA) effect arising from the chirality of the molecules on isotropic surfaces. Here, we propose a strategy for achieving a similar effect: i.e., a surface-enhanced Raman polarization rotation effect arising from the coupling of optically inactive molecules with the chiral plasmonic response of metasurfaces. This effect is due to the optically active response of metallic nanostructures and their interaction with molecules, which could extend the ROA potential to inactive molecules and be used to enhance the sensibility performances of surface-enhanced Raman spectroscopy. More importantly, this technique does not suffer from the heating issue present in traditional plasmonic-enhanced ROA techniques, as it does not rely on the chirality of the molecules.

Enhanced Diffractive Circular Dichroism from Stereoscopic Plasmonic Molecule Array

Artificial nanostructures with large optical chiral responses have been intensively investigated recently. In this work, we propose a diffractive circular dichroism enhancement technique using stereoscopic plasmonic molecule structures. According to the multipole expansion analysis, the z-component of the electric dipole becomes the dominant chiral scattering mechanism during the interaction between an individual plasmonic molecule and the plane wave at a grazing angle. For a periodical structure with the designed plasmonic molecule, large diffractive circular dichroism can be obtained, which can be associated with the Wood-Rayleigh anomaly. Such a diffractive circular dichroism enhancement is verified by the good agreement between numerical simulations and experimental results. The proposed approach can be potentially used to develop enhanced spectroscopy techniques to measure chiral information, which is very important for fundamental physical and chemical research and bio-sensing applications.

Time-Resolved Chiral X-Ray Photoelectron Spectroscopy with Transiently Enhanced Atomic Site Selectivity: A Free-Electron Laser Investigation of Electronically Excited Fenchone Enantiomers

Chirality is widespread in nature, playing a fundamental role in biochemical processes and in the origin of life itself. The observation of dynamics in chiral molecules is crucial for the understanding and control of the chiral activity of photoexcited states. One of the most promising techniques for the study of photoexcited chiral systems is time-resolved photoelectron circular dichroism (TR-PECD), which offers an intense and sensitive probe for vibronic and geometric molecular structure as well as electronic structures, and their evolution on a femtosecond timescale. However, the nonlocal character of the PECD effect, which is imprinted during the electron scattering off the molecule, makes the interpretation of TR-PECD experiments challenging. In this respect, core photoionization is known to allow site and chemical sensitivity to photelectron spectroscopy. Here we demonstrate that TR-PECD utilizing core-level photoemission enables probing the chiral electronic structure and its relaxation dynamics with atomic site sensitivity. Following UV pumped excitation to a 3s Rydberg state, fenchone enantiomers (C10H16O) were probed on a femtosecond scale using circularly polarized soft x-ray light pulses provided by the free-electron laser FERMI. C 1s binding energy shifts caused by the redistribution of valence electron density in this 3s-valence-Rydberg excitation allowed us to measure transient PECD chiral responses with an enhanced C atom site selectivity compared to that achievable in the ground state molecule. This chemical-specific, site-specific, and enantiosensitive observation of the electronic structure of a transiently photoexcited chiral molecule is expected to pave the way toward chiral femtochemistry probed by core-level photoemission.Received 2 March 2022Revised 27 September 2022Accepted 2 February 2023DOI:https://doi.org/10.1103/PhysRevX.13.011044Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasChiralityElectronic structure of atoms & moleculesPhysical SystemsMoleculesTechniquesPhotoionizationUltrafast femtosecond pump probeX-ray photoelectron spectroscopyAtomic, Molecular & Optical

Design of an Electrostatic Frequency Map for the NH Stretch of the Protein Backbone and Application to Chiral Sum Frequency Generation Spectroscopy.

We develop an electrostatic map for the vibrational NH stretch (amide A) of the protein backbone with a focus on vibrational chiral sum frequency generation spectroscopy (chiral SFG). Chiral SFG has been used to characterize protein secondary structure at interfaces using the NH stretch and to investigate chiral water superstructures around proteins using the OH stretch. Interpretation of spectra has been complicated because the NH stretch and OH stretch overlap spectrally. Although an electrostatic map for water OH developed by Skinner and co-workers was used previously to calculate the chiral SFG response of water structures around proteins, a map for protein NH that is directly responsive to biological complexity has yet to be developed. Here, we develop such a map, linking the local electric field to vibrational frequencies and transition dipoles. We apply the map to two protein systems and achieve much better agreement with experiment than was possible in our previous studies. We show that couplings between NH and OH vibrations are crucial to the line shape, which informs the interpretation of chiral SFG spectra, and that the chiral NH stretch response is sensitive to small differences in structure. This work increases the utility of the NH stretch in biomolecular spectroscopy.

Chiral Mesostructured Inorganic Materials with Optical Chiral Response.

Fabricating chiral inorganic materials and revealing their unique quantum confinement-determined optical chiral responses are crucial tasks in the multidisciplinary fields of chemistry, physics, and biology. The field of chiral mesostructured inorganic materials started from the synthesis of individual nanocrystals and evolved to include their assembly from metals, semiconductors, ceramics, and inorganic salts endowed with various chiral structures ranging from atomic to micron scales. This tutorial review highlights the recent research on chiral mesostructured inorganic materials, especially the novel expression of mesostructured chirality and endowed optical chiral response, and it may inspire us with new strategies for the design of chiral inorganic materials and new opportunities beyond the traditional applications of chirality. Fabrication methods for chiral mesostructured inorganic materials are classified according to chirality type, scale, and symmetry-breaking mechanism. Special attention is given to highlight systems with original discoveries, exceptional phenomena, or unique mechanisms of optical chiral response for left- and right-handedness.

Optical Chirality of Gold Chiral Helicoid Nanoparticles in the Strong Coupling Region

The far- and near-field chirality properties are usually characterized by circular dichroism (CD) and optical chirality (OC), respectively. As a light–matter interaction for the hybrid states consisting of plasmons and excitons, the strong coupling interactions can affect the original chiral electromagnetic modes. However, there are few works on this influence process, which prevents an in-depth understanding of chirality. Here, we theoretically investigate both the far-field and near-field characteristics of the chiral plasmonic gold helicoid nanoparticle (GHNP) to explore the chirality mechanism further. We found that the electromagnetic field distribution of GHNP consists of one dark mode and two bright modes. The dark mode is observed more clearly in CD than in extinction spectra. Two bright modes can strongly couple with excitons respectively, which is confirmed by the anticrossing behavior and mode splitting exhibited in the extinction and CD spectra. We also analyzed the near-field OC distribution of the GHNP hybrid system and obtained the chiral responses as well as the spectral correspondence between OC and CD. Furthermore, although the strong coupling interaction changes the energy levels, resulting in mode splitting, the chiral hotspot distributions of both the upper polariton branch and lower polariton branch are consistent with the original bright mode in OC maps. Our findings provide guidance for the design of structures with strong chiral responses and enhance the comprehension of chiral strong coupling systems.

Excitation of Chiral Cavity Plasmon Resonances in Film‐Coupled Chiral Au Nanoparticles

AbstractChiral plasmonic nanostructures have attracted increasing attention because of their superchiral near‐fields as well as strong far‐field chiral optical response. Recently, the development of chemical synthesis methods enabled the large‐scale manufacturing of three‐dimensional colloidal chiral plasmonic nanocrystals. Further improving the chiral optical response of such nanostructures will greatly facilitate their practical applications. In this work, it is found both in calculations and experiments that chiral cavity plasmon resonances can be excited in film‐coupled chiral Au helicoid nanoparticles, enabling the significant enhancement of the nanostructure chiral optical response. In addition, it is demonstrated from simulation that the chiral cavity mode can modulate the emission polarization of a point electric dipole placed in the nanocavity formed by the nanoparticle and the Au film, allowing the emission of almost circularly polarized photons by the linear dipole with the emission circular polarization anisotropy factor reaching as high as 93%. The film‐coupled chiral plasmonic nanoparticles therefore provide a promising platform for the construction of advanced chiral optical devices such as on‐chip nonreciprocal nanoscale light sources, chiral plasmonic sensors, chiral metamaterials, plasmonically enabled valleytronic devices, and nanophotonic circuits for future on‐chip communication applications.