The circular dichroism (ICD) induced from interaction of chiral molecules with achiral plasmonic metal nanostructures has improved sensitivity of molecular chirality detection. Although experimental results have presented several orders magnitude of ICD enhancement, but the cause of ICD induction has not arise from previous studies. In this paper, a theory presented for explain mechanism of ICD induction from near field interaction of chiral molecule with achiral nanostructure. By introducing dipole approaching of chiral molecule and a near field approximation of molecule-nanostructure interaction, we derived an analytical formula for ICD. This theory explains mechanism of ICD is that the dipole radiation of chiral molecules breaks the enantiomorphic relationship of the electric field strengths in the nanostructure. Furthermore, the derived formula exhibits ICD proportional to molecules chirality parameter and number density, and it exponentially increases with the inverse cube of distance between molecule and nanostructure. Most importantly, the formula shows that ICD is proportional to strengths of electric field in nanostructure and magnetic field in molecular region. To verify the analytical results, we studied the ICD properties of graphene nanohole metasurface (GNM) and chiral molecule composed system. Numerical results were found consistent with analytical results. Our investigation results are helpful for understanding mechanism of ICD induction, pushing the limit of ICD enhancement and realizing supersensitive detection of molecular chirality.
Sometimes it appears, sometimes it does not. We present a theoretical study of circular dichroism in the second-harmonic signal of single dielectric nanostructures with different symmetries and noncentrosymmetric materials. We show that this effect is not fully identified only by macroscopic and lattice symmetries, but their relative orientation as well. Based on the cumbersome symmetry and modal analysis, we provide a general and very simple formula to determine whether the dichroism exists, and a table with the most significant cases.
Large optical chirality in the vicinity of achiral high index dielectric nanostructures has been recently demonstrated as useful means of enhancing molecular circular dichroism. We theoretically study the spatial dependence of optical chirality enhancement in the vicinity of high index dielectric nanodisks and highlight its importance for the design of nanophotonic platforms for circular dichroism enhancement. Using a T-matrix framework, we demonstrate that depending on disk aspect ratio chirality is enhanced preferentially along different directions. We employ various statistical procedures (…
Polariton states with high levels of chiral dissymmetry offer exciting prospects for quantum information, sensing, and lasing applications. Such dissymmetry must emanate from either the involved optical resonators or the quantum emitters. Here, we theoretically demonstrate how chiral polaritons can be realized by combining (high quality factor) achiral Fabry-Perot cavities with samples exhibiting a phenomenon known as "apparent circular dichroism" (ACD), which results from an interference between linear birefringence and dichroic interactions. By introducing a quantum electrodynamical theory o…
Optical activity is a fundamental phenomenon originating from the chiral nature of crystals and molecules. While intrinsic chiroptical responses of ordinary chiral materials to circularly polarized light are relatively weak, they can be enhanced by specially tailored nanostructures. Here, nanorod metamaterials, comprising a dense array of vertically aligned gold nanorods, is shown to provide significant enhancement of the circular dichroism response of an embedded material. A nanorod composite, acting as an artificial uniaxial crystal, is filled with chiral mercury sulfide nanocrystals embedde…
We establish a general unified formulation which, using the optical theorem of electromagnetic helicity, shows that dichorism is a phenomenon arising in any scattering -or diffraction- process, elastic or not, of chiral electromagnetic fields by objects either chiral or achiral. It is shown how this approach paves the way to overcoming well-known limitations of standard circular dichroism, like its weak signal or the difficulties of using it with magnetodielectric particles. Based on the angular spectrum representation of optical fields with only right circular or left circular plane waves, we…
Based on a recent unified formulation on dichorism and extinction of helicity on scattering by a small particle, dipolar in the wide sense, magnetodielectric or not, chiral or achiral, we show that such extinction is enhanced not only at resonances of the polarizabilities, but also due to interference between left and right circularly polarized components of the incident wave, which contributes with appropriate parameters of the illuminating field, even if the particle is achiral and is placed at points of the incident field at which the local incident helicity density is zero. This phenomenon…
Sensitivity to molecular chirality is crucial for many fields, from biology and chemistry to the pharmaceutical industry. By generating superchiral light, nanophotonics has brought innovative solutions to reduce the detection volume and increase sensitivity at the cost of a non-selectivity of light chirality or a strong contribution to the background. Here, we theoretically propose an achiral plasmonic resonator, based on a rectangular nanoslit in a thin gold layer behaving as a magnetic dipole, to generate a tunable nanosource of purely superchiral light. This nanosource is free of any backgr…
We derive a set of design requirements that lead to structures suitable for molecular circular dichroism (CD) enhancement. Achirality of the structure and two suitably selected sequentially incident beams of opposite helicity ensures that the CD signal only depends on the chiral absorption properties of the molecules, and not on the achiral ones. Under this condition, a helicity preserving structure, which prevents the coupling of the two polarization handednesses, maximizes the enhancement of the CD signal for a given ability of the structure to enhance the field. When the achirality and heli…
We introduce and theoretically analyze the concept of manipulating optical chirality via strong coupling of the optical modes of chiral nanostructures with excitonic transitions in molecular layers or semiconductors. With chirality being omnipresent in chemistry and biomedicine, and highly desirable for technological applications related to efficient light manipulation, the design of nanophotonic architectures that sense the handedness of molecules or generate the desired light polarization in an externally controllable manner is of major interdisciplinary importance. Here we propose that such…
Chirality is at the origin of life and is ubiquitous in nature. An object is deemed chiral if it is non-superimposable with its own mirror image. This relates to how circularly polarized light interacts with such object, a circular dichroism, the differential absorption of right and left circularly polarized light. According to the common understanding in biology, chemistry and physics, the circular dichroism results from an internal chiral structure or external symmetry breaking by illumination. We show that circular dichroism is possible with simple symmetric optical nanoantennas at symmetri…
The near-field interaction of plasmonic nanostructures and chiral molecules induces circular dichroism in achiral nanostructure. The induced circular dichroism (ICD) strength is several orders greater than the molecular inherent CD (MCD), and it generally appears at visual range (VR) where the CD signal is easily detectable. Therefore, the ICD is considered as a potential method to detect single molecular chirality. In this study, cross shaped nanohole (CSN) is proposed for enhancing the MCD signal. The maximum enhancement of the structure reached 450 folds of molecular inherent MCD. Through a…
We study the magneto-optical response of noble metal nanoparticle clusters. We consider the interaction between the light-induced dipoles of particles. In the presence of a magnetic field, the simplest achiral cluster, a dimer, exhibits circular dichroism (CD). The CD of a dimer depends on the directions of the magnetic field and the light wave vector. The CD of a populous cluster weakly depends on the magnetic field. Upon scattering from the cluster, an incident linearly polarized light with polarization azimuth $φ$, becomes elliptically polarized. The polarization azimuth rotation and ellipt…
Metasurfaces, the two-dimensional analogues of metamaterials, are ideal platforms for sensing molecular chirality at the nanoscale, e.g. of inclusions of natural optically active molecules, as they offer large accessible areas (they are essentially surfaces) and can accommodate the necessary strong resonances for coupling the probing radiation with the chiral inclusions. Here, we examine theoretically achiral isotropic metasurfaces, and we treat them as polarizable surfaces that support resonant electric and magnetic currents, which are coupled via the chiral inclusions. We derive analytically…
In the close vicinity of a chiral nanostructure, the circular dichroism of a biomolecule could be greatly enhanced, due to the interaction with the local superchiral fields. Modest enhancement of optical activity using a planar metamaterial, with some chiral properties, and achiral nanoparticles has been previously reported. A more substantial chirality enhancement can be achieved in the local filed of a chiral nanostructure with a three-dimensional arrangement. Using an embossed chiral nanostructure designed for chiroptical sensing, we measure the circular dichroism spectra of two biomolecule…
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China（电子科技大学光电科学与工程学院电子薄膜与集成器件国家重点实验室）
Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life to materials sciences. Established spectroscopic methods that are sensitive to chirality, such as circular dichroism (CD), exhibit weak signal contributions on an achiral background. Helical dichroism (HD), which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the …
Photoelectron circular dichroism results from one-photon ionization of chiral molecules by circularly polarized light and manifests itself in forward-backward asymmetry of electron emission in the direction orthogonal to the light polarization plane. To expose the physical mechanism responsible for asymmetric electron ejection, we first establish a rigorous relation between the responses of unaligned and partially or perfectly aligned molecules. Next, we identify a propensity field, which is responsible for the chiral response in the electric-dipole approximation, i.e. a chiral response withou…
We present a new design of plasmonic nanoantenna with a slant gap for optical chirality engineering. At resonance, the slant gap provides highly enhanced electric field parallel to external magnetic field with a phase delay of 90 degree, resulting in enhanced optical chirality. We show by numerical simulations that upon linearly polarized excitation our achiral nanoantenna can generate near field with enhanced optical chirality that can be tuned by the slant angle and resonance condition. Our design can be easily realized and may find applications in circular dichroism enhancement.
We experimentally investigate the chiral optical response of an individual nanostructure consisting of three equally sized spherical nanoparticles made of different materials and arranged in \ang{90} bent geometry. Placing the nanostructure on a substrate converts its morphology from achiral to chiral. Chirality leads to pronounced differential extinction, i.e., circular dichroism and optical rotation, or equivalently, circular birefringence, which would be strictly forbidden in the absence of a substrate or heterogeneity. This first experimental observation of the substrate-induced break of s…
Atoms are usually thought of as achiral objects. However, one can construct superpositions of atomic states that are chiral [1]. Here we show how to excite such superpositions with tailored light fields both in the weak-field and strong-field regimes, using realistic laser parameters. First, we use time-dependent Schrodinger equation (TDSE) simulations to demonstrate the creation of a time-dependent bound chiral wavepacket in sodium atoms. Second, we show how the time-dependent handedness of this wavepacket can be probed by photoelectron circular dichroism, in spite of the central symmetry of …
It has been long recognized that the spatial polarization of the electronic clouds in molecules, and the spatial arrangements of atoms into chiral molecular structures, play crucial roles in physics, chemistry and biology. However, these two fundamental concepts - chirality and polarization - have remained unrelated so far. This work connects them by introducing and exploring the concept of polarization of chirality. We show that, like charge, chirality, or handedness, can be polarized, and that such polarization leads to fundamental consequences, demonstrated here using light. First, we analy…
Molecular chirality plays a crucial role in innumerable biological processes. The chirality of a molecule can typically be identified by its characteristic optical response, the circular dichroism (CD). CD signals have thus long been used to identify the state of molecules or to follow dynamic protein configurations. In recent years, the focus has moved towards plasmonic nanostructures, as they show potential for applications ranging from pathogen sensing to novel optical materials. The plasmonic coupling of the individual elements of such chiral metallic structures is a crucial prerequisite t…
Chiral optical effects are generally quantified along some specific incident directions of exciting waves (especially for extrinsic chiralities of achiral structures) or defined as direction-independent properties by averaging the responses among all structure orientations. Though of great significance for various applications, chirality extremization (maximized or minimized) with respect to incident directions or structure orientations have not been explored, especially in a systematic manner. In this study we examine the chiral responses of open photonic structures from perspectives of quasi…
In plasmonic chirality, the phenomenon of circular dichroism for achiral nanoparitcles caused by Coulomb interaction between metal nanoparticles (NPs) and chiral molecules have been studied. At the same time, under the resonance condition, the dye molecules and metal NPs will produce huge Rabi splitting due to strong coupling. If the chiral molecules are at the resonance of the plasmon, what will happen for the strong interaction between the plasmon and molecules with chirality introduced? In this paper, we investigate a spherical core-shell model and analyze its spectral phenomena under the e…
