We consider the optical dipole potential energy, which arises from the interaction of a two-level atom with a circularly polarized Laguerre-Gaussian laser beam of small waist. The beam is characterized by the existence of a longitudinal electric field component which is responsible for the appearance of a chiral term in the optical dipole potential energy. This term reverses sign if either the winding number or the wave polarization of the beam reverses sign. We propose a scheme of a bi-chromatic vortex interaction with the two-level atom in which the resulting optical dipole potential is fully chiral.
Plasmonic chirality exhibits great potential for novel nanooptical devices due to the generation of a strong chiroptical response. Previous reports on plasmonic chirality explanations are mainly based on phase retardation and coupling. We propose a quantitative model similar to the chiral molecules for explaining the mechanism of the intrinsic plasmonic chirality quantitatively based on the interplay and mixing of electric and magnetic dipole modes, which forms a mixed electric and magnetic polarizability. The analysis method is also suitable for small chiral object down to quasi-static limit …
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…
Determination of enantiomeric excess is important and remains challenges. We theoretically propose a new spectroscopic method for this issue based on the chirality-dependent AC Stark effects in cyclic three-level models under the three-photon resonance condition. The enantiomeric excess of the chiral mixture is determined by comparing the amplitudes of the two chosen AC Stark peaks in the Fourier transform spectrum of the induced polarizations, which are (approximately) proportional to the molecule numbers of the two enantiomers, respectively. Comparing with current spectroscopic methods based…
Temporal Faraday effect enabled by Floquet-induced chirality
State Key Laboratory of Radio Frequency Heterogeneous Integration, College of Physics and Optoelectronic Engineering, Shenzhen University（深圳大学物理与光电工程学院射频异质集成国家重点实验室）
Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarisation (C points), with elliptical polarisations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor $\sim 30$), while simultan…
We calculate optical forces and torques exerted on a chiral dipole by chiral light fields and reveal genuinely chiral forces in combining the chiral contents of both light field and dipolar matter. Here, the optical chirality is characterized in a general way through the definition of optical chirality density and chirality flow. We show in particular that both terms have mechanical effects associated respectively with reactive and dissipative components of the chiral forces. Remarkably, these chiral force components are directly related to standard observables: optical rotation for the reacti…
Chirality is an intriguing property of certain molecules, materials or artificial nanostructures, which allows them to interact with the spin angular momentum of the impinging light field. Due to their chiral geometry, they can distinguish between left- and right-hand circular polarization states or convert them into each other. Here, we introduce a novel approach towards optical chirality, which is observed in individual two-dimensional and geometrically mirror-symmetric nanostructures. In this scheme, the chiral optical response is induced by the chosen heterogeneous material composition of …
We present a comprehensive review of recent developments in the field of chiral plasmonics. Significant advances have been made recently in understanding the working principles of chiral plasmonic structures. With advances in micro- and nanofabrication techniques, a variety of chiral plasmonic nanostructures have been experimentally realized; these tailored chiroptical properties vastly outperform those of their molecular counterparts. We focus on chiral plasmonic nanostructures created using bottom-up approaches, which not only allow for rational design and fabrication but most intriguingly i…
Circular dichroism is the differential rate of absorption of right- and left-handed circularly polarized light by chiral particles. Optical vortices which convey orbital angular momentum (OAM) possess a chirality associated with the clockwise or anti-clockwise twisting of their wavefront. Here it is highlighted that both oriented and randomly oriented chiral particles absorb photons from twisted beams at different rates depending on whether the vortex twists to the right or the left through a dipole coupling scheme. This is in contrast to previous studies that investigated dipole couplings wit…
Today we are witnessing the electric-dipole revolution in chiral measurements. Here we reflect on its lessons and outcomes, such as the perspective on chiral measurements using the complementary principles of "chiral reagent" and "chiral observer", the hierarchy of scalar, vectorial and tensorial observables, the new properties of the chiro-optical response in the ultrafast and non-linear domains, and the geometrical magnetism associated with the chiral response in photoionization. The electric-dipole revolution is a landmark event. It has opened routes to extremely efficient enantio-discrimin…
Dark-field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction analogous to true chiral scatterers. Circular dichrosim is experimentally observed at the maximum scattering of single oligomers consisting rotationally symmetric arrangements of gold nanorods, with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the geometric projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark-field that is normally incident…
Chiral spectroscopy is a powerful technique that enables to identify the chirality of matter through optical means. So far, experiments to check the chirality of matter or nanostructures have been carried out using free-space propagating light beams. However, for the sake of miniaturization, it would be desirable to perform chiral spectroscopy in photonic integrated platforms, with the additional benefit of massive parallel detection, low-cost production, repeatability, and portability of such a chiroptical device. Here we show that all-dielectric integrated photonic waveguides can support chi…
Structural chirality can induce counter-intuitive optical forces due to inherent symmetry properties. While optical forces on a single chiral particle in the Rayleigh regime have been well studied, optical forces in coupled chiral particles remain less explored. By using full-wave numerical simulations and analytical methods of source representation and coupled mode theory, we investigated the optical forces induced by a plane wave on two chiral particles coupling with each other via the evanescent near fields. We found that the induced electric and magnetic dipoles of the chiral particles hav…
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 interac…
We theoretically investigate the emergence of lateral (recoil) forces acting on generic dipole-type emitters in the vicinity of a reciprocal translation-invariant substrate. Surprisingly, we find that for linearly-polarized electric dipoles the lateral force invariably vanishes, independent of the anisotropy (e.g., tilted optical axes) or chirality of the substrate. We identify a novel opportunity to have a recoil force relying on a superposition of two linearly-polarized and collinear electric and magnetic dipoles. Counterintuitively, it is shown that when such an emitter stands above a uniax…
Topological phases of light exhibit unique properties beyond the realm of conventional photonics. In particular the valley-Hall topological insulator has been realized in a variety of photonic structures because it can be easily induced by breaking certain lattice symmetries. However, the valley-Chern numbers are usually fixed by design and the corresponding topological edge states are forced to propagate in a fixed direction. Here, we propose a mechanism to induce topological transitions via accidental Dirac points in metasurfaces composed of interacting dipole emitters/antennas. For a fixed …
Tunable high-$Q$ Janus-to-chiral bound states in the continuum in bilayer PhCs
School of Physics and Optoelectronics, Xiangtan University（湘潭大学物理与光电工程学院）；School of Physics and Electronics, Hunan University（湖南大学物理学院）；State Key Laboratory of Millimeter Waves, Southeast University（东南大学毫米波国家重点实验室）；Science, Mathematics and Technology (SMT) Cluster, Singapore University of Technology and Design（新加坡科技设计大学科学、数学与技术学部）
We demonstrate that electric-dipole scatterers can mimic chiral light-matter interaction by generating far-field circular polarization upon scattering, even though the optical chirality of the incident field as well as that of the scattered light is zero. The presented effect originates from the fact that electric-dipole scatterers respond selectively only to the incident electric field, which eventually results in depolarization of the transmitted beam and in generation of far-field circular polarization. To experimentally demonstrate this effect we utilize a cylindrical vector beam with spir…
We theoretically introduce a new type of topological dipole solitons propagating in a Floquet topological insulator based on a kagome array of helical waveguides. Such solitons bifurcate from two edge states belonging to different topological gaps and have bright envelopes of different symmetries: fundamental for one component, and dipole for the other. The formation of dipole solitons is enabled by unique spectral features of the kagome array which allow the simultaneous coexistence of two topological edge states from different gaps at the same boundary. Notably, these states have equal and n…
We study the generic interaction of a monochromatic electromagnetic field with bi-isotropic nanoparticles. Such an interaction is described by dipole-coupling terms associated with the breaking of dual, P- and T-symmetries, including the chirality and the nonreciprocal magnetoelectric effect. We calculate absorption rates, radiation forces, and radiation torques for the nanoparticles and introduce novel characteristics of the field quantifying the transfer of energy, momentum, and angular-momentum in these interactions. In particular, we put forward the concept of 'magnetoelectric energy densi…
