Chiroptical responses in atomic systems are usually weak, as they arise from the interference between electric- and much weaker magnetic-dipole transitions. We show that atoms arranged in chiral geometries can instead exhibit a strong collective chiroptical response mediated entirely by electric-dipole interactions. Using a coupled-dipole framework, we identify a regime of pronounced chiroptical response emerging at subwavelength interatomic separations, which can be tuned by the probe frequency. This enhancement is directly linked to the formation of subradiant collective modes. Our results establish a fundamental connection between geometric chirality and collective light-matter interactions, opening new pathways for engineering and exploiting chiral optical responses in atomic systems.
Journal reference
Phys. Rev. A 114, L021101. Published 19 August, 2026
Chiroptical responses in atomic systems are usually weak, as they arise from the interference between electric- and much weaker magnetic-dipole transitions. We show that atoms arranged in chiral geometries can instead exhibit a strong collective chiroptical response mediated entirely by electric-dipole interactions. Using a coupled-dipole framework, we identify a regime of pronounced chiroptical response emerging at subwavelength interatomic separations, which can be tuned by the probe frequency. This enhancement is directly linked to the formation of subradiant collective modes. Our results establish a fundamental connection between geometric chirality and collective light-matter interactions, opening new pathways for engineering and exploiting chiral optical responses in atomic systems.
Chirality, or handedness, is a geometrical property denoting a lack of mirror symmetry. Chirality is ubiquitous in nature and is associated with the nonreciprocal interactions observed in complex systems ranging from biomolecules to topological materials. Here, we demonstrate that chiral arrangements of dipole-coupled atoms or molecules can facilitate the helicity-dependent superradiant emission of light. We show that the collective modes of these systems experience an emergent spin-orbit coupling that leads to chirality-dependent photon transport and nontrivial topological properties. These phenomena are fully described within the electric dipole approximation, resulting in very strong optical responses. Our results demonstrate an intimate connection between chirality, superradiance, and photon helicity and provide a comprehensive framework for studying electron transport dynamics in chiral molecules using cold atom quantum simulators. Published by the American Physical Society 2024
Collective coupling between dipoles can dramatically modify the optical response of a medium. Such effects depend strongly on the geometry of the medium and the polarization of the light. Using a classical coupled dipole model, here we investigate the simplest case of one-dimensional arrays of interacting atomic dipoles driven by a weak laser field. Changing the polarization and direction of the driving field allows us to separately address superradiant, subradiant, redshifted, and blueshifted eigenmodes, as well as observe strong Fano-like interferences between different modes. The cooperative eigenvectors can be characterized by the phase difference between nearest-neighbor dipoles, ranging from all oscillating in phase to all oscillating out of phase with their nearest neighbors. Investigating the eigenvalue behavior as a function of atom number and lattice spacing, we find that certain eigenmodes of an infinite atomic chain have the same decay rate as a single atom between two mirrors. The effects we observe provide a framework for collective control of the optical response of a medium, giving insight into the behavior of more complicated geometries, as well as providing further evidence for the dipolar analog of cavity QED.
We show that atoms in an optical lattice can respond cooperatively to resonant incident light and that such a response can be employed for precise control and manipulation of light on a subwavelength scale. Specific collective excitation modes of the system that result from strong light-mediated dipole-dipole interactions can be addressed by tailoring the spatial phase profile of the incident light. We demonstrate how the collective response can be used to produce optical excitations at well-isolated sites on the lattice.
An ensemble of atoms coupled to the radiation field exhibits collective behaviours when the interatomic separation is comparable to the wavelength of photons emitted by the atoms. The collective coupling of the atoms with the electromagnetic field gives rise to coherent exchange interactions between the atoms. In addition, the spontaneous decay of photons from the ensemble becomes a collective process that can happen at a much faster (superradiant) or slower (subradiant) rate than that of a single atom. These processes affect the way that fluorescence light is scattered from the system when it is driven by a laser. However, little is known about the response of a dense atomic gas beyond the linear regime of very weak driving, due to the involved complexity of the problem. The first part of this thesis investigates how the presence of strong laser driving affects properties of the far-field fluorescence light emitted by the ensemble, such as the power spectrum and second order coherence. Depending on the strength of the driving laser and the particular arrangement of the atoms, the laser can modify considerably how the collective behaviours of the atomic ensemble are manifested in the fluorescence. The relative strengths of the coherent interaction and collective decay process are intrinsically linked to the way that the atoms couple to the radiation field; one cannot be changed without modifying the other. Altering the environment by, for example, placing the atoms near a surface or a waveguide, can change this relation significantly from that for atoms in free space. Therefore, modified environments represent a platform for the generation of otherwise inaccessible regimes of collective behaviour, with potential applications in quantum state engineering. The second part of this thesis provides a numerical investigation into how the collective properties of a dense atomic system can be manipulated by placing the atoms close to planar surfaces or an optical nanofiber. These structures modify the light-matter coupling by altering the electric field experienced by the atoms. It is shown that surfaces of metallic and dielectric materials can be used to either enhance or suppress the collective behaviours, depending on the particular system arrangement. In particular, the coupling of atoms with bound surface modes, such as the surface plasmon polaritons that are formed at an air-metal interface, is demonstrated to have a strong effect on the interactions. An optical nanofiber supports a set of fundamental guided modes, that propagate along the fiber. If the atoms are suitably polarised, then they will emit primarily into the guided modes travelling in one direction. By calculating and analysing the collective decay states of a chain of atoms placed near a nanofiber, it is demonstrated that the atom-fiber coupling leads to the formation of a superradiant mode that is preserved at large atomic separations. The presence of chiral emission is shown to introduce phase gradients to the collective modes.
Abstract Natural materials typically interact weakly with the magnetic component of light which greatly limits their applications. This has led to the development of artificial metamaterials and metasurfaces. However, natural atoms, where only electric dipole transitions are relevant at optical frequencies, can cooperatively respond to light to form collective excitations with strong magnetic, as well as electric, interactions together with corresponding electric and magnetic mirror reflection properties. By combining the electric and magnetic collective degrees of freedom, we show that ultrathin planar arrays of atoms can be utilized as atomic lenses to focus light to subwavelength spots at the diffraction limit, to steer light at different angles allowing for optical sorting, and as converters between different angular momentum states. The method is based on coherently superposing induced electric and magnetic dipoles to engineer a quantum nanophotonic Huygens’ surface of atoms, giving full 2π phase control over the transmission, with close to zero reflection.
When quantum emitters couple indistinguishably to light, they can synchronize into a collective light matter system with radiative properties profoundly different from those of independent particles. To date, the resulting collective effects have largely been confined to point like or homogeneous ensembles. Here, we open access to a qualitatively new collective regime by realizing geometrically ordered, spatially extended atom arrays with subwavelength spacing. This establishes a fundamentally new platform in which collective emission is no longer confined to a single Dicke mode but instead emerges from an ordered network of photon mediated interactions. We find that 2D atom arrays undergo strong super and subradiant emission. Despite subwavelength spacing, we achieve site resolved imaging and directly observe the buildup of spatial correlations, demonstrating the transformation of cooperative decay into a strongly correlated many-body process. We observe extensive scaling of superradiance, uncover superradiant revivals, and reveal the ferromagnetic nature of superradiance and the antiferromagnetic nature of subradiance. Our results realize a novel programmable platform for exploring and utilizing dissipative many-body quantum physics, opening new possibilities for photon capture, storage, and atom photon entanglement.