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arXiv:2605.08890

Geometrical Tuning of Light-Matter Interaction in Atomic Trimer Antennas: A Symmetry-Resolved Modal Analysis

Linsa G. J., Pushpender Singh, Rohit Dhir, M. Ameen Poyli

Published May 9, 2026• 0 citations
arXivHTMLPDF

Abstract

Atomic trimers constitute the smallest geometry in which collective electric and magnetic responses emerge from coupled electric dipoles. We present a theoretical study of collective mode excitation in atomic trimers as the geometry is continuously tuned from linear to equilateral, using the coupled-dipole method with a multipole expansion formulated about the optimal scattering center. By combining eigenmode analysis and symmetry classification, we provide a complete symmetry-resolved map of the six in-plane and three out-of-plane modes, revealing how symmetry reduction across the $D_{\infty h}$, $C_{2v}$, and $D_{3h}$ configurations governs the evolution of eigenmodes and their spectral features, lifting degeneracies, activating dark modes, and enabling full access to the modal spectrum. Based on this modal understanding, we demonstrate that forward-backward scattering can be switched solely by frequency detuning in a nearly linear trimer, without geometric reconfiguration. Furthermore, a linear trimer under s-polarized excitation supports a magnetic mode with a strongly enhanced magnetic field and a large Purcell factor, making it a promising platform for probing magnetic dipole transitions in atoms, with emission preferentially directed into the transverse plane. These results establish atomic trimers as a minimal platform where symmetry-controlled electric-magnetic mode engineering can be fully resolved and exploited for tailoring light-matter interaction at the atomic level.

DOI
10.48550/arxiv.2605.08890

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Atomic trimers constitute the smallest geometry in which collective electric and magnetic responses emerge from coupled electric dipoles. We present a theoretical study of collective mode excitation in atomic trimers as the geometry is continuously tuned from linear to equilateral, using the coupled-dipole method with a multipole expansion formulated about the optimal scattering center. By combining eigenmode analysis and symmetry classification, we provide a complete symmetry-resolved map of the six in-plane and three out-of-plane modes, revealing how symmetry reduction across the D∞hD_{\infty h}D∞h​, C2vC_{2v}C2v​, and D3hD_{3h}D3h​ configurations governs the evolution of eigenmodes and their spectral features, lifting degeneracies, activating dark modes, and enabling full access to the modal spectrum. Based on this modal understanding, we demonstrate that forward-backward scattering can be switched solely by frequency detuning in a nearly linear trimer, without geometric reconfiguration. Furthermore, a linear trimer under s-polarized excitation supports a magnetic mode with a strongly enhanced magnetic field and a large Purcell factor, making it a promising platform for probing magnetic dipole transitions in atoms, with emission preferentially directed into the transverse plane. These results establish atomic trimers as a minimal platform where symmetry-controlled electric-magnetic mode engineering can be fully resolved and exploited for tailoring light-matter interaction at the atomic level.
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.
By analyzing the parameters of electronic transitions, we show how bosonic Sr atoms in planar optical lattices can be engineered to exhibit optical magnetism and other higher-order electromagnetic multipoles that can be harnessed for wavefront control of incident light. Resonant $\ensuremath{\lambda}\ensuremath{\simeq}2.6\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ light for the $^{3}D_{1}\ensuremath{\rightarrow}{}^{3}{P}_{0}$ transition mediates cooperative interactions between the atoms while the atoms are trapped in a deeply subwavelength optical lattice. The atoms then exhibit collective excitation eigenmodes, e.g., with a strong cooperative magnetic response at optical frequencies, despite individual atoms having negligible coupling to the magnetic component of light. We provide a detailed scheme to utilize excitations of such cooperative modes consisting of arrays of electromagnetic multipoles to form an atomic Huygens' surface, with complete $2\ensuremath{\pi}$ phase control of transmitted light and almost no reflection, allowing nearly arbitrary wavefront shaping. In the numerical examples, this is achieved by controlling the atomic level shifts of Sr with off-resonant ${^{3}P}_{J}\ensuremath{\rightarrow}{^{3}D}_{1}$ transitions, which results in a simultaneous excitation of arrays of electric dipoles and electric quadrupoles or magnetic dipoles. We demonstrate the wavefront engineering for a Sr array by realizing the steering of an incident beam and generation of a baby-Skyrmion texture in the transmitted light via a topologically nontrivial transition of a Gaussian beam to a Poincar\'e beam, which contains all possible polarizations in a single cross-section.
Subwavelength atomic arrays offer a powerful platform for engineering cooperative light-matter interactions and enabling quantum metasurfaces. We demonstrate that a two-dimensional array of three-level atoms operating under electromagnetically induced transparency can function as a tunable dual-band atomic mirror, where two independently controllable reflection bands emerge from the collective optical responses mediated by dipole-dipole interactions. These resonances yield dual reflection bands with asymmetric linewidths, whose spectral positions and bandwidths can be tuned through the control-field parameters, dipole orientation, incident geometry, and lattice constant. We further identify the conditions under which additional diffraction orders emerge, which delineate the operational and tunable range of the atomic mirror via its collective-mode structure. This scheme provides a fully tunable dual-band atomic mirror operating across broad frequency and angular ranges, offering a practical and experimentally accessible pathway toward reconfigurable photonic elements in atomic-array platforms at low energy levels.
By utilizing strong optical resonant interactions in arrays of atoms with electric dipole transitions, we show how to synthesize collective optical responses that correspond to those formed by arrays of magnetic dipoles and other multipoles. Optically active magnetism with the strength comparable with that of electric dipole transitions is achieved in collective excitation eigenmodes of the array. By controlling the atomic level shifts, an array of spectrally overlapping, crossed electric and magnetic dipoles can be excited, providing a physical realization of a nearly reflectionless quantum Huygens' surface with the full 2π phase control of the transmitted light that allows for extreme wavefront engineering even at a single photon level. We illustrate this by creating a superposition of two different orbital angular momentum states of light from an ordinary input state that has no orbital angular momentum.
Tremendous advances in the study of magnetic light-matter interactions have recently been achieved using man-made nanostructures that exhibit and exploit an optical magnetic response. However, naturally occurring emitters can also exhibit magnetic resonances in the form of optical-frequency magnetic-dipole transitions. Here we quantify the magnetic nature of light emission using energy- and momentum-resolved spectroscopy, and leverage a pair of spectrally close electric- and magnetic-dipole transitions in trivalent europium to probe vacuum fluctuations in the electric and magnetic fields at the nanometre scale. These results reveal a new tool for nano-optics: an atomic-size quantum emitter that interacts with the magnetic component of light. Light-matter interactions are generally dominated by electric fields and electric-dipole transitions. This study, however, quantifies magnetic contributions to light emission and so exploits the strong natural magnetic-dipole transitions in lanthanide ions to measure optical-frequency magnetic fields.
The Swing-UP of quantum EmitteR population (SUPER) scheme has recently been proposed as a deterministic method for the preparation of collective radiative states in two strongly dipole-coupled quantum emitters (Phys. Rev. Res. \textbf{8}, 013179 (2026)). Here, we apply this approach to an equilateral subwavelength triangular trimer of dipole-coupled two-level quantum emitters (QEs), loosely inspired by biological light-harvesting ring geometries, and demonstrate that tailored, time-overlapping, red-detuned ultrashort SUPER pulses can selectively prepare collective states in a C3C_3C3​-symmetric system, including energetically degenerate eigenstates that are resolved through site-dependent optical phases. We find that both the state selectivity and the preparation efficiency depend strongly on the inter-emitter spacing. In particular, at deep-subwavelength separations, the symmetric collective state can be deterministically prepared with near-unity efficiency (approximately 94%94\%94%), whereas the inversion efficiency and state selectivity are significantly lower at larger inter-emitter separations. Furthermore, this state preparation technique inherits a certain degree of robustness against reasonable static position imperfections and on-site frequency inhomogeneities of the individual QEs. Our results demonstrate that deep-subwavelength triangular trimers and, more broadly, highly compact ring geometries are excellent candidates for the deterministic preparation of collective radiative states via SUPER excitation. These predictions could be realized with solid-state emitters and molecules. Our findings offer a route toward the direct probing of the `pure' electromagnetic layer of interaction in biological and bio-inspired synthetic nanophotonic ring configurations, with possible relevance in photonics, quantum information processing, and metrology.