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arXiv:1812.01827 · Physical Review A

Coupling of atomic quadrupole transitions with resonant surface plasmons

Eng Aik Chan, Syed Abdullah Aljunid, Giorgio Adamo, Nikolay I. Zheludev, M. Ducloy, David Wilkowski

Published Jun 3, 2019• 15 citations
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Abstract

We report on the coupling of an electric quadrupole transition in atomic vapor with plasmonic excitation in a nanostructured metallic metamaterial. The quadrupole transition at 685 nm in the gas of cesium atoms is optically pumped, while the induced ground-state population depletion is probed with light tuned on the strong electric dipole transition at 852 nm. We use selective reflection to resolve the Doppler-free hyperfine structure of cesium atoms. We observe a strong modification of the reflection spectra at the presence of metamaterial and discuss the role of the spatial variation of the surface plasmon polariton on the quadrupole coupling.

DOI
10.1103/physreva.99.063801

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We report on the coupling of an electric quadrupole transition in atomic vapor with plasmonic excitation in a nanostructured metallic metamaterial. The quadrupole transition at 685 nm in the gas of cesium atoms is optically pumped, while the induced ground-state population depletion is probed with light tuned on the strong electric dipole transition at 852 nm. We use selective reflection to resolve the Doppler-free hyperfine structure of cesium atoms. We observe a strong modification of the reflection spectra at the presence of metamaterial and discuss the role of the spatial variation of the surface plasmon polariton on the quadrupole coupling.
We report on reflection spectra of cesium atoms in close vicinity of a nanostructured metallic meta-surface. We show that the hyperfine sub-Doppler spectrum of the 6(2)S1/2-6(2)P3/2 resonance transition at 852 nm is strongly affected by the coupling to the plasmonic resonance of the nanostructure. Fine tuning of dispersion and positions of the atomic lines in the near-field of plasmonic metamaterials could have uses and implications for atom-based metrology, sensing, and the development of atom-on-a-chip devices.
Light–atom interaction can be engineered by interfacing atoms with various specially designed media, and optical fibers are convenient platforms for realization of compact interfaces. Here, we show that an optical fiber sensor bearing a plasmonic metasurface at its tip can be used to detect modifications of the Doppler-free hyperfine atomic spectra induced by coupling between atomic and plasmonic excitations. We observed the inversion of the phase modulation reflectivity spectra of Cesium vapor in the presence of the metamaterial. This work paves the way for future compact hybrid atomic devices with a cleaved tip as a substrate platform to host various two-dimensional materials.
Electric-dipole-forbidden transitions play an important role in quantum sensing, quantum information, and fundamental tests in physics. As such, the development of novel and sensitive spectroscopic methods is of major interest. Here, we present a Doppler-free selective reflection experiment on the 62S1/2–52D5/2 electric-quadrupole transition of cesium vapor at the vicinity of a sapphire window. The spectrum is obtained by precision experiments overcoming limitations due to the small-signal amplitude of forbidden transitions. The observed sub-Doppler lines allow us to measure the collisional broadening of the electric-quadrupole line. The interaction of cesium atoms with the sapphire surface of the cell is also evidenced, but, due to its weak contribution, a quantitative analysis remains challenging. Our experiment paves the way for further studies of the Casimir–Polder interaction between exotic excited-state atoms and dielectric surfaces.
We investigate the mediation of symmetry-forbidden atomic transitions using plasmonic nanostructures. We show that the excitation of the electric dipole-forbidden, quadrupole-allowed $6{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{-}5{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{5/2}$ transition in cesium may be enhanced by more than 6 orders of magnitude in the intense, inhomogeneous near field of a plasmonic nanoantenna. Using optical reciprocity, the enhancement can be understood to apply to spontaneous emission as well, allowing the fast and efficient optical detection of excited atoms.
Quantum coherence controlled by the resonant condition of electromagnetically induced transparency (EIT) has attracted considerable attention for the excitation of surface plasmon polaritons (SPPs) in various metal–dielectric hybrid structures. However, for all coherent fields operating resonantly in the system, the exact extent to which perfect EIT is effective in establishing coupler-free SPP excitation has not yet been well examined. Here, we investigate the excitation of SPPs in an air–metal–atom composite structure incorporating a three-level Λ-type atomic model that operates in the strong ground-state coherence-dephasing regime. Such decoherence limits the establishment of true EIT under the condition of two-photon Raman resonance. It is found that, although the characteristic narrow resonant field enhancement associated with EIT mediated by the resonant fields disappears, significant SPP-assisted field enhancement survives over a broad off-resonant spectral region. This behavior originates from the preservation of the momentum-matching condition together with the fulfillment of the dielectric requirements for surface plasmon resonance (SPR) in the normal-dispersion regime. If the coupling field is tuned to the SPR condition, the detuned two-photon Raman resonance plays a vital role in structuring a doublet in the spectral response of an initially single-peaked broadband SPP excitation. This observation demonstrates that, although true EIT does not occur in the system, an EIT-like plasmonic spectral signature appears through the off-resonant SPP excitation mechanism. All-optical switching can be realized by turning the detuned condition of the coupling field on and off, thereby exhibiting this coherence feature. Furthermore, we show that the present model remains robust in the Doppler-broadened regime, providing a realistic route toward coherence-controlled coupler-free SPP excitation beyond the conventional ideal-EIT framework.