Engineering strong coupling with molecular coatings in optical nanocavities

Abstract

Quantum emitters near the surface of silver nanoparticles undergo Rabi oscillations in electronic population dynamics due to strong coupling with near-field multipole modes that are not radiative. Low-frequency nanoparticle dipole modes are radiative but do not couple strong enough to quantum emitters. These features limit the observation of strong coupling. Using macroscopic quantum electrodynamics theory within a Lorentzian pseudo-mode approximation for the non-Markovian interaction kernel, we demonstrate that by coating spherical silver nanoparticles with a thin molecular J-aggregate layer, the resulting core-shell plexciton resonance restructures the local electromagnetic vacuum at dipole-mode frequencies to enable Rabi oscillations for quantum emitters that otherwise would only undergo exponential population decay. Specifically, we show for quantum dot emitters in the near field of silver nanospheres of 20 nm radius, that weak-to-strong coupling crossovers can be induced using 2 nm J-aggregate shells. Our work demonstrates the potential of molecular aggregates to enable deep sub-wavelength structuring of the vacuum field for the observation of coherent quantum dynamics in optical nanocavities.

Figures (6)

• Figure 1: Schematic of the core--shell plexcitonic nanocavity. A silver nanosphere of radius $a=20$ nm is coated with a J-aggregate shell of thickness $h$ and suspended in a homogeneous medium ($n_b=1.3$, $\epsilon_b=1.69$). A $z$-oriented quantum dot ($d_{eg}=24$ D) is placed on the symmetry axis at a fixed metal-to-emitter gap of $d=3$ nm. The coated configuration features a shell thickness of $h=2$ nm ($\omega_\mathrm{ex}=\omega_\mathrm{sp}$, $f=0.3$, $\gamma_\mathrm{ex}=50$ meV).
• Figure 2: Material permittivities. (a) Real (solid) and imaginary (dashed) parts of the Drude permittivity $\epsilon_m(\omega)$ of the silver core. The horizontal line marks the quasistatic Fröhlich condition $\mathrm{Re}[\epsilon_m] = -2\epsilon_b$. The vertical dash-dotted line indicates the retarded dipole resonance $\omega_\mathrm{sp}\approx 3.07$ eV for the $a=20$ nm sphere, slightly red-shifted from the quasistatic intersection due to electrodynamic retardation. (b) Real and (c) imaginary parts of the J-aggregate shell permittivity $\epsilon_\mathrm{sh}(\omega)$ for varying nominal oscillator strengths $f$. In both (b) and (c), the exciton is tuned to the bare plasmon resonance ($\omega_\mathrm{ex} = \omega_\mathrm{sp}$) with linewidth $\gamma_\mathrm{ex} = 50$ meV, and the vertical dotted line marks $\omega_\mathrm{ex}$.
• Figure 3: Extinction and scattering cross sections of the Ag/J-aggregate core--shell nanoparticle (core radius $a=20$ nm, shell thickness $h=2$ nm). Solid curves represent our Lorenz--Mie calculations using the Drude core and Lorentz shell permittivities with the oscillator strength rescaling described in the text. Symbols denote results from Ref. Antosiewicz2014. The three peaks correspond, in order of increasing frequency, to the lower polariton, the geometric shell resonance, and the upper polariton.
• Figure 4: Kernel spectrum $\mathcal{K}(\omega)$ on a logarithmic scale for a $z$-oriented quantum dot ($d_{eg}=24$ D) placed at a fixed metal-to-emitter gap of $d=3$ nm. (a) Spectrum for the bare Ag nanosphere of radius $a=20$ nm. (b) Spectrum for the Ag/J-aggregate core--shell plexciton with the same core radius and a shell thickness of $h=2$ nm. Thick solid curves (gray for the bare sphere, blue for the plexciton) represent the Mie theory calculation. Dashed red lines denote the total multi-Lorentzian fit defined in Eq. (\ref{['eq:kernel lorentzian']}). Solid orange lines show the individual Lorentzian components comprising the pseudo-mode expansion. A vertical dashed line marks the frequency of the geometric mode (Geo) at 3.14 eV.
• Figure 5: Broadband emitter dynamics and coherence spectra as a function of transition frequency $\omega_e$ at a fixed gap of 3 nm. (a) Excited-state population dynamics $|C_{e0}(t)|^2$ for the bare silver nanosphere at different dipole frequencies $\omega_e$. (b) Normalized excited-state coherence spectra for the bare sphere, exhibiting a single avoided crossing near the Near-UV multipole at 3.65 eV. (c) Population dynamics $|C_{e0}(t)|^2$ for the coated core--shell system at different dipole frequencies $\omega_e$. (d) Coherence spectra for the core--shell system. The molecular coating induces two distinct regions of coherent exchange: a blue-shifted multipole near 3.75 eV, and a merged multi-branch avoided crossing in the visible spectrum. This merged feature demonstrates that the geometric mode (3.14 eV) and the lower polariton (2.98 eV) act collectively as a dense, overlapping multi-mode continuum rather than independent modes.