Extreme polaritonic interactions in a room-temperature deterministic sub-nanocavity quantum electrodynamic platform

TL;DR

We address the challenge of realizing extreme nano-cQED by engineering a deterministic sub-nanocavity platform (NPcoM) that embeds sub-10 nm Au seeds in a nanoparticle-on-mirror geometry and couples them to monolayer MoS$_2$ excitons. The approach yields a deeply subwavelength mode volume of $V_m\approx 55\ \mathrm{nm^3}$ and strong room-temperature coupling with Rabi frequencies approaching $208$–$210\ \mathrm{meV}$, giving $\Omega/\Gamma \approx 1$. This leads to ultrabright polaritonic PL (up to $\sim 2.5\times10^4$ fold) and robust multibranch Rabi splitting, outperforming conventional NPoM cavities and enabling quantum-optical phenomena such as photon blockade with predicted $g^{(2)}(0) = 0.16$ for a two-exciton NPcoM. Overall, NPcoM sub-nanocavities provide a deterministic, room-temperature nano-cQED testbed with broad implications for single-molecule spectroscopy and quantum nano-optics.

Abstract

Pushing nanoscale optical confinement to its ultimate limits defines the regime of nano-cavity quantum electrodynamics (nano-cQED), where light--matter interactions approach the fundamental quantum limits of individual atoms, e.g., picocavities. However, realizing such extreme confinement in a stable and controllable manner remains a key challenge. Here, we introduce a van der Waals material-based nano-cQED platform by coupling monolayer MoS2 excitons to plasmonic sub-nanocavities formed via assembly of ultrasmall gold clusters (3-5 nm) in the nanogap of a nanoparticle-on-mirror nanocavity. These clusters emulate the field-confining role of atomic protrusions of the picocavities through a resonance-insensitive lightning-rod effect, achieving deep-subwavelength mode volumes. In this nano-cQED testbed, we observe pronounced multi-branch Rabi splittings and ultrastrong lower-branch polaritonic photoluminescence with up to 10^4-fold enhancement. This deterministic architecture provides a controllable pathway to access picocavity-like behavior and opens new opportunities for single-molecule spectroscopy and the exploration of nano-cQED.

Figures (4)

• Figure 1: Schematics (a1, b1, c1), scattering spectra (a2, b2, c2) and field enhancements (a3, b3, c3, c4) of three different configurations: (a1-3) 100 nm nanoparticles-on-mirror (NPoM) nanocavity, (b1-3) 100 nm NPoM with 1 nm diameter semispherical protrusion, i.e., emulating the picocavity. (c1-4) 100 nm NPoM hosting a small gold cluster inside the gap with 5 nm diameter, forming a NPcoM sub-nanocavity. In panels a3, b3, and c3, the blue regions schematically highlight the dimension of the electromagnetic hotspots, indicating a pronounced field confinement within the picocavity and sub-nanocavity cases. The near-uniform field enhancement ($E_{\rm host}$) in a nanocavity (a3) explains the electric field baselines in (b3 and c3). Local-response model (solid lines) and nonlocal model (hydrodynamic theory, lighter lines) are compared for all results. The gold protusions and seeds (i.e., clusters) are assumed on the axis to implement cylindrical coordinates. c4 shows the 3D field enhancement of the NPcoM. Scalebars represent 10 nm (a1), 0.5 nm (b1), and 2.5 nm (c1), respectively.
• Figure 2: (a) 3D schematic of the NPcoM structure, with the inset showing its 2D cross-section. (b) A representative TEM image of densely assembled 5 nm Au nanoseeds, with their size distribution shown in (c). For the full TEM image from which the data in (c) are extracted, see SI Fig. S3. (d) SEM images of NPcoM structures on and off monolayer MoS$_2$, with corresponding schematics shown as insets. From bottom to top, the layers are Si substrate, 80 nm Au film, 5 nm nanoseeds, with or without a monolayer MoS$_2$, and a 100 nm Au nanoparticle. (e, f) Bright- and dark-field optical images of the same NPcoM region. (g) TEM images of the bare NPcoM sub-nanocavity's vertical cross-section confirming the presence of nanoclusters beneath the nanoparticle. The scalebars represent 20 nm. (h, i) Scattering spectra of NPcoM without (h) and with (i) a monolayer MoS$_2$ excitons, respectively. The absorption of the monolayer MoS$_2$ in (i) shows two peaks of A and B excitons.
• Figure 3: (a) Example of scattering and PL from the NPcoM exciton–polariton, showing three distinct branches. The statistical distribution of the enhanced PL peak positions is overlaid. (b) Multibranch anti-crossing dispersion with two Rabi splittings, $\Omega_1 = 208$ meV and $\Omega_2 = 210$ meV, fitted from the polaritonic frequency gathered from different nanoparticles. (c) Comparison of our results with the literature of plasmon-monolayer TMDC hybrid systems. The referred literature is summarized in Table S1 in the SI. (d) Enhanced polaritonic PL from a single NPcoM sub-nanocavity (red), and from an NPoM nanocavity (green). (e1) The histogram of peak locations of the PL from NPcoM-MoS$_2$ hybrid (orange) and the respective bare MoS$_2$ (blue) nearby. See full statistics in SI Fig. S7. The PL from the hybrid is generally redshifted relative to the bare MoS$_2$ nearby (see (e2) for the statistical differences in frequency), manifesting as a mix of lower-branch polaritonic PL. (f) Statistical analysis of PL enhancement across multiple devices illustrates the variability of NPcoM systems and their potential for realizing ultrabright light sources.
• Figure 4: (a) Three-dimensional geometry of the NPcoM structure consisting of seven compact nanoseeds assembled inside a nanocavity formed by the facet. Vacuum electric field distributions are shown for the (b) in-plane and (c) out-of-plane components. The in-plane component can couple with in-plane excitons, such as the bright exciton exA in MoS2 ($\boldsymbol{\mu} = 7.36$ D yang2022strong), while the out-of-plane component couples efficiently with emitters possessing a vertical dipole moment, such as molecules or J-aggregates ($\boldsymbol{\mu} = 33.6$ D liu_strong_2017). Single-exciton coupling strength $g_0=\mathbf{E}_{\rm vac}\cdot\boldsymbol{\mu}$ with MoS2(b) and J-aggregates (c) are shown, respectively. Photon blockade behavior is demonstrated in (d), where two J-aggregate excitons coupled to the NPcoM sub-nanocavity photons exhibit (d1) mode splitting in the normalized population and (d2) anti-bunching behavior in the second-order correlation function $g^{(2)}$.