Evanescent-Vacuum-Enhanced Photon-Exciton Coupling and Fluorescence Collection

An evanescent optical mode existing in various nanophotonic structures always acts as a cavity mode rather than an electromagnetic vacuum in the study of cavity quantum electrodynamics (CQED). Here we show that taking the evanescent mode as an electromagnetic vacuum in which the nanocavity is located is possible through the optical mode design. The proposed evanescent vacuum enables us to enhance both the reversible photon-exciton interaction and fluorescence collection. By embedding the custom-designed plasmon nanocavity into the evanescent vacuum provided by a metallic or dielectric nanowire, the photon-exciton coupling coefficient can achieve 4.2 times that in vacuum due to the exponential decay of the evanescent wave, and spontaneously emitted photons with Rabi splitting can be guided by an evanescent wave with a collection efficiency of 47% at most. Electromagnetic vacuum engineering at subwavelength scale holds promise for controlling the light-matter interaction in quantum optics, CQED, and on-chip quantum information.

Figure 1

Nano-CQED system embedded in EV. (a) Schematic diagram of AgNR nanocavity near the nanowire. (b) Evanescent wave of Ag nanowire. At $\lambda =780\mathrm{nm}$, the propagation constant $k_z/k_0=1.747$, which is consistent with the wavelength of evanescent wave of 447 nm. (c),(d) Electric field distributions of single excitation of the same size AgNR with and without a nanowire. With a Ag nanowire, the resonant length of the AgNR with $D_{\mathrm{rod}}=20\mathrm{nm}$ is $a_{\mathrm{rod}}=38.6\mathrm{nm}$. While without a nanowire, the resonant wavelength is 670.7 nm.

Figure 2

Mechanism of enhancing the coupling coefficient via EV. (a),(b) $g_0$ along the central axis of AgNR for a Ag nanowire and dielectric nanowire systems. (c) Schematic diagram of the AgNRs with variable size in the evanescent wave.

Figure 3

Reversible interaction in EV. (a) Coupling coefficients $g_0^{\mathrm{mid}}$ and decay rates $\gamma$ of a single emitter at the center of the nanoscale gap, and decay rates $\kappa$ of a AgNR nanocavity in the Ag nanowire system. Insets in (a) show the expectation values of ${\rho }_{nn}$ and ${\rho }_{ee}$ for $\mu =0.15\mathrm{enm}$ and 0.5 enm. (b) Fluorescence spectra for different dipole moments as a function of the detuning ${\mathrm{\Delta }}_p=\omega -{\omega }_e$.