Fig. 3. Evidence of long-range dipole-dipole interactions across metamaterials.

(A) The sample types used to isolate RDDI in various material systems (donors, Alq3, shown green). (B to D) The transmitted continuous-wave excitation PL spectra (CW PL) is shown for the donor and acceptor separated by dielectric, metal, and metamaterial. The spectra are in units of spectrometer charge-coupled-device (CCD) counts. We note that energy transfer is visible in all three material systems; that is, the donor-excited state is causing the acceptor to be excited and subsequently relax and emit a photon. This is concluded by noting an increased intensity of acceptor emission and a quenched donor emission when the emitters are placed in the hybrid geometry (black curve) relative to the donor-only (blue curve) and acceptor-only (red curve) control systems (see the Supplementary Materials for quantitative differential spectra). (E to G) The time-resolved donor fluorescence for donor-only (blue) and hybrid (black) samples are shown for the three material systems. For the donors/acceptors separated by 100 nm of SiO₂ or Ag (E and F), the hybrid decay traces reveal no additional lifetime reduction compared to the donor-only case, indicating no long-range RDDI. When the donor and acceptors are separated by a 100-nm Ag/SiO₂ multilayer metamaterial (G), we observe a marked excited-state lifetime reduction when the acceptor molecules are present, providing evidence of long-range super-Coulombic RDDI. The percentage change in the signal is plotted in the inset. It is seen that the signal is many SDs away from the uncertainty level and the background throughout the time trace. We emphasize that the net time integrated difference in counts is far greater, providing conclusive evidence of the emergence of super-Coulombic interactions.