Fig. 2. Manipulation of the degree of superposition by Stark effect.

a, b Excitation spectra of a pair of molecules with nearly parallel dipoles in the J-configuration and V ∼ γ₀, recorded with an excitation intensity of 0.25 W cm⁻². The applied electric field shifts the molecular detuning from 10γ₀ (a) to 4γ₀ (b). Lorentzian fits (red curves) of the subradiant (higher-energy) ZPL gives a FWHM linewidth of 22 MHz in (a) and 13 MHz in (b). The inset in b displays the spectral trails of this pair as the voltage is swept from 0 (top) to 150 V (bottom). c, d Zoomed-in ZPLs of the subradiant (c) and the superradiant (d) states of another molecular pair with nearly parallel dipoles in the H-dipole configuration and V ~ 15γ₀. The powers of the Gaussian illumination are respectively 0.1 nW and 0.3 nW. The red curves are Lorentzian fits with FWHM linewidths (13.9 ± 0.6) MHz and (33.0 ± 1.6) MHz, respectively. e Saturation plots (blue disks) and fits (red curves) of these ZPL linewidths. Radiative rates γ₋/2π = 15.3 ± 0.5 MHz and γ₊/2π = (33.0 ± 0.5) MHz are derived from the fitting saturation intensities. The horizontal dashed line at 23 MHz marks the average linewidth of single DBATT molecules at low excitation intensities. f, g Experimental (f) and simulated (g) Stark-tuned spectral trails of a coupled pair of molecules under an excitation intensity of 200 W cm⁻². The experimental spectra have been recentered on the two-photon transition, while the raw spectral trails are presented in Supplementary Fig. 7. The applied voltage is limited to the range −150 to 150 V by electrode breakdown. As the system is tuned towards the anti-crossing point, the two-photon transition gains weight, the superradiant ZPL widens and the subradiant ZPL narrows with vanishing fluorescence amplitude. The simulations are performed for a H-dipole configuration leading to maximal superposition at the anti-crossing point.