Single-Molecule Vacuum Rabi Splitting: Four-Wave Mixing and Optical Switching at the Single-Photon Level

A single quantum emitter can possess a very strong intrinsic nonlinearity, but its overall promise for nonlinear effects is hampered by the challenge of efficient coupling to incident photons. Common nonlinear optical materials, on the other hand, are easy to couple to but are bulky, imposing a severe limitation on the miniaturization of photonic systems. In this Letter, we show that a single organic molecule acts as an extremely efficient nonlinear optical element in the strong coupling regime of cavity quantum electrodynamics. We report on single-photon sensitivity in nonlinear signal generation and all-optical switching. Our work promotes the use of molecules for applications such as integrated photonic circuits operating at very low powers.

Figure 1

Schematics of the experimental setup. Details can be found in Refs. [17, 20] and the SM [21]. An external Fabry-Perot cavity (FC) serves as a narrow-band filter for the molecular emission. BS: beam splitter; Asph: aspheric lens; $\mu \mathrm{M}$: micromirror; PM: planar mirror; DBT/AC: dibenzoterrylene-doped anthracene crystal; QWP: quarter-wave plate; HWP: half-wave plate; POL: polarization filter; FM: flip mirror; APD: avalanche photodiode. Left inset: An enlargement of the microcavity. Right inset: Jablonski diagram for DBT. The 00ZPL takes place at a wavelength of $\lambda \sim 785\mathrm{nm}$. $\gamma$ stands for the total decay rate of the excited state. The triplet state is denoted by $|t⟩$.

Figure 2

Transmission spectrum of the bare cavity (a) and the coupled molecule-cavity system (c). Ring-down temporal signal of the bare cavity (b) and the coupled molecule-cavity system (d). See the SM [21] for information on the influence of the detector response time [shown by the gray area in (b)] and on the fits to the experimental data (solid orange curves). (e) Transmission through the cavity on resonance with the molecule and the laser as a function of the excitation power. Green and blue present the same data for different horizontal axis scalings. The vertical dotted line marks $S=1$. (f) Intensity autocorrelation of the light transmitted through the cavity on resonance with the molecule and the laser. The maximum value of 250 is limited by residual background light, which was accounted for by the theoretical fit (orange curve). The side peak expected at a delay of about 0.6 ns caused by the Rabi oscillation is washed out since the cavity frequency was not stabilized in this measurement to avoid residual background from the lock laser and due to the contribution of the detector response function (see Fig. S2 in the SM [21]).

Figure 3

Transmission spectrum of the coupled system under excitation at two frequencies separated by 300 MHz at different powers (see legend). The spectra reveal the generation of higher harmonics. The theory curves reproduce the features. The vertical lines point to the regular frequency spacing of the observed signals.

Figure 4

(a) Transmission of the probe beam as a pump beam excites the molecule at a detuning of 300 MHz. The symbols and the solid curve present the experimental and theoretical data, respectively. Measurements close to full transmission involve smaller spectral features and thus lead to larger error bars. (b) Calculated transmission of a probe beam as a function of the pump frequency detuning from the cavity resonance and the pump power. The horizontal dashed line indicates the conditions for measurements presented in (a).