Reversible Switching of Ultrastrong Light-Molecule Coupling

We demonstrate that photochromic molecules enable switching from the weak- to ultrastrong-coupling regime reversibly, by using all-optical control. This switch is achieved by photochemically inducing conformational changes in the molecule. Remarkably, a Rabi splitting of 700 meV is measured at room temperature, corresponding to 32% of the molecular transition energy. A similar coupling strength is demonstrated in a plasmonic structure. Such systems present a unique combination of coupling strength and functional capacities.

Figure 1

(a) Absorption spectra for a neat film in the SPI (dashed line) and MC (solid line) forms. (b) Cavity normal-incidence transmission with the molecules in the SPI (dashed line) and MC (solid line) forms. The dotted lines in (a) and (b) were taken at a few intermediate states. (c) All-optical reversible switching of SC, showing the evolution of the transmission spectrum during conversion. The time scale for photoconversion is $\sim 10\min $ at $\sim 1\mathrm{mW}/{\mathrm{cm}}^2$, compared to 1 day for thermal backconversion at room temperature.

Figure 2

Cavity dispersion, measured by angle-dependent transmission (TE polarization, 0–60°) in the SPI (a) and MC (b) forms, showing the splitting into the upper and lower polariton branches. The solid line marks the molecular transition, and the dashed line is the SPI-state dispersion of (a).

Figure 3

Numerical simulations for an SPI (a) and MC (b),(c) cavity. (a) and (b) show the field distribution, with the structure being illuminated from the top. The white lines represent the edges of the Ag layers. The graphs show the transmission intensity (solid line) and total absorption (dash-dotted line). (c) Dissipation rate ${\mathit{\Pi }}_D$ calculated for the MC (strongly coupled) cavity. The graphs show the $z$-direction integrated dissipation rate for the whole structure (dashed line) and the molecule layer alone (solid line). (d) Experimentally measured spectra under SC. The transmission is multiplied by 5 for ease of comparison.

Figure 4

Dispersion of a hole-array–photochrome system in noncoupled SPI form (a) and MC form (b), where strong coupling is observed as the splitting into two branches. The dispersion was obtained by measuring transmission spectra for a series of hole arrays of different periodicities ($P=180,190,\dots ,340\mathrm{nm}$), milled on a 200 nm thick Ag film, with a hole diameter of $P/2$. The vertical dashed line marks the transition energy of the molecule at the MC form, while the dashed line is a parabolic fit to the bare plasmon mode of (a).