Cavity-Free Ultrastrong Light-Matter Coupling

Abstract

Strong coupling between light and matter can occur when the interaction strength between a confined electromagnetic field and a molecular resonance exceeds the losses to the environment, leading to the formation of hybrid light-matter states known as polaritons. Ultrastrong coupling occurs when the coupling strength becomes comparable to the transition energy of the system. It is widely assumed that the confined electromagnetic fields necessary for strong coupling to organic molecules can only be achieved with external structures such as Fabry-Pérot resonators, plasmonic nanostructures, or dielectric resonators. Here we show experimentally that such structures are unnecessary and that a simple dielectric film of dye molecules supports sufficiently modified vacuum electromagnetic fields to enable room-temperature ultrastrong light-matter coupling. Our results may be of use in the design of experiments to probe polaritonic chemistry and suggest that polaritonic states are perhaps easier to realize than previously thought.

Figure 1

(a) Sample design showing a thin film of SPI/MC on a silicon substrate. (b) Schematic of ellipsometer arrangement for the optical measurements. (c) Real (solid lines) and imaginary (dashed lines) of the permittivities of SPI (black) and MC (blue), as derived from ellipsometry for a film of thickness 109 nm.

Figure 2

Experimental (a,b) and calculated (Fresnel approach) (c,d) dispersion plots constructed using the ellipsometric parameter Ψ for (a,c) SPI and (b,d) MC films over a range of thicknesses L at fixed angle θ = 65°. The lines in (b) indicate the positions of the uncoupled MC resonance (dashed white line), TE leaky modes (dashed black lines) and upper and lower polariton bands (solid black lines) calculated using the coupled oscillator model (Ω = 550 meV).

Figure 3

(a) Maximum electric field enhancement as a function of incident angle associated with the second-order Fabry–Pérot cavity resonance in a Si substrate/Ag(40 nm)/SPI(294 nm)/Ag(40 nm)/air structure (blue line) and the second-order TE leaky mode in a Si substrate/SPI(294 nm)/air structure (black line). (b) Electric field profile associated with the second-order TE leaky mode in a Si substrate/SPI (294 nm)/air structure for light incident at θ = 65°. The different colored regions indicate each layer of the structure (Si, purple; SPI, green; air, blue).