Controlling the strong light-matter coupling in metal-dielectric optical resonators using spin-crossover molecules

. We report the observation of (ultra)strong light-matter coupling, in the UV spectral region, between optical modes of a metal/dielectric bilayer nanocavity and the electronic excitations of spin-crossover (SCO) molecules. By thermally switching the SCO molecules between their low-spin and high-spin states, we demonstrate the possibility of fine-tuning the light-molecule hybridization strength, allowing a reversible switching between strong-(with Rabi splitting values of up to 550 meV) and weak-coupling regimes within a single photonic resonator. As a result, we show that spin-crossover molecular compounds constitute a novel, promising class of active nanomaterials in the context of tuneable and reconfigurable polaritonic devices.


Introduction
The field of "polaritonic chemistry" has seen a spectacular progress in the past decade and represents today a powerful strategy for controlling the physico-chemical properties of molecules [1].In particular, within this field, the use of switchable, stimuli-responsive molecules is of great interest [2], as it opens interesting perspectives for the active control of nanophotonic devices (e.g., by modulating the light-molecule hybridization strength) and, on the other way around, to explore the effects of the strong coupling on the molecular bistability.
From this point of view, spin-crossover (SCO) molecules represent a promising class of switchable molecular materials.Indeed, these transition metal ion complexes can reversibly switch between their so-called low-spin (LS) and high-spin (HS) electronic configurations (see Fig. 1a) through the application of a variety of external perturbations, including a temperature change, the application of pressure, of an intense magnetic field, or by light irradiation [3].The switching of the molecular spin state is known to involve a drastic alteration of the optical properties of the material (absorption coefficient and refractive index), which has been effectively harnessed to develop spectrally tuneable resonant cavities [4] or plasmonic nanostructures [5].Here, we show that SCO nanomaterials can be used as an active medium, that can be strongly coupled to optical modes of purposefully-designed nanocavities to form switchable hybrid light-matter states [6].

Results
The fabricated optical resonator, operating in the total internal reflection (TIR) geometry, consists of a bilayer structure composed of a thin (16 nm) aluminum layer coated with a 138-nm-thick film of the SCO complex [Fe(HB(1,2,4-triazol-1-yl)3)2] 1 (see Fig. 1b).In this bilayer structure, optical resonances can be excited using the Kretschmann method, employing a fused-silica prism as a coupling medium.Interestingly, the presence of the 138-nm-thick dielectric SCO layer affords for the resonant excitation of both TM (transverse magnetic; p) and TE (transverse electric; s) optical modes.Films of 1 undergo a transition between the LS and HS states for temperatures spanning over a range of ~12 °C , 05001 (2023) around TSCO = 64 °C (see Fig. 1d) [7].As shown in Fig. 1c, complex 1 exhibits three intense, charge-transfer absorption bands in the LS state within the ultraviolet (UV) spectral range (at 317, 305 and 272 nm).Upon heating the molecular film above the spin-transition temperature, the UV absorption bands are completely bleached, and the film becomes transparent within the 250-400 nm spectral region.The fabricated SCO-based optical resonator was investigated by spectroscopic reflectometry measurements as a function of incident angle, temperature, and polarization of the incoming light.Fig. 2a displays the typical angular dispersion of the resonator obtained by acquiring angle-dependent RTM/RTE spectra at two different temperatures, T = 60 °C (LS state) and T = 80 °C (HS state).In the LS state, we clearly observe that the dispersion curve of the TM optical mode (dark-blue band) is split into two main branches at wavelengths corresponding to the absorption bands of the SCO complex.Such splitting features in the dispersion are the typical signature of a light-matter hybridization between the molecules and the resonant optical modes.Importantly, when heating the sample above the spintransition temperature, as the molecules are converted into the optically inactive HS state, no splitting is observed, as expected for a non-absorbing dielectric layer.The effect of the spin transition is better shown in Fig. 2b, where the temperature evolution of the TM optical mode (recorded at a fixed angle of incidence, θair = 74°), clearly shows a crossover between split and unsplit regimes at TSCO = 64 °C.A detailed analysis of the dispersion (energy vs. in-plane wave vector) curves reveals Rabi splitting energies of up to 550 meV at room temperature (in the full LS state).Interestingly, this coupling energy value corresponds to ca. 14 % of the molecular excitation energy, which indicates an ultrastrong-coupling regime.Fig. 3 displays the temperature evolution of the measured Rabi splitting energy, showing the possibility of fine-tuning the lightmolecule coupling strength using the SCO phenomenon.Besides, given the strong resilience and the exceptionally high switching endurance (>10 7 thermal switching events [8]) of films of 1, the conversion between the (ultra)strong-and weak-coupling regimes can be achieved in a reproducible manner a large number of times.

Conclusion
In conclusion, switchable strong light-matter coupling has been demonstrated between optical modes of a metal/dielectric bilayer resonator and SCO molecules, with Rabi splitting energies of up to 550 meV (i.e., 14 % of the molecular excitation energy).As a result, SCO compounds are deemed to constitute an attractive class of switchable molecular materials for both switchable polaritonic chemistry and active photonics.This work received financial support from the Agence Nationale de la Recherche (project SCOPOL, ANR-22-CE09-0019), the University of Toulouse III (project AO Tremplin "MaCaPeSuMO") and the Centre National de la Recherche Scientifique (project MITI "CMTS-SLM").The fabrication was partly done within the LAAS-CNRS cleanroom facilities, a member of the national RENATECH platform network.

Fig. 1 .
Fig. 1.(a) Representation of the 3d electron configurations for a Fe 2+ SCO complex in its low-spin (LS, S = 0) and high-spin (HS, S = 2) state.(b) Scheme of the fabricated SCO-based optical resonator formed by an Al (16 nm)/1 (138 nm) bilayer deposited on a prism.(c) Optical absorbance spectra of the 138-nm-thick film of 1, acquired in the LS (20 °C) and HS (120 °C) states.The inset shows the molecular structure of the complex.(d) Thermal spin-transition curve of the film of 1.

Fig. 2 .
Fig. 2. Angular dispersion of the resonator at T = 60 °C (LS state) and T = 80 °C (HS state).The three main absorption maxima of the SCO complex in the LS state are depicted by horizontal white dashed lines.(b) Temperature evolution of the TM mode at a fixed angle of incidence, θair = 74°.The vertical white dashed line marks the transition temperature TSCO = 64 °C.

Fig. 3 .
Fig. 3. Evolution of the Rabi splitting energy measured in the SCO-based optical resonator as a function of the temperature.