Reversible Switching of Strong Light-Matter Coupling Using Spin-Crossover Molecular Materials.

The formation of hybrid light-matter states through the resonant interaction of confined electromagnetic fields with matter excitations has emerged as a fascinating tool for controlling quantum-mechanical states and then manipulating the functionalities and chemical reactivity landscape of molecular materials. Here we report the first observation of switchable strong light-matter coupling involving bistable spin-crossover molecules. Spectroscopic measurements, supported by transfer-matrix and coupled-oscillator simulations, reveal Rabi splitting values of up to 550 meV, which at 15% of the molecular excitation energy enter the regime of ultrastrong coupling. We find that the thermally induced switching of molecules between their low-spin and high-spin states allows fine control of the light-matter hybridization strength, offering the appealing possibility of reversible switching between the ultrastrong- and weak-coupling regimes within a single photonic structure. Through this work, we show that spin-crossover molecular compounds constitute a promising class of active nanomaterials in the burgeoning context of tunable polaritonic devices and polaritonic chemistry.

High-Sensitivity Microthermometry Method Based on Vacuum-Deposited Thin Films Exhibiting Gradual Spin Crossover above Room Temperature.

We report on the fabrication, characterization, and microthermometry application of high-quality, nanometric thin films, with thicknesses in the range 20-200 nm, of the molecular spin-crossover complex [Fe(HB(1,2,3-triazol-1-yl)3)2]. The films were obtained by vacuum thermal evaporation and characterized by X-ray diffraction, UV spectrophotometry, and atomic force microscopy. The as-deposited films are dense and crystalline with a preferred [011] orientation of the monoclinic crystal lattice normal to the substrate surface. The films exhibit a gradual spin conversion centered at ca. 374 K spanning the 273-473 K temperature range, irrespective of their thickness. When deposited on a microelectronic device, these films can be used to enhance the UV-light thermoreflectance coefficient of reflective surfaces by more than an order of magnitude, allowing for high-sensitivity thermoreflectance thermal imaging.