Thermo‐Optical Switches Based on Spin‐Crossover Molecules with Wideband Transparency

Abstract In this work, a thermally tunable optical resonator operating in the visible wavelength range is reported, which consists of a bilayer structure composed of a thin silver layer and a dielectric coating of [Fe(HB(1,2,4‐triazol‐1‐yl)₃)₂] switchable spin‐crossover (SCO) molecules. White light is coupled to the resonant structure using a prism and the resulting resonance spectra are investigated as a function of the incident angle and temperature. Switching the SCO molecules from the low‐spin to the high‐spin state gives rise to a substantial blueshift of the resonances reaching up to 30 nm (associated with a reflectance modulation of up to 70%), which can be linked, through transfer‐matrix simulations, to the variation of the optical thickness of the molecular layer. Interestingly, the study demonstrated that the large thermal tunability of the device also gives rise to a photothermal nonlinearity, which can be leveraged for achieving optical limiting applications. Overall, through the present study, it is shown that molecular SCO nanomaterials can be superior to commonly used thermo‐optical switches and well‐established optical phase‐change materials for applications requiring high broadband optical transparency in the visible spectral domain.