A molecular spin-crossover film allows for wavelength tuning the resonance of a Fabry-Perot cavity

We harness the remarkable refractive index switching (0.04 < Δn(λ) < 0.2) between the low-spin and high-spin states of the molecular spin-crossover (SCO) complex [Fe(HB(1,2,4-triazol-1-yl)3)2] to realize a wavelength-tunable Ag/SCO/Ag multilayer Fabry–Perot cavity. This tuneability is coupled with low absorption losses (k ≤ 10−4) between 350 and 1000 nm providing scope for reconfigurable and self-adaptive photonics applications in the visible spectral region.