Unveiling Spin Transition at Single-Particle Level in Levitating Spin Crossover Nanoparticles.

The ability to control and understand phase transitions of individual nanoscale building blocks is key to advancing the next generation of low-power reconfigurable nanophotonic devices. To address this critical challenge, molecular nanoparticles (NPs) exhibiting spin crossover (SCO) phenomenon are trapped by coupling a quadrupole Paul trap to a multispectral polarization-resolved scattering microscope. This contact-free platform simultaneously confines, optically excites, and monitors the spin transition in Fe(II)-triazole NPs in a pressure-tunable environment, eliminating substrate artifacts. Thus, we demonstrate light-driven manipulation of the spin transition in levitating NPs, enabled by laser heating and free of substrate-induced effects. Using the robust spin bistability near room temperature of our SCO system, we quantify reversible optovolumetric changes of up to 10%, revealing precise switching thresholds at the single-particle level. Independent pressure modulation produces a comparable volume increase, confirming mechanical control over the same bistable transition. These results constitute full real-time control and readout of spin states in levitating SCO NPs, with operating conditions compatible with ultralow-power optical switching, data storage, and nanoscale sensing.