Observation of photonic paramagnetic to spin-glass transition in a specially designed TiO₂ particle-based dye-colloidal random laser.

Colloidal-based random lasers (RLs) are highly efficient and have been exploited in a wide range of geometries. However, in the particular case of ethanol solutions of rhodamines and TiO₂ particles, the RL behavior is quite unstable due to the fast precipitation of the particles. In this Letter, specially designed amorphous TiO₂ particles were synthesized by a sol-gel method, preventing the degradation of the RL for long operating lifetimes of over 10⁵ shots. As a consequence, this modified colloidal RL allowed the observation of a clear replica-symmetry-breaking phase transition from the paramagnetic fluorescent to spin-glass RL behavior, which has not been observed in the system with nonfunctionalized TiO₂ particles.