Solvent-triggered off-to-on circular polarization activation via scattering modulation in polymer-based chiral photonic particles for encryption and authentication.

Photonic crystals have attracted significant attention for their structural colors and stimuli-responsive optical properties. However, conventional systems often suffer from undesirable "on-to-off" switching behavior, where structural color fades upon environmental stimulation, limiting their dynamic functionality. To overcome this limitation, we introduce a polymer-based chiral photonic crystal that exhibits an unconventional "off-to-on" switching mechanism governed by scattering modulation. In its dried state, circularly polarized (CP) light is concealed due to strong scattering, rendering the chiral signal optically inactive. Upon solvent infiltration, a reduced refractive index contrast suppresses scattering, activating vivid chiral structural colors. This scattering-driven concealment and reversible activation offer new opportunities for secure optical systems. We demonstrate its application in photonic security labels, where solvent exposure selectively reveals CP light-based patterns that remain hidden under ambient conditions and become fully visible under a polarizer. Furthermore, the inherent variability from particle assembly enables physically unclonable functions for instrument-based authentication, with encoding derived from structural randomness and CP light responses. Together, this system provides a robust, solvent-responsive and polarization-stealth photonic platform for secure information storage and anti-counterfeiting technologies.