Electrostrictive Structural Transitions and Tensorial Electro-Optics in Mesomorphic Blue Phase Crystals

Electrostriction is present in all dielectrics, but the electrostriction itself is generally minuscule and is even insignificant in low-dielectric-constant materials. Herein, extraordinary electrostriction is found in mesomorphic blue phase (BP) crystals, which are fluidic “giant” crystals with a lattice constant of several hundred nanometers and contain 10 7 –10 8 molecules in each unit cell. In situ optical observations revealed that the BP crystals exhibit −19.5% to 15.7% electrostrictive actuation at weak field strengths (<3 V/μm), during which a transient monoclinic phase is found. The monoclinic phase occurs in the form of twinning and single crystals successively, which acts as an intermediate state to dominate the pathway of electrostrictive structural transition. Furthermore, we experimentally confirmed that a tensorial crystal-optic effect is induced in BPs by electrostriction, which updates our conventional understanding about liquid crystalline materials that the electro-optics has always been considered as a scalar effect. This work reveals the structural polymorphism in electrostrictive soft crystals, which, in the meantime, provides additional degrees-of-freedom for their advanced applications in nano or micro actuators, flexible electronics, and photonics of next generation, by precisely manipulating the soft crystalline structures and their crystal-optic characteristics.