Highly stable and reversible birefringence switching triggered by thermally stimulated phase transition

Abstract

Manipulating light polarization in birefringent crystals is central to photonic applications. However, controlling intrinsic optical anisotropy in a noninvasive manner remains challenging. Here, we report our discovery of the polymorphism of the inorganic birefringent crystal KCe(SO₄)₂. P and P2₁/c-KCe(SO₄)₂ crystals can be obtained through polymorph-controllable synthesis. Interestingly, P-KCe(SO₄)₂ exhibits reversible phase transition during which the birefringence value varies by up to 50%. The material is highly stable and the birefringence alteration stimulated by thermally induced phase transition is reversible for over 10 cycles, demonstrating its potential as a birefringence switch. We reveal that the optical anisotropy of KCe(SO₄)₂ polymorphs primarily stems from the arrangement of [CeO₉] and [KO₁₀] structural units, as evidenced by the dipole moments and first principles calculations. Our work not only provides new insights into the controlled synthesis of polymorphic materials but also advances the development of tunable birefringent materials for integrated photonics.