Reversible Hydration Tuning of Polar Order and Large Nonlinear Optical Response in a 2D van der Waals Crystal, LiInP ₂ S ₆

ABSTRACT

Typically, moisture degrades the performance of commercial optical crystals that are hygroscopic. In contrast, a hydration‐driven symmetry transformation is discovered in a layered van der Waals compound that induces a new polar phase that exhibits a large nonresonant nonlinear optical response. Water intercalation converts the chiral‐nonpolar phase of pristine (point group 32 and an indirect gap of 3.0 eV) into that exhibits a chiral‐polar structure (point group 3 and an indirect gap of 3.15 eV), stabilized by sub‐angstrom lattice distortions. This structural change results in new symmetry, allowing nonresonant optical second‐harmonic generation (SHG) coefficients in the hydrated phase of pm/V and pm/V at the 1550 nm telecom wavelength; these are up to twice as large as other well‐known materials with similar bandgaps. Density functional theory calculations predict the emergence of a polar mode consistent with the trigonal point group 3 in hydrated , as well as inform the emergence of large SHG coefficients. The reversible structural transformation via hydration and dehydration is confirmed through temperature‐dependent SHG, x‐ray diffraction, and differential scanning calorimetry. These findings demonstrate intercalation as a powerful means for tuning polar order, enhancing the nonlinear optical response and on‐demand creation and erasure of tunable optical elements for photonic integrated circuits.