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

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 ${\mathrm{LiInP}}_2{S}_6$ (point group 32 and an indirect gap of $\sim$ 3.0 eV) into ${\mathrm{LiInP}}_2{S}_6·3H_{2}O$ that exhibits a chiral-polar structure (point group 3 and an indirect gap of $\sim$ 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 $d_{33}\approx 47$ pm/V and $d_{15}\approx 35$ 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 ${\Gamma }_2^{-}$ polar mode consistent with the trigonal point group 3 in hydrated ${\mathrm{LiInP}}_2{S}_6$ , 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.