Reversible Hydration Tuning of Polar Order and Large Nonlinear Optical Response in a 2D van der Waals Crystal, LiInP₂S₆.

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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><msub><mi>LiInP</mi> <mn>2</mn></msub> <msub><mi>S</mi> <mn>6</mn></msub> </mrow> </math> (point group 32 and an indirect gap of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>∼</mo> <mspace></mspace></mrow> </math> 3.0 eV) into <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><msub><mi>LiInP</mi> <mn>2</mn></msub> <msub><mi>S</mi> <mn>6</mn></msub> <mo>·</mo> <mn>3</mn> <msub><mi>H</mi> <mn>2</mn></msub> <mi>O</mi></mrow> </math> that exhibits a chiral-polar structure (point group 3 and an indirect gap of <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>∼</mo></math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><msub><mi>d</mi> <mn>33</mn></msub> <mo>≈</mo> <mn>47</mn></mrow> </math> pm/V and <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><msub><mi>d</mi> <mn>15</mn></msub> <mo>≈</mo> <mn>35</mn></mrow> </math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>Γ</mi> <mn>2</mn> <mo>-</mo></msubsup> </math> polar mode consistent with the trigonal point group 3 in hydrated <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><msub><mi>LiInP</mi> <mn>2</mn></msub> <msub><mi>S</mi> <mn>6</mn></msub> </mrow> </math> , 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.