Second-Harmonic-Generation\nSwitching via Pressure-Suppressed\nDynamical Disorder

Second-harmonic-generation\n(SHG) switching is an emerging phenomenon\nwith potential applications in bistable storage and optical switches\nwhile also serving as a sensitive probe for inversion-symmetry. Temperature-induced\ndisorder–order phase transition has been proven to be a rational\ndesign strategy for achieving SHG bi-state switching; however, pressure-sensitive\nSHG switching via a disorder–order structural transition mechanism\nis rarely reported and lacks sensitivity and cyclicity as practical\nswitching materials. Herein, we demonstrate the pressure-induced “dynamical\ndisorder–order” phase transition as an effective strategy\nfor triggering SHG and SHG switching in NH<sub>4</sub>Cl. The “dynamical\ndisorder–order” phase transition of NH<sub>4</sub>Cl\noccurring at as low as 1 GPa is confirmed by comprehensive <i>in situ</i> high-pressure XRD, molecular vibrational spectra,\nand Brillouin scattering spectra. The pressure-induced SHG is responsive\nto a wide excitation wavelength region (800–1500 nm), and the\n“off–on” switching is reversible for up to 50\ncycles, setting a record for pressure-driven switching materials.\nIt is worth noting that when pressure is further increased to 14 GPa,\nNH<sub>4</sub>Cl exhibits another SHG “on–off”\nswitching, which makes it the first triplet SHG “off–on–off”\nswitching material. Molecular dynamics simulations reveal the key\nrole of N–H···Cl hydrogen bonding in the pressure-induced\n“dynamic disorder–order” mechanism. Finally,\nwe verified that chemical pressure and physical pressure can jointly\nregulate the SHG switching behavior of NH<sub>4</sub>X (X = Cl, Br).\nThe pressure-driven “dynamic disorder–order”\ntransition mechanism sheds light on the rational design of multistable\nSHG switching materials for photoswitches and information storage.