Second-Harmonic-Generation Switching via Pressure-Suppressed Dynamical Disorder

Second-harmonic-generation (SHG) switching is an emerging phenomenon with potential applications in bistable storage and optical switches while also serving as a sensitive probe for inversion-symmetry. Temperature-induced disorder–order phase transition has been proven to be a rational design strategy for achieving SHG bi-state switching; however, pressure-sensitive SHG switching via a disorder–order structural transition mechanism is rarely reported and lacks sensitivity and cyclicity as practical switching materials. Herein, we demonstrate the pressure-induced “dynamical disorder–order” phase transition as an effective strategy for triggering SHG and SHG switching in NH4Cl. The “dynamical disorder–order” phase transition of NH4Cl occurring at as low as 1 GPa is confirmed by comprehensive in situ high-pressure XRD, molecular vibrational spectra, and Brillouin scattering spectra. The pressure-induced SHG is responsive to a wide excitation wavelength region (800–1500 nm), and the “off–on” switching is reversible for up to 50 cycles, setting a record for pressure-driven switching materials. It is worth noting that when pressure is further increased to 14 GPa, NH4Cl exhibits another SHG “on–off” switching, which makes it the first triplet SHG “off–on–off” switching material. Molecular dynamics simulations reveal the key role of N–H···Cl hydrogen bonding in the pressure-induced “dynamic disorder–order” mechanism. Finally, we verified that chemical pressure and physical pressure can jointly regulate the SHG switching behavior of NH4X (X = Cl, Br). The pressure-driven “dynamic disorder–order” transition mechanism sheds light on the rational design of multistable SHG switching materials for photoswitches and information storage.