advantages, such as large NLO effects, ultrafast responses, and high laser-induced damage thresholds. Several organic molecules and metal-organic compounds have displayed NLO switching in solution, by utilizing the chemical procedures of protonation, two-photon absorption, or oxidation/reduction. [ 2 ] However, the search for highly effi cient solid-state switches remains a challenge, particularly for second-harmonic generation (SHG) activities, because the macroscopic noncentrosymmetric (NCS) arrangement of NLO moieties must be reversibly controlled in a collective manner. [ 3 ] As far as we are aware, only a few solid-state SHG switches have been reported, including organometallic thin fi lms for redox switching, photochromic anil crystals and polymer fi lms, etc. [ 4 ] The switching-induced irreversible changes of the initial chromophore alignment severely limits the number of "on"/"off" cycles in these cases. Moreover, their inferior resistance to laser-induced damage also suppresses the NLO switching effi ciency, [ 5 ] since materials should suffer from high-power laser radiation and thus high laser-induced damage thresholds are essential.Crystalline materials with phase transitions (PTs) have attracted great interest, [ 6 ] since they display outstanding performances around the transition point ( T c ) and their bulk physical properties can be modulated. [ 7 ] In this context, thermally driven structural deformations during PTs afford a great possibility for high-performance SHG switches. Using this conceptually new scheme, simple organic salts and organic-inorganic hybrids have been reported to be potential NLO switches. [ 8 ] During our investigation of novel SHG modulators, H-bonded materials with long-range order of their dipolar arrangement provide us with promising SHG switch candidates. One of our strategies is to discover the hidden pseudo-symmetry induced by hydrogen bonds, [ 9 ] which may be destroyed during PTs and arouse macroscopic dipole moments, corresponding to the generation of SHG signals. [ 10 ] In the present work, a new supramolecular crystal, bis-(imidazolium hydrochlorate) dihydrate 18-crown-6 ( 1 ), is fi rstly reported as a solid-state SHG modulator with a superior switching contrast (∼12) and remarkable switching reversibility. Strikingly, its laser-induced damage threshold (up to ∼8.9 GW/cm 2 ) is much higher than other materials, which guarantees its conversion effi ciency under the high-power laser. As far as we Organic nonlinear optical (NLO) materials are anticipated as the competitive successors to inorganic compounds in the highly controllable photonic devices. Here, a new supramolecular crystal, bis(imidazolium hydrochlorate) dihydrate 18-crown-6 ( 1 ), behaves as a potential modulator of quadratic NLO responses. It is found that ( 1) possesses a superior NLO switchingcontrast of up to ∼12, a remarkable switching reversibility, and a quite high laser-induced damage threshold (up to ∼8.9 GW/cm 2 ), which may guarantee its device application. In addition, mechanism stu...