Thermo-mechanical control of second harmonic generation in a ferroelectric plastic crystal

Abstract

Multi-step nonlinear optical (NLO) switching opens avenues for photonic and optoelectronic technologies, but remains limited by insufficient mechanistic insight and poor tunability at the macroscopic level. Here, we report a ferroelectric plastic crystal, (TMIPA)[InBr₄] (1, TMIPA⁺ = N,N,N-trimethyl-2-propanaminium), that exhibits four discrete second harmonic generation (SHG) states through an “on–off–on–off” switching pattern over a wide thermal window (120–500 K), enabled by sequential phase transitions driven by progressive freezing of TMIPA⁺ cation dynamics and cooperative interactions with [InBr₄]⁻ anions. Capitalizing on the plastic nature of 1, we fabricated a hot-pressed tablet with a remarkably high density (2.42 g cm⁻³, 98% of crystallographic density), achieving significant SHG enhancements of 2.8 and 3.4 times greater in the low- and intermediate-temperature phases, respectively, relative to loose powder. Notably, deep potential molecular dynamics simulations, complemented by hyperpolarizability calculations, reveal atomic-level insight into the interplay between dynamic disorder and NLO activity, a regime inaccessible to traditional molecular dynamics simulations or density functional theory. By bridging molecular motion with macroscopic SHG behavior, this work established a framework for engineering programmable NLO switches in plastic crystals.