Organic THz Crystals Based on Off‐Diagonal Optical Nonlinearity with Optimal In‐Plane Polar Axis

Abstract Highly efficient off‐diagonal THz generators based on novel organic nonlinear optical single crystals with optimal polar axis orientation in the as‐grown crystal morphology are introduced. The newly developed fluorinated and non‐fluorinated cationic chromophores exhibit a non‐centrosymmetric crystal structure with P2₁ space group symmetry in the crystalline phase. In particular, PB5FB‐T (5‐fluoro‐2‐(4‐(2‐(hydroxymethyl)pyrrolidin‐1‐yl)styryl)‐3‐methylbenzothiazol‐3‐ium 4‐methylbenzenesulfonate) crystals have an in‐plane polar axis, along with a top‐level off‐diagonal component of the nonresonant effective first hyperpolarizability of ≈100 × 10⁻³⁰ esu. This stands in stark contrast to previously reported organic THz crystals, which either show a small off‐diagonal optical nonlinearity with an in‐plane polar axis or a large off‐diagonal optical nonlinearity with an almost out‐of‐plane polar axis. Employing an off‐diagonal THz generation configuration, PB5FB‐T crystals generate a THz electric field approximately four times higher than previously reported benchmark analogous crystals with a polar axis nearly out‐of‐plane. Moreover, PB5FB‐T crystals yield smooth THz spectral shapes without significant absorption dimples up to 6 THz when utilizing 130‐fs pump pulses at the wavelength of 1300 nm.