Constructing Broadband THz Optical Crystals by Extending the Length of the Alkyl Chain

Chalcone crystals, known for their significance as molecular nonlinear optical (NLO) materials, exhibit exceptional output properties within the terahertz frequency range. In this study, three chalcone-based crystals, (E)-3-(4-methoxyphenyl)-1-(4-(trifluoromethyl)phenyl) prop-2-en-1-one (TFMOC), (E)-3-(4-ethoxyphenyl)-1-(4-(trifluoromethyl)phenyl) prop-2-en-1-one (TFEOC), and (E)-3-(4-propoxyphenyl)-1-(4-(trifluoromethyl)phenyl) prop-2-en-1-one (TFPOC), were designed by extending the alkyl chains spatial structure and successfully grown. The results indicated that TFEOC and TFPOC were crystallized in noncentrosymmetric space groups. Optimization of the carbon chain (−CH₃) length effectively altered the molecular arrangement. The molecular stacking arrangement, band gap size, specific atomic contributions to the density of states, and NLO coefficients were analyzed to understand the origin of nonlinearity. The powder second harmonic generation (SHG) efficiencies of TFEOC and TFPOC were 4.32 and 8.72 times greater than KDP, respectively. It is worth noting that both crystals have large bandgaps and wide transparent ranges. Furthermore, THz output from both crystals was obtained for the first time via optical rectification, with the spectral width of the TFPOC crystal, designed by elongating the alkyl chain, slightly exceeding that of trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST).