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

Chalcone\ncrystals, known for their significance as molecular nonlinear\noptical (NLO) materials, exhibit exceptional output properties within\nthe terahertz frequency range. In this study, three chalcone-based\ncrystals, (E)-3-(4-methoxyphenyl)-1-(4-(trifluoromethyl)phenyl) prop-2-en-1-one\n(TFMOC), (E)-3-(4-ethoxyphenyl)-1-(4-(trifluoromethyl)phenyl) prop-2-en-1-one\n(TFEOC), and (E)-3-(4-propoxyphenyl)-1-(4-(trifluoromethyl)phenyl)\nprop-2-en-1-one (TFPOC), were designed by extending the alkyl chains\nspatial structure and successfully grown. The results indicated that\nTFEOC and TFPOC were crystallized in noncentrosymmetric space groups.\nOptimization of the carbon chain (−CH<sub>3</sub>) length effectively\naltered the molecular arrangement. The molecular stacking arrangement,\nband gap size, specific atomic contributions to the density of states,\nand NLO coefficients were analyzed to understand the origin of nonlinearity.\nThe powder second harmonic generation (SHG) efficiencies of TFEOC\nand TFPOC were 4.32 and 8.72 times greater than KDP, respectively.\nIt is worth noting that both crystals have large bandgaps and wide\ntransparent ranges. Furthermore, THz output from both crystals was\nobtained for the first time via optical rectification, with the spectral\nwidth of the TFPOC crystal, designed by elongating the alkyl chain,\nslightly exceeding that of trans-4′-(dimethylamino)-<i>N</i>-methyl-4-stilbazolium tosylate (DAST).