A New Class of Organic Crystals with Extremely Large Hyperpolarizability: Efficient THz Wave Generation with Wide Flat-Spectral-Band

Abstract

In organic pi-conjugated crystals, enhancing molecular optical nonlinearity of chromophores (e.g., first hyperpolarizability beta >= 300 x 10(-30) esu) in most cases unfortunately results in zero macroscopic optical nonlinearity, which is a bottleneck in organic nonlinear optics. In this study, a new class of nonlinear optical organic crystals introducing a chromophore possessing an extremely large first hyperpolarizability is reported. With newly designed 4-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-1-(pyrimidin-2-yl)pyridin-1-ium (PMPR) chromophore, incorporating a head-to-tail cation-anion O-HMIDLINE HORIZONTAL ELLIPSISO hydrogen-bonding synthon and an optimal selection of molecular anion into crystals results in extremely large macroscopic optical nonlinearity with effective first hyperpolarizability beta iiieff$\beta _{{\rm{iii}}}<^>{{\rm{eff}}}$ of 335 x 10(-30) esu. This is in sharp contrast to zero beta iiieff$\beta _{{\rm{iii}}}<^>{{\rm{eff}}}$ value for previously reported analogous crystals. An ultrathin PMPR crystal with a thickness of approximate to 10 mu m exhibits excellent terahertz (THz) wave generation performance. Both i) broadband THz wave generation with a wide flat-spectral-band in the range of 0.7-3.4 THz defined at -3 dB and high upper cut-off generation frequency of > 7 THz as well as ii) high-generation efficiency (5 times higher THz amplitude than ZnTe crystal with a mm-scale thickness) are simultaneously achieved. Therefore, new PMPR crystals are highly promising materials for diverse applications in nonlinear optics and THz photonics.