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

Abstract

In organic π-conjugated crystals, enhancing molecular optical nonlinearity of chromophores (e.g., first hyperpolarizability β ≥ 300 × 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H⋯O hydrogen-bonding synthon and an optimal selection of molecular anion into crystals results in extremely large macroscopic optical nonlinearity with effective first hyperpolarizability β iii eff of 335 × 10 −30 esu. This is in sharp contrast to zero β iii eff value for previously reported analogous crystals. An ultrathin PMPR crystal with a thickness of ≈10 µm 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.

Design of High‐Performance Organic Nonlinear Optical and Terahertz Crystals by Controlling the van der Waals Volume

“…In many nonlinear optical organic salt crystals with top‐level macroscopic optical nonlinearity, the volume ratio between cations and anions is very narrow. [ 13 ] For example, stilbene‐type chromophores, 4‐(4‐(dimethylamino)styryl)−1‐methylpyridinium (DAS), 2‐(4‐hydroxy‐3‐methoxystyryl)−1‐methylquinolinium (HMQ), 2‐(4‐hydroxy‐3‐methoxystyryl)−3‐methylbenzothiazol‐3‐ium (HMB), 2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)−3‐methylbenzothiazol‐3‐ium (PMB) and 1‐ethyl‐2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)−3,3‐dimethyl‐3H‐indolium (EHPSI) cations and their analogous cations exhibit a high order parameter when crystallized with benzenesulfonate (and naphthalene sulfonate) anions having a single phenyl (and naphthalene) ring. [ 14,23–28 ] Using relatively bigger anions such as stilbene‐type 4‐((4‐(dimethylamino)phenyl)diazenyl)benzenesulfonate) (MO), these cations are centrosymmetrically aligned in crystals.…”

“…In many nonlinear optical organic salt crystals with top-level macroscopic optical nonlinearity, the volume ratio between cations and anions is very narrow. [13] For example, stilbene-type chromophores,…”

“…[1][2][3][4][5][6][7][8][9][10] Compared to inorganic crystals, many organic crystals exhibit a superior second-order nonlinear optical coefficient when highly nonlinear optical chromophores assemble in a non-centrosymmetric structure with a high-order parameter (i.e., close to parallel alignment of chromophores in the crystalline state). [11][12][13] This superior macroscopic optical nonlinearity of organic crystals generally leads to a significantly higher optical-to-THz conversion efficiency, which is often accompanied by a broader THz spectral bandwidth, [6,[13][14][15][16][17][18][19] supported by better phase matching conditions and weaker absorption resonances in the THz frequency range. [7,20] Organic nonlinear optical crystals consist of molecular components including highly polar nonlinear optical chromophores.…”

Local Rigidity by Flexibility: Unusual Design for Organic THz‐Device Materials

“…In many benchmark organic nonlinear optical salt crystals showing parallel-type cation-anion self-assembly with high order parameter (close to maximum value of 1.0), the ratio of V anion /V cation is extremely small as shown in Figure 1c. [29,30] The V anion and V cation in crystals were obtained from Hirshfeld surface analysis. [31] In OHQ-based crystals with high order parameter (≥0.94), the ratio of V anion /V cation is in the range of 0.57-0.66, whereas the V anion is in the range of 187-223 Å 3 .…”

“…Many OHQ-based THz crystals show a relatively narrow ratio between the crystallographic volumes (from Hirshfeld surface analysis) of cations and anions (V anion /V cation ) in the range of 0.57-0.66, considering OHQ-based crystals with a high order parameter (≥0.94). [24][25][26][27][28][29] vibrations that might result in excellent THz generation performance with small self-absorption of THz waves.…”