Abstract
AbstractNew molecular salt crystals based on linear‐shaped polymer‐like cation–anion assembly exhibiting extremely large nonlinear optical response and high THz generation efficiency are reported. Two hydroxy benzothialzolium PMB (2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐3‐methylbenzo[d]thiazol‐3‐ium) crystals with different benzenesulfonate counter anions provide isomorphic crystal structure with acentric monoclinic Cc space group symmetry. In contrast to previously reported benchmark nonlinear optical salt crystals with a parallel‐type cation–anion assembly, newly developed PMB‐based crystals exhibit a series‐type cation–anion assembly mediated by strong bidentate‐like hydrogen‐bonds. Such series‐type cation–anion assembly results in perfect alignment of highly nonlinear PMB cations in the crystalline state, leading to extremely large diagonal component of the second‐order nonlinear optical coefficient exceeding that of the state‐of‐the‐art nonlinear optical crystals. In THz wave generation experiments based on optical rectification, a 0.33 mm thick PMB crystal generates intense THz pulses with peak‐to‐peak THz electric field of 430 kV cm−1 and extremely broad flat spectrum with upper cut‐off frequency of above 8.0 THz. In addition, compared to inorganic standard 1.0 mm thick ZnTe crystals, the PMB crystal delivers a 24 times higher THz electric field and about 3 times broader bandwidth. Therefore, hydroxy benzothialzolium PMB crystals are highly desired novel materials for various nonlinear optical applications including THz photonics.
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