Alkynyl Pyridinium Crystals for Terahertz Generation

Abstract

The design and synthesis of three organic nonlinear optical crystals is presented for terahertz (THz) generation that incorporates the optimal characteristics of known organic nonlinear optical crystals 4‐(4‐(dimethylamino)styryl)‐1‐methylpyridinium 4‐methylbenzenesulfonate (DAST) and 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 2,4,6‐trimethylbenzenesulfonate (HMQ‐TMS). The three crystals feature the 4‐((4‐(dimethylamino)phenyl)ethynyl)‐1‐methylpyridin‐1‐ium (4DEP) cation and three different anion combinations. Gas phase ab initio calculations of the cation show that 4DEP has a larger hyperpolarizability than cations from both DAST and HMQ‐TMS. To obtain the molecular packing needed for large nonlinear optical susceptibility and efficient THz generation, three anions are tested that result in noncentrosymmetric crystals: 4‐methylbenzenesulfonate, 3‐nitrobenzenesulfonate, and napthalene‐2‐sulfonate. After synthesis and crystallization, the crystal structure is characterized via X‐ray diffraction (XRD) analysis, and the relative THz generation efficiency for ideal crystals is predicted. 4DEP‐N2S shows a roughly 30% larger relative second‐order nonlinear optical susceptibility compared to HMQ‐TMS and DAST and, therefore, shows promise as a new advanced THz generation crystal.

X‐Shaped Alignment of Chromophores: Potential Alternative for Efficient Organic Terahertz Generators

“…X‐shaped alignment of nonlinear optical chromophores has been previously reported and their off‐diagonal optical nonlinearities are often larger than diagonal ones 1,12,29,30,31,38,39. As far as we know, the off‐diagonal component of the effective hyperpolarizability in PFB‐CBS and PFB‐BTS crystals (105 and 106 × 10 −30 esu, respectively) presents one of the largest values of off‐diagonal optical nonlinearity in organic crystals based on quasi‐1D chromophores,28 and is comparable to the values for diagonal components in state‐of‐the‐art crystals (>100 × 10 −30 esu).…”

“…Organic nonlinear optical crystals offer high potential for various electro‐optic and nonlinear optical applications, including intense broadband or narrowband THz wave generation 1–7. In optical‐to‐THz conversion processes,8,9 benchmark organic nonlinear optical crystals simultaneously achieve high optical‐to‐THz conversion efficiency and broad bandwidth by using a diagonal optical nonlinearity from the nearly parallel alignment of nonlinear optical chromophores 10–16. For example, compared to widely used inorganic electro‐optic ZnTe crystals, organic ionic nonlinear optical crystals exhibit one order of magnitude higher THz generation efficiency, and over two times broader bandwidth, with an upper cut‐off frequency of >5 THz 17–20.…”

“…Most state‐of‐the‐art organic THz generators are based on optical configurations using a diagonal optical nonlinearity of benchmark organic crystals 10–20. The use of an off‐diagonal component of optical nonlinearity has rarely been investigated 21,22.…”

“…In many cases, off‐diagonal nonlinear optical components are negligibly small. Consequently, large diagonal optical nonlinearity with negligible off‐diagonal optical nonlinearity is commonly considered as optimal for electro‐optic and THz applications 12,15,19. For this reason, most previous studies for developing new organic THz generators are mainly targeted at achieving perfectly (or nearly) parallel alignment of nonlinear optical chromophores in solid state.…”

Fluorinated Organic Electro‐Optic Quinolinium Crystals for THz Wave Generation

“…Organic second‐order nonlinear optical crystals have been highly considered as potential materials for various photonic applications such as broadband and narrowband terahertz (THz) wave generation and detection, integrated optics for high‐speed telecommunications, photorefraction, and frequency conversion . Among them, organic ionic‐type crystals based on nonlinear optical molecular cations provide benchmark macroscopic second‐order optical nonlinearity (e.g., effective hyperpolarizability in the crystal of over 90 × 10 −30 esu) .…”