Wide‐Bandgap Organic Crystals: Enhanced Optical‐to‐Terahertz Nonlinear Frequency Conversion at Near‐Infrared Pumping,Advanced Optical Materials

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Wide‐Bandgap Organic Crystals: Enhanced Optical‐to‐Terahertz Nonlinear Frequency Conversion at Near‐Infrared Pumping
Advanced Optical Materials ( IF 8.0 ) Pub Date : 2020-03-16 , DOI: 10.1002/adom.201902099
Deokjoong Kim ₁ , Won Tae Kim ₂ , Jae‐Hyun Han ₁ , Ji‐Ah Lee ₁ , Seung‐Heon Lee ₁ , Bong Joo Kang ₂ , Mojca Jazbinsek ₃ , Woojin Yoon ₄ , Hoseop Yun ₄ , Dongwook Kim ₅ , Stein Bezouw ₆ , Jochen Campo ₆ , Wim Wenseleers ₆ , Fabian Rotermund ₂ , O‐Pil Kwon ₁

Affiliation

Enhanced terahertz (THz) wave generation is demonstrated in nonlinear organic crystals through refractive index engineering, which improves phase matching characteristics substantially. Unlike conventional low‐bandgap nonlinear organic crystals, the newly designed benzimidazolium‐based HMI (2‐(4‐hydroxy‐3‐methoxystyryl)‐1,3‐dimethyl‐1H‐benzoimidazol‐3‐ium) chromophore possesses a relatively wide bandgap. This reduces the optical group index in the near‐infrared, allowing better phase matching with the generated THz waves, and leads to high optical‐to‐THz conversion. A unique feature of the HMI‐based crystals, compared to conventional wide‐bandgap aniline‐based crystals, is their remarkably larger macroscopic optical nonlinearity, a one order of magnitude higher diagonal component in macroscopic nonlinear susceptibility than NPP ((1‐(4‐nitrophenyl)pyrrolidin‐2‐yl)methanol) crystals. The HMI‐based crystals also exhibit much higher thermal stability, with a melting temperature T_m above 250 °C, versus aniline‐based crystals (116 °C for NPP). With pumping at the technologically important wavelength of 800 nm, the proposed HMI‐based crystals boost high optical‐to‐THz conversion efficiency, comparable to benchmark low‐bandgap quinolinium crystals with state‐of‐the‐art macroscopic nonlinearity. This performance is due to the excellent phase matching enabled by decreasing optical group indices in the near‐infrared through wide‐bandgap chromophores. The proposed wide‐bandgap design is a promising way to control the refractive index of various nonlinear organic materials for enhanced frequency conversion processes.

中文翻译：

宽带隙有机晶体：近红外泵浦时增强的光学-太赫兹非线性频率转换

通过折射率工程在非线性有机晶体中证明了增强的太赫兹（THz）波产生，这大大改善了相位匹配特性。与传统的低带隙非线性有机晶体不同，新设计的基于苯并咪唑的HMI（2-（4-羟基-3-甲氧基苯乙烯）-1,3-二甲基-1H-苯并咪唑-3-鎓）发色团具有相对较宽的带隙。这样可以降低近红外的光学组指数，从而可以更好地与产生的太赫兹波进行相位匹配，并导致高的光到太赫兹转换。与传统的基于宽带隙苯胺的晶体相比，基于HMI的晶体的独特之处在于它们的宏观光学非线性度显着更大，宏观非线性磁化率中的对角线成分比NPP（（1-（4-硝基苯基）吡咯烷-2--2-基）甲醇）晶体高一个数量级。基于HMI的晶体在熔化温度下也表现出更高的热稳定性与苯胺基晶体相比，T _m高于250°C（NPP为116°C）。通过在技术上重要的800 nm波长处进行泵浦，拟议的基于HMI的晶体可提高高光学至THz的转换效率，可与具有最新宏观非线性的基准低带隙喹啉晶体相比。这种性能是由于通过宽带隙发色团降低近红外的光学基团指数而实现了出色的相位匹配。拟议的宽带隙设计是控制各种非线性有机材料的折射率以增强频率转换过程的一种有前途的方法。

更新日期：2020-03-16

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