Fluororesin coating for enhancing the ambient stability of DAST crystals in terahertz wave emission

4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystals, as organic nonlinear optical crystals, are excellent sources of terahertz radiation due to their high second-order nonlinear coefficients. However, as ionic crystals, they are susceptible to deliquescence in air, which affects their optical properties, and there are fewer studies on the related processes for improving their environmental stability. In this study, the FCS-130L fluororesin coating method is adopted to address this problem. The protective effect of the FCS-130L coating was characterized by testing the contact angle, water vapor transmission rate, surface energy, degree of wetting, optical transmittance, and the environmental stability of DAST crystals. The results show that the FCS-130L coating is hydrophobic (distilled water contact angle of 122.1°) and has a low surface energy (22.7 mN/m), which can reduce water vapor transmission. The FCS-130L coating also has a good optical transmittance and no significant absorption of terahertz waves. In the environmental stability test, the FCS-130L-coated DAST crystals can still generate terahertz waves after 60 days at 80% humidity. The service life is significantly longer than that of the uncoated crystals, indicating that the FCS-130L coating can effectively improve the environmental stability of DAST crystals.