Low-Temperature Flexibility of Chiral Organic Crystals with Highly Efficient Second-Harmonic Generation

Flexible crystals with unique mechanical properties have presented potential for applications in optoelectronics, soft robotics and sensors. However, there have been no reports of low-temperature-resistant flexible crystals with second-order nonlinear optical properties (NLO). Here, we report flexible chiral Schiff-base crystals capable of efficient second harmonic generation (SHG). Both enantiomers and the racemic form of these crystals are mechanically flexible in two directions at both room temperature and at 77 K, although their mechanical responses differ. The enantiomers display SHG with an intensity of up to 12 times that of potassium dihydrogenphosphate (KDP) when pumped at 980 nm, and they also have high laser-induced damage thresholds (LDT). Even when bent, the crystals retain strong second harmonic generation, although with a different intensity distribution depending on the polarization, compared to when they are straight. This work describes the first instance of flexible organic single-crystalline material with NLO properties and lays the foundation for the development of mechanically flexible organic NLO materials.