Photostable second-harmonic generation from a single KTiOPO₄ nanocrystal for nonlinear microscopy

A study was conducted to report the observation and the characterization of nanometric-sized crystals extracted by centrifugation from KTP (potassium titanyl phosphate) powder. In situ atomic force microscopy (AFM) analysis of its size and analysis of its second-harmonic emission properties were performed for a well-isolated single nanocrystal. The highly efficient nonlinear response leads to the emission of several SHG photons and in a photostable and blinking-free manner due to non-resonant coherent interaction. The study retrieved the in situ three-dimensional crystal orientation by recovering the radiation pattern from the recorded defocused images. Solution-based chemical synthesis of KTP nanocrystals with a monodisperse size controlled by capping agents should lead to optimized KTP nanocrystallites and to a more accurate estimate of the size-detection threshold. Fully characterized nano-KTPs are also attractive for the development of novel schemes of nonlinear microscopy.