KTiOPO4 single nanocrystal for second-harmonic generation microscopy

Nonlinear second-harmonic generation (SHG)microscopy has become a commonly used technique for investigating interfacial phenomena(Kemnitz et al., 1986; Shen, 1989) and imaging biological samples.(Moreaux et al., 2000) Different non-centrosymmetric nanometric light sources have been recently studied in this context, e.g. organic nanocrystals.(Shen et al., 2001; Treussart et al., 2003) For those systems, resonant optical interaction leads to an enhancement of the nonlinear response but also to parasitic effect that is detrimental for practical applications, namely photobleaching due to two-photon residual absorption.(Patterson et al., 2000) Conversely, inorganic non-centrosymmetric materials with far-off resonance interaction avoid this limitation.(Johnson et al., 2002; Long et al., 2007) Recent achievements have been obtained using KNbO3 nanowires as a tunable source for sub-wavelength optical microscopy(Nakayama et al., 2007) and Fe(IO3)3 nanocrystallites as promising new SHG-active particles with potential application in biology.(Bonacina et al., 2007) However, either the dimensions of the used crystals are still of the order of themicrometer along one axis,(Nakayama et al., 2007) or the corresponding bulk material is not easily grown,(Bonacina et al., 2007) so that the crystal characteristics are not directly available. A complementary approach consists in considering a well-known SHG-active bulk material and investigating its properties in nanoparticle form. Different materials have been considered, e.g. BaTiO3 (Hsieh et al., 2009) and ZnO (Kachynski et al., 2008). In this view, we were among the pioneers in this domain, considering the well-known KTP material. Potassium titanyl phosphate (KTiOPO4, KTP) is a widely used nonlinear crystal.(Zumsteg et al., 1976) Studies on this material have focused on the optimized growth of large-size single crystals, which have found numerous applications in laser technology for efficient frequency conversion.(Driscoll et al., 1986) Here we show that KTP particles of nanometric size (nano-KTPs) are an attractive material for SHGmicroscopy. Under femtosecond excitation and in ambient conditions, a single nano-KTP with a size around 60 nm independently determined with atomic force microscopy (AFM), generates a perfectly stable blinking-free second-harmonic signal which can be easily detected in the photon-counting regime. Furthermore, we demonstrate that this single nanocrystal can 7