Second-Harmonic Generation of Single BaTiO₃ Nanoparticles down to 22 nm Diameter

We investigate the second-harmonic generation (SHG) signal from single BaTiO₃ nanoparticles of diameters varying from 70 nm down to 22 nm with a far-field optical microscope coupled to an infrared femtosecond laser. An atomic force microscope is first used to localize the individual particles and to accurately determine their sizes. Power and polarization-dependent measurements on the individual nanoparticles reveal a diameter range between 30 and 20 nm, where deviations from bulk nonlinear optical properties occur. For 22 nm diameter particles, the tetragonal crystal structure is not applicable anymore and competing effects due to the surface to volume ratio or crystallographic modifications are taking place. The demonstration of SHG from such small nanoparticles opens up the possibilities of using them as bright coherent biomarkers. Moreover, our work shows that measuring the SHG of individual nanoparticles reveals critical material properties, opening up new possibilities to investigate ferroelectricity at the nanoscale