Spontaneous polarization in ultrasmall lithium niobate nanocrystals revealed by second harmonic generation

Nanocrystals of lithium niobate (LiNbO3) with diameters of between 5 and 50 nm were produced by a combination of sol-gel synthesis and wet-chemical etching. The phase purity and nearly spherical shape of the crystals were verified by transmission electron microscopy. Power and polarization dependences of the second harmonic signal of single nanocrystals reveal that the nanocrystals have the noncentrosymmetric point group 3m and that the second-order nonlinear optical coefficients known from the bulk material are fully present in all nanocrystals studied. The volume of the crystals is still the main source of frequency conversion and they are spontaneously polarized.