  Second Harmonic Generation (SHG) from BiFeO3 nanocrystals is investigated for
the first time to determine their potential as biomarkers for multiphoton
imaging. Nanocrystals are produced by an auto-combustion method with TRIS as a
fuel. Stable colloidal suspensions with mean particle diameters in the range
100-120 nm are then obtained after wet-milling and sonication steps. SHG
properties are determined using two complementary experimental techniques,
Hyper Rayleigh Scattering and nonlinear polarization microscopy. BiFeO3 shows a
very high second harmonic efficiency with an averaged <d> coefficient of 79+-12
pm/V. From the nonlinear polarization response of individual nanocrystals,
relative values of the independent dij coefficients are also determined and
compared with recent theoretical and experimental studies. Additionally, the
particles show a moderate magnetic response, which is attributed to gamma-Fe2O3
impurities. A combination of high nonlinear optical efficiency and magnetic
response within the same particle is of great interest for future bio-imaging
and diagnostic applications.
