We present a near-field optical technique for second-harmonic imaging by use of tapered optical fiber tips externally illuminated with femtosecond laser pulses. Enhancement of the electric field at the tip of the fiber results in enhanced second-harmonic (SH) generation from the sample region near the tip. This SH emission is collected by the same tapered fiber. The spatial distribution and polarization properties of SH generation from thin ferroelectric films and a poled single crystal of BaTiO<sub>3</sub> have been studied. A spatial resolution of the order of 80 nm was achieved. Symmetry properties of the near-field SH signal allow us to recover the local poling direction of individual ferroelectric domains in the film. Thus the technique provides a novel tool for nanometer-scale crystal analysis of polycrystalline samples.
© 2000 Optical Society of America
I. I. Smolyaninov, H. Y. Liang, C. H. Lee, C. C. Davis, S. Aggarwal, and R. Ramesh, "Near-field second-harmonic microscopy of thin ferroelectric films," Opt. Lett. 25, 835-837 (2000)