Near-field nonlinear optical spectroscopy of Langmuir-Blodgett films

Using an uncoated fiber tip as a light source and a multilayer Langmuir-Blodgett film of 2-docosylamino-5-nitropyridine as a sample, we obtain near-field images at the fundamental-harmonic (FH) and second-harmonic (SH) wavelengths for different polarizations and wavelengths of the pump light. The spatial resolution of similar to 100 nm is achieved in near-field SH images that are not correlated with either the topographical or the FH images. It is directly demonstrated that SH generation is most concentrated inside submicrometer-sized (800-500 nm) crystallike domains, which are oriented differently but predominantly in the dipping direction and whose SH efficiency is maximum for pump wavelengths in the range of 770-800 nm. (C) 1998 Optical Society of America.