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 ∼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 (300–500 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.