Abstract

Local probing of second-order susceptibilities with a scanning near-field optical microscope and a conventional far-field microscope arrangement is demonstrated for domain structures in a periodically poled LiNbO₃ crystal and a multilayer Langmuir-Blodgett film of 2-docosylamino- 5-nitropyridine (DCANP). In the first configuration, an uncoated fiber tip is used as a light source, and the transmitted radiation at the fundamental and second harmonic (FH and SH) wavelengths is detected while simultaneously recording surface topography. The experimental setup used consists of a stand-alone illumination scanning near-field optical microscope combined with a shear force based feedback system. The linearly polarized light beam from a mode-locked Ti-Sapphire laser (λ_ω~800 nm, P_ω^av ≈ 200 mW, repetition rate f ≈ 80 MHz, pulse duration τ ~ 200 fs) is coupled into a single-mode fiber terminated with a sharp etched tip. FH radiated from an uncoated fiber tip (and transmitted through a sample) and SH generated in a sample are collected by a microobjective and detected with a photodiode and a photomultiplier (connected with a photon counter), respectively. The tip can be scanned along the sample surface at a constant distance (~5 nm) by virtue of shear force feedback, thus resulting in a topographical image of the sample surface. In the far-field microscope arrangement, the SH radiation, that is generated in a small region illuminated by a focused laser beam (directed from the same laser), is collected with the help of a microobjective (N.A. = 0.4) to form a SH image registered with a CCD camera. The CCD camera is connected with a TV monitor to view and a computer to storage the images.

© 1998 IEEE