We present an experimental study of near-field optical interactions between an optical probe and sample objects with different dielectric properties. The interaction strongly affects the radiation emitted at angles beyond the critical angle of total internal reflection in the substrate (the forbidden light regime). Such an effect has been predicted theoretically. Our experimental data show that if a conducting object is close to the optical probe, p-polarized optical fields are deflected away from the object. On the other hand, s-polarized fields are deflected toward dielectric objects. The experimental results show good qualitative agreement with numerical simulations. The described effects have a strong influence on image formation in scanning near-field optical microscopy and thus have to be taken into account for image analysis.

Polarization contrast scanning near-field optical microscopy (SNOM) provides information on the orientation of molecules and molecular aggregates on surfaces. Other than in transmission or fluorescence SNOM, the control of polarization demands for SNOM probes having extinction ratios better than 20:1 in at least two perpendicular polarization directions. Most common SNOM probes consist of tapered and metal-coated fiber tips. While the birefringence of fibers can be compensated, the tapering often exhibits depolarizing effects that make good qualitative or even quantitative measurements difficult. In an attempt to improve the quality of the apertures, we have modified metal-coated fiber probes with a focused ion beam (FIB). As an example of investigations where a very good polarization control is needed, we present and discuss polarization-modulation SNOM (PM-SNOM) measurements on dye crystals. The modulation of the direction of linearly polarized light and the use of lock-in techniques allows the simultaneous detection of absorption together with magnitude and orientation of optical anisotropy.

We have studied glasslike cholesteric liquid crystals by means of scanning near-field optical microscopy. A periodic modulation of the optical polarization was used in order to achieve an optical contrast due to the birefringence of the liquid crystal. Variation of the cholesteric pitch indicates an optical resolution of ≈200 nm for a wavelength of 633 nm.

Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland.

Recent advances in the understanding of light propagation in small dimensions as well as in instrumentation make scanning near-field optical microscopy (SNOM) a very promising tool for studying optical phenomena on a nanometer scale.In this talk, we will demonstrate experiments carried out with the recently developed tunneling near-field optical microscope. We found superior image contrast, as compared with images taken with conventional aperture SNOM, along with the high resolution commonly achieved with fiber probes. This work was motivated by the theoretical investigations presented in Dr. Pohl's talk.We will further describe two recently built instruments. The first is a scanning tunneling optical microscope combined with a scanning force microscope. The second instrument is an aperture-type SNOM mounted on the sample stage of a conventional inverted optical microscope. Of particular interest to us is imaging with polarization contrast. One of the goals is to study liquid-crystal films which have been micropatterned with the help of a force microscope tip. These samples are promising as waveguides and potential electro-optical devices. Additionally, they represent very convenient test samples for polarization SNOM.

The concepts of near-field optical microscopy and experimental and theoretical work carried out in Switzerland over the last 10 years are reviewed. After a description of the pioneering experiments of the mid-1980s, we focus on the recent efforts of the three Swiss laboratories currently working in the field in close collaboration. This newly refreshed initiative in near-field optics is supported by the Swiss Priority Program Optique.