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A far-infrared rotating-analyzer ellipsometer which uses a step-tunable, optically pumped gas laser as its light source is described herein. As polarizers novel metal grids with 10 000:1 polarization contrast were used. The instrument determines the complex dielectric function in the spectral range between 10 and 150 cm−1. A cryostat allows both reflection and transmission measurements from 10 to 330 K. Measurements of the birefringence of crystalline quartz, of both the carrier density and the scattering frequency of doped semiconductors, and of the low energy excitations of high-TC YBaCuO ceramics are presented herein.

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According to classical optics the spatial resolution that can be obtained in microscopy is limited by diffraction, to about half the illumination wavelength λ. Scanning near-field microscopy (SNOM) defeats this limitation by exploiting the evanescent field propagated through a subwavelength aperture of a tapered fiber tip, yielding a spatial resolution approximately equal to the aperture diameter. However, the resolution achievable remains limited by power loss as the aperture becomes smaller than λ/10. This is a severe restriction, especially when doing microscopy with mid-infrared wavelengths.Scattering-type SNOM (s-SNOM) uses the optical near-field interaction between an illuminated metal or dielectric probe tip and the sample surface [1][2][3][4][5]. Its spatial resolution is not limited by diffraction but rather by the actual size of the scattering probe tip (< 20 nm). We show that it is possible to distinguish between material classes at a resolution of approx. 30 nm already in a singlewavelength experiment (Figure 1). We image a three-component test-sample (Au, Si ,PS) at two widely separated wavelengths. Our results are evidence that the imaging process of s-SNOM is wavelength-independent, namely that the resolution is mainly determined by the tip's properties, and that the contrast is given by the refractive index of the sample. This categorizes s-SNOM contrast into the material classes of metals, semiconductors, and polymers enabling a simple, high resolution material-specific mapping of nanosystems [6,7].