Entwicklung eines optischen Tieftemperatur-Raster-Nahfeld-Mikroskops

Within the frame of this work a scanning near-field optical microscope (SNOM) for investigations under ultrahigh vacuum conditions, strong magnetic fields and cryogenic temperatures was developed. The microscope offers force microscopy with shear-force detection as well as near-field and confocal investigations in the visible spectral region. A spectrograph with a liquid nitrogen cooled CCD-camera is attached to the optics allowing spectroscopic experiments on specific positions on the sample surface. Due to its suitability for cryogenic experiments in vacuum environment, a parabolic mirror with a high Numerical Aperture (NA ≈ 1) is used for illumination of a small spot (≈ 0.73 ·λ) on the sample surface. The mirror is also used for collecting of the scattered or emitted radiation. For coarse positioning of the sample and the approach of the tip, stepper motors were built. In cooperation with Physik Instrumente, Waldbronn, a 3-axis scanning stage was developed to ensure a linearized and reproducible fine positioning of the sample. The stage offers a scan range of 27 µm in x- and y-direction and 2 µm in z-direction at cryogenic temperatures (8.5 K). As an application of the microscope, the temperature dependence of the optical response from Rhodamine 6G (R6G) dye molecules dispersed on silver nano-particles or nano clusters were investigated. For optical excitation of the molecules, either an aperture probe or a focused laser spot in confocal arrangement were employed. Raman spectra were recorded in the wave number range between 300 cm −1 and 3000 cm −1 at room temperature down to 8.5 K. Many of the observed Raman lines can be associated with the structure of the adsorbed molecule. Intensity fluctuations in spectral sequences were observed in the confocal arrangement down to 77K indicating a single-molecule detection sensitivity of the apparatus.