Implementation of an Apertureless Scanning Near-Field Optical Mircroscope for the Infrared Spectrum

In this thesis, a new apertureless scanning near-field optical microscope (aSNOM) with the ability to resolve optical properties of surfaces with a lateral resolution better than 50 nm in a broadband infrared (IR) spectrum is described. The high resolution beyond the diffraction limit is achieved by using the sharp tip of an atomic force microscope (AFM) to probe the local electromagnetic field of a sample situated at nanometer distance. A continuous wave (cw) laser beam is focused onto the apex of a metalized or dielectric AFM tip with an apex radius of about 10 nm. The backscattered light is mixed with a reference beam of the same wavelength and detected with a nitrogen cooled InSb-photovoltaic diode. This interferometric scheme allows to enhance the signal level and to detect both the amplitude and phase of the scattered light. The obtained signal is influenced by two components: the light scattered in a small vicinity of the tip apex containing near-field information from the sample, and a background field scattered from the body of the tip and from the sample. To suppress the background component, the AFM is used in non-contact mode with a small vibration amplitude, and the signal is demodulated at higher harmonics of the tip vibration frequency with a lock-in amplifier. By raster scanning the sample under the AFM tip, near-field information from the sample is obtained simultaneously with the topography. Imaging in a broad infrared spectrum is achieved by using a cw optical parametric oscillator (cw-OPO) as light source. The OPO has been carefully characterized and gives the instrument the capability to image in a spectral region from 1.5 to 3.8 μm with a small gap between 2.0 and 2.3 μm. By using spherical mirrors instead of lenses, the setup is nearly achromatic. As applications of the infrared aSNOM (IR-aSNOM), images of two different sub-wavelength gold structures on glass surfaces were taken. The first is a test sample that shows the capability of the instrument to image material contrast with a lateral resolution better than 50 nm, corresponding to λ/50. The second application shown is the investigation of split-ring resonators, which are of interest for the study of negative index materials. Maps of optical near-fields at the resonantly excited plasmonic structures are shown — with similar resolution.