Apertureless SNOM: a new tool for nano-optics
In this thesis a new scanning near field optical microscope based on an apertureless scattering technique is introduced for resolving optical properties of surfaces with lateral resolution reaching 10 nm and better. The construction of the instrument is based on a dynamic mode operating atomic force microscope (AFM) which is coupled with a sophisticated heterodyne interferometric optical detection system. A continuous wave (cw) laser beam is focused onto the apex of the metallic or dielectric AFM tip. The backscattered light is collected and interfered with a reference beam which is slightly shifted in frequency with respect to the scattered beam. The interfering signals are detected by a fast avalanche photodiode. The result is a temporal beat modulation at the shift frequency. The scattered light consists of two parts of different spatial origin. One of them is the near field that contains information belonging to a very small vicinity of tip apex interacting with surface. The second part is the far field part which comes from parasitic scattering along the illuminated tip body and the sample surface. By demodulating the beat signal at higher harmonics of the tip vibration, the far field part can be suppressed effectively, leaving only the near field information of the surface-tip interaction. By raster scanning the sample under the AFM tip, information about the amplitude and phase of the near field belonging to the surface is obtained simultaneously with topography. This new apertureless scanning near field optical microscope (a-SNOM) features several advantages over the well-known aperture SNOM: High resolution limited essentially only by the tip apex dimension, and effective background suppression. Particular care has been taken in the operation settings of the AFM, since they are shown to be one of the sources of artifacts in the detected signal due mechanical nature of the AFM. When proper conditions are met, these mechanical interaction artifacts are minimal and the a-SNOM produces essentially only optical information. The demonstration of the a-SNOM operation on Au pattern on glass surface and Ag colloid on Si surface systems show that a high sensitivity to material contrast as well as a high spatial resolution is achieved.