Development of a near-field scanning optical microscope and its application in studying the optical mode localization of self-affine silver colloidal films

A scanning photon tunneling microscope (PSTM) combined with shear-force atomic microscope (AFM) was designed and built, which allows the topography and the optical response of a surface to be investigated simultaneously on a sub-microscopic scale. The lateral resolution attained by our instrument in topographic images is 30 to 50 nm, varying with the fiber tip. While operating in the near-field region, the optical resolution can overcome the traditional diffraction limit, producing a resolution typically of about 100 run. Thus, it is possible to obtain site-specific spectroscopic information. The system was employed to study the optical behavior of fractal silver colloidal films.; Near-field microscopy with simultaneous topographic imaging show that the electromagnetic fields excited in a self-affine film are localized in small portions of the film. The pattern of high-field areas varies greatly with excitation wavelength and does not correlate simply with the topography.; Near-field spectroscopic study further complemented the microscopic study. Tunable CW lasers were used to excite the silver colloidal thin films. When scanning the incident laser, site specific spectra showed that distinct resonance arises from different spots apart on the same sample.; This observation validates a recent theory of the optical response of self-affine, fractal objects and also indicates that the locus of optical effects, such as surface enhanced Raman, does not reside in special topographic elements such as interstices. Additionally, it is shown that the conclusions of the theory, which was based on a quasi-static approximation, are qualitatively valid for self-affine films considerably larger than the exciting optical wavelength.