Development of a scanning near-field optical microscope with a probe consisting of a small spherical protrusion

The probe of the scanning near-field optical microscope (SNOM) is a dielectric sphere 500 nm in diameter on a transparent substrate. The probe sphere is illuminated by evanescent waves which are formed by the incidence of a He-Ne laser with the wavelength of 632.8 nm under the condition of total internal reflection. The light from the probe is collected by a conventional microscope through the substrate. The detected light intensity varies markedly when a sample is brought into the near-field around the probe. The variation of the detected light intensity in the near-field depends on the complex index of refraction of samples; the smaller the real part of the refractive index, the more marked the increase of the detected light intensity. This result is explained through the use of an electric dipole model for the electromagnetic interaction between the probe and sample. The vertical and lateral resolutions of about 1 nm and 10 nm, respectively, are obtained for a standard sample which is prepared by vacuum evaporation of metal.