Micro-Machining by Nearfield Optics

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The probe of this scanning nearfield optical microscope (SNOM) is a dielectric sphere 500nm 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 632.8nm 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 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 detected light intensity. This result is explained through use of an electric dipole model for the electromagnetic interaction betweem probe and sample. The vertical and lateral resolutions of about 1nm and 10nm, respectively, are obtained for a standard sample which is prepared by vacuum evaporation of metal.We develop an SNOM that achieves high resolution which is not affected by a diffraction limit and that uses a polystirene latex sphere 500nm in diameter located on a quartz substrate as a probe. To investigate the spatial resolution of the SNOM,we prepared a standard sample and measured the shape of its surface. As a result, the vertical resolution of SNOM was found to be about 1nm and the lateral resolution about 10nm. Therefore the spatial resolution of the SNOM is much higher than that of a conventional optical microscope.Although the limit of the spatial resolution of the SNOM has not been theoretically determined, it will depend on the technique by which the probe is produced. Future improvement of the spatial resolution will require reliable and reproducible fabrication technology in order to produce a probe with a smaller tip curvature radius.