Near-field scanning optical microscopy (NSOM) was developed from the conceptual stage to the point where optical images with resolution well in excess of the classical limit of diffraction could be obtained routinely. In the simplest embodiment of NSOM, the light transmitted through a submicron aperture in an opaque screen is used to form a subwavelength light source which is then scanned over the object to generate the desired superresolution image. Several different theoretical models for near -field diffraction by subwavelength structures were considered, all of which indicated that, to achieve tenth wavelength resolution with visible light, an ~500A aperture had to be placed ~200A away from the sample with a precision of ~ 200A. Consequently, the implementation of NSOM required developments in several areas: subwavelength aperture fabrication; vibration isolation; nanometer level positioning; distance regulation; and low light level detection. To demonstrate that these challenges could be met, a simple prototype capable of producing one dimensional linescans was constructed. The prototype instrument proved that features could be seen with a sharpness superior to even the theoretical limits of conventional optical systems. Superresolution fluorescence linescans were also obtained. These results justified the construction of a considerably more elaborate instrument, capable of producing two dimensional images with much greater speed, reliability, and flexibility. Improvements were made in all technical areas during the development of this microscope. Many images were generated with the new instrument. A true resolution of better than 1000A and an edge sharpness of less than 400A was obtained. A high degree of reproducibility was also demonstrated. The resolution was found to be roughly equal to the greater of the aperture size and aperture to sample separation. Several different contrast mechanisms were explored, including polarization dependent contrast. Other images indicated that further design improvements are possible. Nevertheless, the current instrument could be fruitfully applied in several areas. The results obtained thus far demonstrate that NSOM has considerable potential, because it promises to combine the high resolution of electron microscopy with the non-destructiveness, high speed, low cost, reliability, ease of use, and wealth of contrast mechanisms characteristic of conventional optical microscopy.
- Pub Date:
- VOLUMES 1 AND 2;
- Physics: Optics; Biophysics: General; Engineering: General