NSOT: Near-field Scanning Optical Tomography

Overview:

Near-field optical microscopy has developed dramatically in recent years. The first proposal of a method to circumvent the Rayleigh-Abbe resolution limit was put forward by Synge in 1928. Synge proposed that a thin sample be illuminated through a subwavelength aperture. By recording the transmitted light as a function of aperture position, a subwavelength resolved image of the sample may be acquired. Today this method is known as near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM); it is practiced in many variations including the reciprocal arrangement in which the sample is illuminated by a source in the far zone of the sample and light is collected through a small aperture. The role of the small aperture is now played by the tip of a tapered optical fiber, a technique not known to Synge.

NSOM has attracted considerable attention as a technique to obtain images of surfaces with subwavelength resolution. This achievement is particularly important for imaging structures where spectroscopic concerns or sample handling requirements dictate the use of lower frequency fields and yet high spatial resolution is still required. Applications range from the inspection of organic and biological samples to semiconductors. Various experimental modalities are in practical use. Two prominent examples are collection mode NSOM and illumination mode NSOM. In illumination mode NSOM, a tapered fiber probe with a sub-wavelength size aperture serves as a source of illumination in the near-zone of the sample. The scattered field intensity is then measured and recorded as a function of the probe position while the probe is scanned over the sample. In collection mode NSOM, the fiber probe serves to detect the total field in the near-zone as the sample is illuminated by a source in the far zone.

There are certain limitations of NSOM as currently practiced. Despite the fact that the sample may present a complicated three-dimensional structure, NSOM produces only a two-dimensional image. Indeed, rather than being an imaging method, it is more accurate to say that NSOM maps the sub-wavelength structure of the optical near-field intensity in some plane above the sample. Under certain simplifying assumptions, such as homogeneity of the bulk optical properties of the sample, the images produced in these experiments may be related to the sample structure. However, for the more general case in which the topography of the sample and the bulk optical properties both vary, the relationship between the near-field intensity and the sample structure has proven ambiguous.

Essential to the near-field modality of NSOM is the presence of inhomogeneous, or evanescent, modes of the illumination field. Specifically, the illuminating field consists of a superposition of plane waves including the high spatial frequency evanescent plane waves. These waves are super-oscillatory parallel to some reference plane and are exponentially decaying away from the plane.

To resolve the ambiguity in near-field images it is desirable to solve the near-field inverse scattering problem. By solving the ISP two issues are resolved. The ambiguity in the relationship between the sample properties and the measured data is removed, and simultaneously three-dimensional, tomographic images of the sample are obtained. The inverse problem is ill-posed, in particular the reconstructed image changes dramatically with small changes in the data. However, the inverse kernel may be regularized and meaningful reconstructions computed as has been shown within the framework of a scalar model for a variety of modalities.

References:

• P S Carney and J C Schotland, "Inverse scattering for near-field microscopy," Appl. Phys. Lett. 77, 2798-2800 (2000). PDF


• P S Carney and J C Schotland, "Near-field tomography," in Inside Out, Gunther Uhlman, ed. (Cambridge University Press, Cambridge, 2003). PDF