Spectroscopic Near-Field Scanning Optical Tomography

 

The development of near-field scanning optical microscopy (NSOT) has been driven by the need for an imaging technique that retains the various contrast mechanisms afforded by optical microscopy methods while attaining spatial resolution beyond the classical optical diffraction limit.

NSOT, as an extension of near-field scanning optical microscopy (NSOM), amounts to inverting the scattering data collected by the NSOM and reconstructing the sample, instead of taking the raw data as an image of the sample. This invention is a new method to simplify realization of NSOT and reduce mechanical and other noises. This invention also proposes an alternative NSOT modality to improve the experimental feasibility of NSOT.

For ultra-high optical resolution, near-field scanning optical microscopy (NSOM) is currently the photonic instrument of choice. Near-field imaging occurs when a sub-micron optical probe is positioned a very short distance from the sample and light is transmitted through a small aperture at the tip of this probe. The near-field is defined as the region above a surface with dimensions less than a single wavelength of the light incident on the surface. Within the near-field region evanescent light is not diffraction limited and nanometer spatial resolution is possible. This phenomenon enables non-diffraction limited imaging and spectroscopy of a sample that is simply not possible with conventional optical imaging techniques.

In addition, rather than using multiple observation angles or multiple probes, it is possible to collect data in a third dimension using the spectral degree of freedom. The idea of constructing an image in N spatial dimensions by collecting data in (N - 1) spatial dimensions and a spectral dimension has found application in techniques such as optical coherence tomography and synthetic aperture radar.

Applications

  • Any high magnification microscopy, with requirements short of a scanning electron microscope.

Benefits 

  • This method clarifies the ambiguity between the sample and its NSOM image, and by inverting multiple NSOM images of a three-dimensional sample, a three-dimensional tomography of the sample may be obtained.
  • Multiple images corresponding to different wavelengths can be collected from each probe scan and are therefore inherently co-registered.¬†