Apertureless Coherent Interferometry

Apertureless Coherent Interferometry

Since the introduction of SPM, many different techniques have been invented. Among them is the Near-Field Scanning Optical Microscopy (NSOM) which was first demonstrated by Prof. A. Lewis (CEO and Founder of Nanonics) to image beyond the diffraction resolution limit.

It the classical approach, the light is confined within a cone and illuminated onto the sample through an aperture smaller than the wavelength of light hence the illumination is in the near field.

In the apertureless (aNSOM) approach, a laser spot illuminates the sample. A small metallic tip in the near vicinity of the sample enhances the localized electrical field. The tip scatters the light according to its polarizability. The presence of the sample under the metallic tip modifies the polarizability of the tip according to the dielectric function of the sample. A signal is then scattered and collected in the far-field.

In order to distinguish between the background scattering and the near-field scattering, the tip is kept oscillating above the sample in the tapping mode operation, and the near-field signal is extracted at the resonance frequency harmonics using an interferometer and a lock-in amplifier. This is due to the non-linear characteristics of the near-field in comparison to the elastic scattering of the non-near-field background signal.

By scanning the metallic probe over the sample with an AFM feedback in intermittent contact mode, one can use the scattered far-field signal, obtained through lock-in detection, to form a correlated image of topography and near-field characteristics.

Apertureless NSOM is particularly advantageous method for infrared light source & THz optical frequencies, high resolution surface imaging.

Apertureless NSOM has been successfully applied to imaging of various features in condensed phase materials, including the metal-to-insulator transition of correlated electronic materials, metallic plasmonic structures, surface phonon polaritons, graphene plasmons, peptide and proteins and many other applications.

Our Apertureless technique can be combined inside a Cryogenic or High Vacuum chamber along with simultaneous Raman & Femto-second Imaging.

 

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