## 1 Introduction  
Imaging of nanoscale features has been a long standing challenge in optical microscopy. In the late 19 th century an apparent fundamental limit on the maximum resolution of an optical image was formulated based on the diffraction theory of light [1]. Although this formulation suggested that optical microscopy was limited to micron-scale objects, the diffraction limit has since been overcome through the development of super resolution techniques operating in the far-field [1; 2; 3; 4; 5; 6; 7]. Far-field super resolution strategies make use of enhanced contrast using a secondary process such as fluorescence or scattering to image subwavelength nanostructures. Fluorescence imaging was one of the first super resolution techniques to be developed. It utilizes fluorescence emitted from a fluorophore used to mark a sample rather than diffracted light, allowing for nanoscale images to be produced down to single-molecule resolution [2]. Fluorescence imaging forms the backbone of super resolution techniques capable of nanoscale imaging, that includes stimulated emission depletion (STED) microscopy, photo-activated localization microscopy, structured illumination spectroscopy, and stochastic optical reconstruction microscopy (STORM) [2; 6; 7]. STED is a variation of fluorescence imaging that achieves higher resolution by selectively quenching fluorophores used for labeling. Through judicious choice of fluorophore and excitation laser, resolution in the tens of nanometerscan be achieved [2]. STORM employs a variation of STED utilizing time-resolved selective switching of fluorophores. Selective switching in this manner is used to collect a series of images that can be combined to produce images with nanometer resolution. Through careful choice of measurement optics, STORM can also be used to detect fluorescence from below a sample surface to construct 3D images [7]. The major drawback to any fluorescence based technique is the potential for photo bleaching and sample damage brought on by the requirement for highly focused light sources. Diffraction limited resolution can be avoided without the need for sample labeling, and the dangers of photo bleaching though the use of scattering techniques such as optical coherence tomography (OCT) which can achieve sub-micron resolution [3; 4; 5]. Researchers realized that OCT could be enhanced with interferometry, which combines a series of images to produce an interference pattern containing information about sub-wavelength surface features. OCT typically employs longer wavelength light which has the added benefit of penetrating deeply into biological tissue. This gives OCT the ability to collect 3D images at depths of hundreds of microns beneath a sample's surface [4]. Light scattering techniques of this type are advantageous over fluorescence in that no labeling of the samples are necessary, but at the cost of lower resolving power. Scanning near-field microscopy (SNOM or NSOM) is the most recently developed super resolution technique based on light scattering that is also capable of obtaining nanoscale images. SNOM is a photon-in photon-out nano-optical imaging technique that uses near-field light to construct nanoscale images. To make use of the near-field, a nanoscale probe is brought into the near-field of a surface or object to be investigated, causing near-field evanescent waves to be scattered into the far-field to be detected. Evanescent light experiences an exponential decay in the propagation direction away from the originating surface that has the consequence of localizing the near-field to a volume within \(\sim\)100 nm nanometers of the originating emitter. This localization produces a high intensity near-field very near the surface, which contains information about the surface properties beyond the diffraction-limited maximum resolution of the far-field.