## 3 Aperture-Type SNOM  
### c-SNOM Spectroscopy  
In aperture-type near-field microscopy, near-field radiation can be collected through the probe tip where the size and shape of the aperture, as well as the composition of the tip can be tailored towards the specific application over a wide range of wavelengths (200-1700 nm). This allows a host of  
Figure 1: The near-field and types of SNOM tips. (**a**) The far and near-field microscopy arrangements are defined by the proximity of the probe to the nano object being probed. When at a distance (\(r\)) from the emitting object that is much less than the wavelength of light used, evanescent light dominates the light field; (**b**) Aperture SNOM where an evanescent light field created by a nanoscale opening at the end of a tip is scattered off of the surface. Light scattered from the surface is detected in the far-field; (**c**) Apertureless SNOM utilizes near-field light emitted from the surface due to external illumination that is scattered off of a sharp tip into the far-field to be detected.  
different spectroscopy techniques to be implemented alongside SNOM systems to allow spectroscopy and imaging at sub-wavelength resolution. Techniques with which aperture-type SNOM has been coupled include Fourier-transform infrared spectroscopy (FTIR) [8], fluorescence [16], and Raman spectroscopy [17; 18].  
The lateral resolution of fluorescence from single molecules can be enhanced through near-field imaging of strong fluorescent emitters. Fluorescence SNOM microscopy has been very successful as a contrast method to enhance resolution using transmission, reflection, or phase contrast modes [19; 20]. In Fluorescence microscopy, fluorophores such as engineered organic fluorescent molecules [21] or fluorescent proteins [22] possessing different characteristic absorption and emission wavelengths are used to label different parts of a sample. The ability to distinguish the contrast between fluorescent signals in a given sample then depends on the type of optics and detector used to collect the image. SNOM techniques have been employed as a natural extension of far-field microscopy to increase the resolution, and c-SNOM has been employed in biological, thin film, and device imaging [23; 24].  
Aperture-type SNOM has also been applied with great success in Raman spectroscopy. Although a review on SNOM Raman is beyond the scopes of this article, we cannot omit to report that much research has been focused on increasing the Raman cross-section to the single molecule detection limit. The sensitivity can be improved greatly by combining SNOM with other techniques such as coherent anti-Stokes Raman scattering (CARS), stimulated Raman scattering [25], or tip enhanced Raman scattering (TERS) [26]. More recently surface enhanced Raman scattering (SERS) has demonstrated label-free single molecule resolution [18; 27]. Coupling Raman spectroscopy with c-SNOM systems overcomes the limitations of the other sensitivity enhancing techniques and can achieve nanoscale-SNOM enhanced Raman spectroscopy [17]. Nano Raman is a c-SNOM technique making use of aperture type tips placed within the near-field of a sample to be imaged. This technique has been used to study inorganic crystals, polymer thin films, liquid-liquid interfaces at the nanoscale, as well as mapping of single molecules on surfaces [17] an investigation that is not possible with conventional far-field Raman.