*2.4.2 Lateral nanoscopic localization*

A near-field geometry in which the illuminated sample is located within a fraction of the wavelength of light emitted by the source or aperture, can be used to achieve lateral nanoscale light confinement [8, 10] and an electric field propagation around a nanoscopic system generates spatially localized optical interactions. Further, owing to the virtual values of wavevector-like characteristics, the distribution of the spatially localized electric field has a significant evanescent form, that is, it decays exponentially. To study near-field geometry, a near-field scanning optical microscope (NSOM) with aperture based and appertureless configurations may be used, and an aperture-based NSOM uses a submicron-sized 50–100 nm aperture, near to the tapered optical fiber's opening tip, utilized to keep light confined, while an apertureless structure maximizes the local field by a nanoscopic metal tip or a metallic nanoparticle [11].
