SLIDE NO. = 2/14

In the "traditional" optics used for conventional spectroscopy, we employ the familiar properties of light propagating through space. Lens or mirror systems are used to focus and condense the light. Here, light is treated as "wave", and so diffraction restricts the minimum spot size of a condensed light beam to about its wavelength. This also implies that the spatial resolution of conventional optics cannot exceed the wavelength, a concept that is called the "diffraction limit" or the "Abbe limit". However, near-field optics effectively transcends this diffraction limit because the wave-like propagation of light no longer determines the optical system. By inserting a sub-wavelength physical aperture into the optics, near-field light is localized near the aperture.

Near-field light (also called the "evanescent field") possesses some unique and interesting features:

1) the wavenumber vector is perpendicular to the pointing vector, thus, near-field light cannot propagate through space by itself.
2) the size of the near-field depends only on the aperture size and not on the wavelength. Note that, "far-field" normal light can pass through sub-wavelength apertures, but at intensities too low to employ in spectroscopy.

The principle merit of using near-field light for spectroscopy is mainly due to point 2) above. Point 1) is potentially a demerit of the technique because it means that conventional detectors cannot be used. However, we can circumvent this problem by scattering the near-field by the sample and thus converting to far-field "conventional" light. Point 2) also brings another problem: to illuminate the sample by near-field light, we must control the aperture-probe separation at the size of the aperture. (See slide 8)