In Scanning Near-field Optical Microscopy, the excitation laser light is focused through an aperture with a diameter smaller than the excitation wavelength, resulting in an evanescent field (or near-field) on the far side of the aperture.
When the sample is scanned at a small distance below the aperture the optical resolution of transmitted or fluorescent light is limited only by the diameter of the aperture. The surface of the ample is scanned under the aperture and the transmitted or reflected light is detected point by point and line by line in order to generate an optical image. The aperture itself is located at the apex of a hollow pyramid on the micro-fabricated WITec SNOM cantilever. The optical resolution attainable is in the range of 50 – 100 nm.