Near-field optical microscopy Principles of near-field optical microscopy
Traditional optical microscopes are composed of optical lenses that can magnify objects up to thousands of times to observe details. Due to the diffraction effect of light waves, it is impossible to infinitely increase the magnification because it will encounter the obstacle of the diffraction limit of light waves. The resolution of traditional optical microscopes cannot exceed half of the wavelength of light. For example, using green light with a wavelength of λ=400nm as the light source, only two objects separated by 200nm can be distinguished. In practical applications, when λ>400nm, the resolution is lower. This is because general optical observations are made at a distance (>>λ) from the object.
Near field optical microscopy, based on the detection and imaging principles of non radiative fields, can break through the diffraction limit of ordinary optical microscopes and perform nanoscale optical imaging and spectroscopy research at ultra-high optical resolution.
A near-field optical microscope consists of a probe, signal transmission device, scanning control, signal processing, and signal feedback system. Principle of near-field generation and detection: When incident light shines on an object with many tiny and fine structures on its surface, the reflected waves generated by these fine structures under the action of the incident light field include evanescent waves confined to the surface of the object and propagating waves that propagate to a distance. Evanescent waves come from subtle structures in objects (objects smaller than the wavelength). And the propagating waves come from the rough structures in the object (objects larger than the wavelength), which do not contain any information about the subtle structures of the object. If a very small scattering center is used as a nanodetector (such as a probe) and placed close enough to the surface of an object, the evanescent wave will be excited, causing it to emit light again. The light generated by this excitation also contains undetectable evanescent waves and propagating waves that can be detected at a distance, completing the near-field detection process. The conversion between the evanescent field and the propagation field is linear, and the propagation field accurately reflects the changes in the evanescent field. If a scattering center is scanned on the surface of an object, a two-dimensional image can be obtained. According to the principle of reciprocity, the interaction between the illumination light source and the nano detector is swapped. A nano light source (evanescent field) is used to illuminate the sample. Due to the scattering effect of the object's fine structure on the illumination field, the evanescent wave is converted into a propagating wave that can be detected at a distance, and the result is completely the same.
