Principles of near-field optical microscopy
Traditional optical microscopes are composed of optical lenses that can magnify objects to thousands of times to observe details. Due to the diffraction effect of light waves, it is impossible to increase the magnification infinitely because it will encounter the obstacle of the diffraction limit of light waves. Traditional optics The resolution of a microscope cannot exceed half the wavelength of light. For example, using green light with a wavelength of λ=400nm as a light source, it can only distinguish two objects that are 200nm apart. In practical applications, λ>400nm, the resolution is lower. This is because general optical observations are performed far away from the object (>>λ).
Based on the detection and imaging principles of non-radiative fields, near-field optical microscopes can break through the diffraction limit of ordinary optical microscopes and can conduct nanoscale optical imaging and nanoscale spectral research at ultra-high optical resolution.
Near-field optical microscopes are composed of probes, signal transmission devices, scanning control, signal processing and signal feedback systems. Principle of near field generation and detection: Incident light irradiates an object with many tiny structures on the surface. Under the action of the incident light field, the reflected waves generated by these structures include evanescent waves limited to the surface of the object and propagated far away. propagating waves. Evanescent waves originate from tiny structures in objects (objects smaller than the wavelength). The propagating wave comes from the rough structure of the object (objects larger than the wavelength), which does not contain any information about the fine structure of the object. If a very small scattering center is used as a nanodetector (such as a probe) and is placed close enough to the surface of the object, the evanescent wave will be excited and make it emit light again. This excited light also contains undetectable evanescent waves and propagated waves that can propagate to distant locations for detection. This process completes near-field detection. The conversion between the evanescent field and the propagating field is linear, and the propagating field accurately reflects the changes in the evanescent field. If a scattering center is used to scan the surface of an object, a two-dimensional image can be obtained. According to the principle of reciprocity, the roles of the illumination light source and the nano-detector are interchanged, and the nano-light source (evanescent field) is used to illuminate the sample. Due to the scattering effect of the fine structure of the object on the illumination field, the evanescent wave is converted into a signal that can be detected at a distance. The results of the detected propagating waves are exactly the same.
