Principles of optical microscopy in the near-field
The optical microscope of the principle of near-field optical microscope consists of optical lenses, which can magnify the object up to thousands of times to observe the details. Due to the diffraction effect of light waves, it is impossible to increase the magnification indefinitely because the obstacle of the diffraction limit of light waves will be encountered, and the resolution of the traditional optical microscope can not be more than half of the wavelength of the light. For example, with a wavelength of λ = 400nm of green light as a light source, can only distinguish between two objects that are 200nm apart. In practice λ>400nm, the resolution is somewhat lower. This is due to the fact that optical observation in general is made at a great distance from the object (>>λ).
Near-field optical microscopy, based on the principle of non-radiation field probing and imaging, is able to break through the diffraction limit to which ordinary optical microscopes are subjected, allowing nanoscale optical imaging and nanoscale spectroscopic studies to be carried out at ultra-high optical resolution.
Near-field optical microscope consists of probe, signal transmission device, scanning control, signal processing and signal feedback system. Near-field generation and detection principle: incident light irradiation to the surface of the object with many tiny microstructures, these microstructures in the role of the incident light field, the resulting reflected wave contains a sudden wave confined to the surface of the object and propagation waves to the distance. Sudden waves come from the fine structures in the object (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 centre is used as a nanodetector (e.g. a probe), placed close enough to the surface of the object to excite the swift wave, causing it to emit light again. The light produced by this excitation also contains undetectable swift waves and propagating waves that can be propagated to distant detections, and this process completes the detection of the near field. The transition between the swift field and the propagating field is linear, and the propagating field accurately reflects the changes in the hidden field. If a scattering centre is used to scan over the surface of an object, a two-dimensional image can be obtained. According to the principle of reciprocity, the roles of the irradiating light source and the nano-detector are switched with each other, and the sample is irradiated with a nano-light source (abrupt field), and due to the scattering of the irradiating field by the fine structure of the object, the abrupt wave is converted into a propagating wave which can be detected at a distance, and the result is exactly the same.
