Principi no tuvā lauka optiskā mikroskopija of tuvā lauka optiskā mikroskopija
The traditional optical microscope consists of optical lenses that can magnify an object up to thousands of times to observe the details. Due to the diffraction effect of light waves, an infinite increase in magnification is not possible because the obstacle of the diffraction limit of light waves will be encountered, and the resolution of the traditional optical microscope cannot be more than half of the wavelength of 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.
Near-field optical microscopy consists of point-by-point scanning and point-by-point recording by a probe on the surface of the sample followed by digital imaging. Figure 1 shows the imaging schematic of a near-field optical microscope. In the figure, the x-y-z coarse approximation method can adjust the distance from the probe to the sample with an accuracy of tens of nanometres; while the x-y scanning and z control can be used with an accuracy of 1nm to control the probe scanning and z direction feedback follow. The incident laser, shown in the figure, is introduced into the probe through an optical fibre, and the polarisation state of the incident light can be changed according to requirements. When the incident laser irradiates the sample, the detector can separately collect the transmission and reflection signals modulated by the sample and amplified by the photomultiplier tube, and then directly by the analogue-to-digital converter through the computer acquisition or through the spectroscopy system into the spectrometer to get the spectral information. System control, data acquisition, image display and data processing are completed by the computer. From the above imaging process, it can be seen that the near-field optical microscope can simultaneously collect three types of information, i.e., the surface morphology of the sample, the near-field optical signal and the spectral signal.
