Principles of optical microscopy in the near field
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.
Near-field optical microscopy uses a probe to scan point by point on the sample surface and record it point by point before digital imaging. Figure 1 is the imaging principle diagram of a near-field optical microscope. In the figure, the x-y-z rough approximation method can adjust the distance between the probe and the sample with an accuracy of tens of nanometers; while the x-y scanning and z control can control the probe scanning and the feedback follow-up in the z direction with an accuracy of 1nm. The incident laser in the figure is introduced into the probe through an optical fiber, and the polarization 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 signal and reflection signal modulated by the sample, which are amplified by the photomultiplier tube, and then directly converted from analog to digital and then collected by a computer or entered into the spectrometer through a spectroscopic system to obtain the spectrum. information. System control, data collection, image display and data processing are all completed by computers. It can be seen from the above imaging process that near-field optical microscopes can collect three types of information at the same time, namely the surface morphology of the sample, near-field optical signals and spectral signals.
