Principle of Near-Image Optical Microscopy
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.
Near field optical microscopy is a digital imaging technique that involves scanning and recording a probe point by point on the surface of a sample. Figure 1 is an imaging principle diagram of a near-field optical microscope. The x-y-z coarse approximation method in the figure can adjust the distance between the probe and the sample with an accuracy of tens of nanometers; And x-y scanning and z-control can be used to control probe scanning and feedback tracking in the z-direction with a precision 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 modulated transmission signal and reflection signal of the sample, amplify them by a photomultiplier tube, and then directly convert them from analog to digital and collect them through a computer or enter the spectrometer through a spectroscopic system to obtain spectral information. System control, data acquisition, image display, and data processing are all completed by computers. From the above imaging process, it can be seen that near-field optical microscopy can simultaneously collect three types of information, namely the surface morphology of the sample, near-field optical signals, and spectral signals.
