Applications of Near-field Optical Microscopy:
Due to its ability to overcome the low resolution of traditional optical microscopes and the damage caused to biological samples by scanning electron microscopes and scanning tunneling microscopes, near-field optical microscopes have been increasingly widely used, especially in biomedical, nanomaterial, and microelectronics fields.
Scanning Near Field Optical Microscopy (SNIM) is a branch of SNOM, which is the application of SNOM technology in the infrared field. Microprobes used for positioning, scanning, and near-field detection are crucial components in SNIM for obtaining high-resolution information. There are many forms of microprobes, roughly divided into two categories: small hole probes and non hole probes, with small hole probes often being fiber optic probes. When the distance between the fiber optic probe and the measured sample is constant, the size of the optical aperture of the fiber optic probe and the cone angle shape of the needle tip determine the resolution, sensitivity, and transmission efficiency of SNIM. However, it is quite difficult to make infrared optical fibers for SNIM and microprobes. Compared with the preparation of fiber probes in the visible light band, on the one hand, there are too few types of fibers suitable for the mid infrared band (2.5-25mm); On the other hand, existing infrared optical fibers are relatively brittle, with poor ductility and flexibility, and their chemical properties are not ideal. It is quite difficult to produce high-quality infrared fiber probes to reduce light attenuation.
Some foreign institutions studying SNIM have adopted other methods of optical probes in terms of probes, such as the spherical prism probe developed by Kawata et al. in Japan, the tetrahedral probe developed by Fischer et al. in Germany, and the most recent non porous scattering probe made of semiconductor (such as silicon) polymers, such as KNOLL. The above microprobe solution is unlikely for us because it requires a high level of manufacturing technology and specialized equipment. Additionally, due to the reflective mode chosen in our SNIM design, we ultimately adopted the fiber optic probe solution.
In the development process of microprobes, two aspects need to be considered: on the one hand, it is necessary to make the light hole of the optical probe as small as possible, and on the other hand, it is necessary to make the light flow through the light hole as large as possible to achieve high signal-to-noise ratio. For fiber optic probes, the smaller the diameter of the needle, the higher the resolution, but the transmittance will decrease. At the same time, it is required that the tip of the probe be as short as possible, because the longer the tip, the farther the light propagates through a waveguide smaller than its wavelength, resulting in greater light attenuation. So, the goal pursued in the production of fiber optic probes is to obtain a needle tip with a small needle size and a short cone tip.
