Overview and applications of scanning near-field optical microscopy
Because near-field optical microscopy can overcome the shortcomings of traditional optical microscopes such as low resolution and damage to biological samples by scanning electron microscopes and scanning tunneling microscopes, it has become more and more widely used, especially in biomedicine, nanomaterials and microelectronics. fields of study.
Scanning near-field optical microscopy (SNIM) is a branch of SNOM and an application of SNOM technology in the infrared field. In order to obtain high-resolution information, microprobes used for positioning, scanning, and near-field detection are very critical parts of SNIM. There are many forms of microprobes, which are roughly divided into two categories: small hole probes and non-hole probes, and small hole probes are often fiber optic probes. When the distance between the optical fiber probe and the sample being measured is constant, the size of the light-passing hole of the optical fiber probe and the shape of the cone angle of the tip determine the resolution, sensitivity and transmission efficiency of SNIM. But it is more difficult to make infrared optical fibers for SNIM and microprobes. Compared with the preparation of optical fiber probes in the visible light band, on the one hand, there are too few types of optical fibers suitable for the mid-infrared band (2.5~25mm); on the other hand, existing infrared optical fibers are relatively brittle and have poor ductility and flexibility. And the chemical properties are not ideal. In order to reduce light attenuation, it is difficult to make high-quality infrared optical fiber probes.
Some foreign institutions researching SNIM have adopted other forms of optical probes in probes, such as the spherical prism probe developed by Kawata and others in Japan, the tetrahedral probe developed by Fischer and others in Germany, and most recently, KNOLL and others using semiconductors (such as Non-porous scattering probes made of silicon) polymers, etc. The above-mentioned microprobe solution is impossible for us because it requires a high level of manufacturing technology and requires specialized equipment. And because our SNIM design chose the reflection mode, we finally adopted the optical fiber probe solution. .
In the development process of microprobes, two aspects must be considered: on the one hand, the light-passing aperture of the optical probe must be made as small as possible; on the other hand, the light flux through the light-passing aperture must be as small as possible. large to obtain a high signal-to-noise ratio. For fiber optic probes, the smaller the diameter of the needle, the higher the resolution, but the light transmittance will become smaller. At the same time, it is required that the cone tip of the probe be as short as possible, because the longer the cone tip is, the farther the light will propagate through a waveguide smaller than its wavelength, so the light attenuation will be greater. Therefore, the goal pursued in the production of fiber optic probes is to obtain a needle tip with a small needle size and a short taper tip.
