Applications of near-field optical microscopy
Since near-field optical microscopy can overcome the shortcomings of traditional optical microscopy such as low resolution and the damage to biological samples caused by scanning electron microscopy and scanning tunneling microscopy, it has been more and more widely used, especially in the fields of biomedicine as well as nanomaterials and microelectronics.
Scanning near-field optical microscopy (SNIM) is a branch of SNOM, which is an application of SNOM technology in the infrared field. In order to obtain high-resolution information, microprobes for localization, scanning, and near-field probing 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 from the fiber optic probe to the sample under test is certain, the size of the through-hole of the fiber optic probe and the shape of the cone angle of the tip determine the resolution, sensitivity and transmission efficiency of SNIM. However, it is difficult to make infrared fiber for SNIM and microprobe. Compared with the preparation of fiber optic probes in the visible wavelength band, on the one hand, there are too few types of optical fibers suitable for the mid-infrared wavelength band (2.5-25 mm); on the other hand, the existing infrared optical fibers are brittle, with poor ductility and flexibility, and with unsatisfactory chemical properties. In order to reduce the light attenuation, it is difficult to make a high-quality infrared fiber probe.
Some foreign research SNIM institutions in the probe used in other ways of light probe, such as Japan's Kawata and other development of spherical prism probe, Germany's Fischer and other tetrahedral probe, and zui recently KNOLL and other use of semiconductors (eg, silicon) polymers made of non-porous scattering probes and so on. The above microprobe solution is unlikely for us, because of the high level of production process required, requiring specialized equipment, and due to our SNIM design chooses the reflection mode, zui finally adopted the fiber optic probe solution.
In the microprobe development process, there are two aspects to consider: on the one hand, it is necessary to make the optical probe through the small hole as small as possible, on the other hand, to make the light flow through the small hole as large as possible, in order to obtain a high signal-to-noise ratio. For fiber optic probes, the smaller the diameter of the needle part, the higher the resolution, but the luminous flux will become smaller. At the same time, the probe tip part of the shorter the better, because the longer the tip, the light propagation through a waveguide smaller than its wavelength is also the farther, so that the light attenuation is the greater. Therefore, the fiber optic probe production in the pursuit of the goal is to obtain a small needle size and the tip of the short tip.
