Research On Tip Position Control And Signal Collection Technology Of Scanning Near-field Optical Microscope

Scanning near-field optical microscope (SNOM) is a new type of microscope that came out in the late eighties of last century. Compared with classical optical microscope, SNOM can obtain the image of sub-wavelength spatial resolution far beyond the diffraction limit. While compared with no-optical microscope, such as AFM (Atom Force Microscope), SNOM uses photos as information transmission carrier, so it scans the sample surface without touching it at all, and releases the restriction to the samples, such as electrical conductivity and the physical phase type. In addition, it can acquire rich physical and chemical characters of the samples by its optical effects such as absorbing, scattering, reflecting, polarizing and so on. Attributing to all these merits mentioned above, SNOM is widely employed in biomedicine, information technology, and some other traditional industry.After deeply analyzing the reason of limitation to classical optical microscope resolution, the principles and existing condition of the evanescent wave, which breaks through the resolution of diffraction limit, are discussed in this paper. Based on this theory, the principle of SNOM was proposed and a simple experiment was set up in this thesis. The whole system can be divided into two parts: mainframe and subordinate circuit. Mainframe completes the mission of anti-vibration, sample-tip position control, automatically scanning, and evanescent collection. While subordinate circuit aims at providing reference signal, processing electric signal and controlling the motion drivers through computer.In this work, we calculated the field distribution from a fiber probe with different diameters and cone angles by means of a two-dimensional finite different time domain (FDTD) method. The influences of different diameters and cone angles on field distribution were proposed here, providing theory foundation for choosing the proper fiber probe in latter experiment. Based on piezoelectric effect, controlling the sample-tip distance within wavelength, by shear force detection using tuning fork and piezoelectric ceramics separately as the detector, was constructed. After comparing these two methods, we chose the latter one for further use.Studying on the theory of weak signal detection and noise restraint, the optical detecting circuit using photomultiplier (PMT) and lock in amplifier (LIA) was realized. We made the optical chopper ourselves to provide reference signal for LIA. The circuit meets the requirement of the system.A simple experiment system was established and operated smoothly at last. This system controls the sample-tip distance based on shear force and collects near field signal successfully.