The Refractive Index Scaling And To Improve The Scanning Speed And The Contrast In SNOM

With respect that the theoretical resolutions limit of conventional optical microscopy is 200 to 300 nanometers. The scanning near-field optical microscopy (SNOM) had broken this limit. Our institute had successfully developed an atomic force combined with photon scanning tunneling microscopy (AF/PSTM) prototype in 2002, and later an atomic force combined with reflection scanning near-field optical microscopy (AF/RSNOM) prototype has also been developed in 2003. However, the absolute value of the sample's refractive index in AF/PSTM can not be acquired and the scanning speed is slow. Furthermore the contrast should be improved in AF/RSNOM. This dissertation first summarizes the earlier research in SNOMs of our group, and then highlights the following three innovative works:First is to scale the sample's refractive index by solving six equations which are abtained by adjusting the intensity of the two illumination lights. Second is to exploit the frequency demodulation circuits and manufacture the micro-cantilever/optical fiber tip to improve the scanning speed and sensitivity which are two key perfermance of the AF/PSTM. Thirdly we propose a method that the tip vibrates with small amplitude and the scattered light signal is filter by a PLL to eliminate the scattering background. The scattering background is a DC component. By this way the images'contrast in AF/RSNOM can be improved. The above three major improvements have been proved to be successful by experiments. This dissertation mainly includes the following five parts:In the first part scanning tunneling microscopy (STM) and the atomic force microscopy (AFM) which has close relationship with the SNOM are introduced. The SNOM is introduced from the three aspects:the first is the developed history of the instrument; the second is the mechanism that the instrument can break the limitation of the diffraction and the relations between evanescent wave and sub-wavelength structure; and the third is the structure and the classification of SNOMs.The emphasis of the second part is put on the common components of the SNOM and the way to build them. The most critical part of SNOM is the microprobe which determines the quality of the image. We apply heating integrating with dynamic and static etching method to fabricate the optical fiber tips. And the tip has an apex diameter of smaller than 100nm and cone angle of 60°～90°. The hardware and software are implemented according to modular design method, which enable flexibility and scalability for the system. Imaging in SNOM is needed to deal with pre and post-processing the role of which are to improve the image quality. In hardware the sample is illuminated with two symmetry light beams to eliminate the optical spurious image, and in software convolution filter, Fourier filter and K-L filter are use to filter noises.The third part discusses the refractive index scaling of the transparent sample in near field and the building of a new type of experimental AF/PSTM to realize this idea. In theory, we deduce if we six intensities of the evanescent field can be detected (acquiring six irrelevant equations) the refractive index of samples can be scaled. In order to acquire equations satisfied the requirement, we design a scheme to change the intensity of the illumination with the tip's position. In this microscopy, the illumination intensity is controlled by a feedback loop, the vertical vibrating tip scans the sample with a constant distance and the sample is illuminated by two symmetry light beams. Altering of the illumination intensity and the sampling of the local intensity are controlled by DSP to be synchronized with the vibration of the tip. A group of extreme values of the tunneling light are obtained in one oscillation period. In another oscillation period, changing of any illumination beam will modify the near field distribution. Then, another group of extreme values of the tunneling light will be obtained. The refractive index can be computed from groups of extreme values that obtained from multi-oscillation periods. From the experiment it can be concluded that the instrument not only can measure samples' refractive index effectively but also acquire the angle of inclination of the object.The fourth part introduces the method to decrease the scanning time and improve the sensitivity of the AF/PSTM. The probe of the SNOM operates in tapping-mode and there are two governing modes to do distance regulations between the tip and the sample, one is the amplitude-modulated (AM) mode and the other is the frequency-modulated (FM) mode. In AM mode the stable time of the instrument with hight quality factor tip is increased and the scanning speed is dropped. The new experimental AF/PSTM operates with FM mode and the Q of the tip does no affect on the system, the scanning speed of the instrument with this mode increases comparing that with AM mode. Then we design a positive feedback system to cause the probe vibrating at its resonant frequency. Furthermore the circuit structure of the instrument and the method to design the phase loop (PLL) which is used to detect the frequency of the system are introduced. The experiment shows using FM mode the AF/PSTM's bandwidth is about 50Hz, and it is an order of magnitude larger than that attained by using AM mode. In order to reduce the response time of the SNOM and improve its sensitivity at the same time, we manufactured a piezoelectric micro-cantilever composed with optical fiber tip. The micro-machine techniques to fabricate the piezoelectric micro-cantilever and the process to assemble the combined probe are presented. The combined probe has a resonant frequency of about 80kHz with a Q factor of 1500. The combined probe which has high quality factor was used in a photon scanning tunneling microscopy to observe the erythrocytes membrane sample, and it can be proved the system with this combined probe has higher sensitivity than that with the optical fiber probe inspirited by bimorph.Methods to increase the images'signal to noise ratio in the atomic force combined with reflection scanning near-field optical microscopy (AF/RSNOM) are presented in the fifth part. The new AF/RSNOM in our experiment uses a beam splitter with metal film and the angle of the film is about 45 degree. The scattering background in the new instrument is smaller than that in our earlier instrument who used a Y-type beam splitter. The scattering background is a DC component, if the tip vibrates with a small amplitude and the evanescent wave collected goes throught a phase lock loop (equal to a bandpass filter whose center frequency is equal with the vibrating frequency of the tip), the scattering background can be eliminated. An ion etched metal grating was imaged by the new RSNOM and it has been proved that the images'signal to noise ratio increased.At the end of this dissertation we summary the achievement of our research, put forward prospect and point out the problems and the method to resolve them.