Research On The Near-field Optics Theory And Key Techniques Of Non-probe Near-field Optical Microscope

As a new type of optical instrument, the scanning near-field optical microscope (SNOM) employs an optical probe tip to collect the evanescent wave existing in immediate proximity to the surface of the observed sample, and obtains the image of subwavelength spatial resolution far beyond the diffraction limit. Since the invention of the SNOM, it has attracted extensive attention and stimulated considerable research. Combined with the principle of SNOM and our research of the Hadamard Transform imaging, a non-probe multi-channel near-field optical microscope (NPNOM) has been proposed. This new type of microscopy offers a new approach to reduce the difficulty of the precision positioning in experiment. In this dissertation, the near-field optics theory and non-probe multi-channel near-field optical microscopy are studied systematically, which is mainly made up of two parts: the theoretical analysis and the experimental study. The theoretical analysis also includes two aspects, one is by applying macroscopical electromagnetic method without considering the interaction between the probe and the sample, and the other by microcosmic electromagnetic method with considering the interaction. In the second part of this dissertation, the experimental design of NPNOM is discussed in detail. Main achievement and creative work made in this dissertation are summarized as follows. About the theoretical research, based on the macroscopical electromagnetic theory, a theoretical model for the observed sample in the scanning near-field optical microscopy is proposed. Applying the vector diffraction theory, we have established a set of equations to describe the near-field of the sample. The complex amplitudes of reflection and transmission modes of all orders near-field distribution are obtained conveniently using perturbation theory in the case of small roughness. This offers an effective way of calculating the near-field distribution of the sample surface. The calculating results are beneficial to the comprehension of the near-field optical distributing properties of the sample surfaces. Based on the dipole diffraction theory of small circular apertures developed by Bethe and the waveguide theory, the near-field optical distributing formula of the nanometer-small circular apertures in a thick, perfectly conducting screen is developed, a rigorous near-field electromagnetic distribution of the small circular aperture is obtained. The results are significant to the understanding of the near field distributing properties of the nanometer-small apertures, and can also be directly applied to the design of mask in NPNOM and the probe tip in SNOM. A theoretical model for scanning near-field optical microscope based on the consideration of the interaction between the probe tip and the sample is proposed. In the present model, both the optical probe tip and the sample protrusions are represented by polarizable dipole spheres respectively. The induced polarization effects on the sample surface can be replaced by the image dipoles in the circumstance of quasi-electrostatic approximation. Applying the radiation theory of the dipole, we have established a set of equations to describe the field distribution at the sites of the probe tip and the sample protrusions. This set of field equations can be solved self-consistently. The results are completely the same as those by means of the dyadic electromagnetic propagator formalism and also the derivation procedure is relatively simple. This method permits us to analyze the physical mechanisms of the interaction between the probe tip and the surface in SNOM intuitively. Applying this approach, some optical effects about SNOM are discussed in detail. For example, how the imaging quality of the SNOM can be influenced by the polarization of the incident light. The configurational resonance evoked by the interaction between the probe tip and the sample in SNOM is discussed. On the basis of this approach, we further discussed the characteristic of the near-field imaging and the localized optical material resonance of the metallic sample. In the discussion of the configurational resonance in SNOM, we improved the configurational resonance theory of point dipole developed by Keller,Girard et al.. After considering the actual size of the sample dipoles and the probe dipole, we investigated how the field enhancement effects in SNOM depend on various system parameters. These researches give us an essential comprehension of the properties of the near-field optics, and also give us an indispensable instruction for subsequent experiment. About the experimental research, according to the near-field theory and the Hadamard transform imaging theory, we improved the conventional SNOM and accomplished the experimental design of NPNOM. Based on this new type near-field microscopy, we designed the experimental non-probe multi-channel near-field infrared optical microscope at the 10.6μm wavelength. The methods to determine and construct encoding matrix are discussed. The encoding designs based on cyclic S-matrix are presented. After the choice of the infrared material, we have accomplished the fine polishing and metallic coating of the substrate. Utilizing the optical lithography technology, the 9*7 encoding mask on which the dimension of mask aperture is 4μm was manufactured. A coarse -fine combined positioning platform was developed to drive the mask accurately. Both the motor and the piezoelectric actuator were adopted to ensure a larger travel range and higher precision. The circuit of the PZT power supply and the amplifier circuit of pyroelectric infrared sensor are designed and carried out. The high precision A/D and D/A chip were adopted, and the control card based on the EPP protocol of PC was designed. Using these circuits, we fulfilled the signal collection, the control of the motor and the PZT. According to the fine-sample technology applied in code aperture image, the fine-sample method and fast δHT (FDHT) decoding algorithm were established. On the basis of the design of the instrument, we developed the experimental NPNOM which yields a resolution of 4μm at the 10.6μm wavelengh. At last, some preliminary tests were carried out, and the theoretical analysis and the experimental design of this thesis were verified.