Nanoscale measurement by using the near-field polarization

• Main Author: Liu, Zhuang.
• Other Authors: Soh Yeng Chai
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/53737
• Institution: Nanyang Technological University

This thesis studies the polarization representations of near-field light for 3D nano-scale measurement and resolution improvement of SNOM imaging. The work is performed from two aspects. One aspect is to achieve optical measurements and thin film characterizations by using polarization-based SNOM. The other one is to investigate light-matter interactions and to make use of the interactions to improve the resolution of SNOM imaging. The representation of light polarization in the near-field is first formulated. They are described by complex 3D vectors. It is found that the measurement of the polarization in the near-field could be affected by some factors, including the geometries of the SNOM probe and the sample-probe separation. A polarization heterodyning SNOM system is established based on the defined representation of the polarization. The amplitude and phase information of the orthogonal components of the light are measured through heterodyning detection, and the polarization state is constructed based on the measured information. This technique can attain nano-scale measurement of the light polarization with a high detection sensitivity. Further, to achieve a precise sample-probe separation control, we propose a new AFM scheme by operating the tuning fork probe in its second-resonance regime. This method delivers better sensitivity for the separation control and yields an improved lateral resolution. Through these two techniques, we are able to accurately measure nano-scale polarization of near-field light. Leveraging on the polarization measurement capability, we then develop a near-field ellipsometry technique to characterize thin films properties. The technique is first established on a transmission-configuration of the SNOM. The near-field ellipsometry equation is formulated by using measured light polarization. The topographic and optical signals that are simultaneously measured by the SNOM are utilized to solve the ellipsometry equation. A nano-scale lateral resolution and corrected artifacts are demonstrated by using the proposed method for thin film characterization. Simulation and experimental studies are performed to verify the performance of the proposed method. In addition, the near-field ellipsometry technique is extended to the characterization of opaque thin film based on a reflection-configuration of the SNOM. A modified ellipsometry equation is formulated to fit the new configuration. In the modified equation, the sample-probe interaction and the sample's topography information are taken into consideration. It is shown that the technique is able to accomplish nano-scale characterization of the opaque thin film as verified by both simulation and experiment studies. Furthermore, we developed a new scheme to characterize the porosity of thin films. This scheme also relies on the polarization in near-field. The porosity information of the sample is encoded in its effective dielectric constant through the effective medium theory. This method retrieves the localized sample porosities from the effective dielectric constants that are calculated from the near-field polarization. The effectiveness of the proposed method is also verified by both experiment and simulation studies. Hence, we have established practical and effective techniques for the characterization of different kinds of thin film samples. We further explore light-matter interactions in the near-field. The interactions are analyzed by using the Fresnel diffraction and the bound charge effect. The polarization-dependent edge effect is observed in SNOM imaging. Based on this effect, a new SNOM imaging scheme is developed. The images are first recorded by polarization-dependent detections, and these images are fused into one resulting image, which contains optimized resolution over all of the source images. Simulation and experimental studies are carried out to demonstrate the performance of the proposed scheme. Through the development of the above techniques, we are able to systematically study light polarization in the near-field, employ the polarization to achieve nano-scale optical measurements and characterizations, and improve the resolution of SNOM imaging. These techniques have broad potential applications in nano-technology.