Multi-harmonic scanning near-field optical microscopy

• Main Author: Dong, Zhaogang
• Other Authors: Soh Yeng Chai
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: http://hdl.handle.net/10356/48037
• Institution: Nanyang Technological University

This thesis focuses on the development of a multi-harmonic scanning near-field optical microscopy (SNOM) system. The system is demonstrated to have the capabilities of improving optical imaging resolution, identifying and reducing artifacts, and achieving near-field tomography. The key contributions of this PHD thesis can be summarized as follows: Firstly, a multi-harmonic SNOM technology is proposed by utilizing the intrinsic harmonics in the mechanical oscillation of the tuning fork probe. The technology is demonstrated on a system using an aperture-SNOM probe. It is shown that the proposed multi-harmonic SNOM system is able to enhance optical imaging resolution. The multi-harmonic SNOM system is also demonstrated on an apertureless-SNOM system implementation, where the probe used is a metal-implanted protrusion-type scattering-SNOM probe. It is shown that in the apertureless-SNOM system, the collected harmonic SNOM signals also enhance optical imaging resolution when compared to the traditional SNOM systems. Secondly, based on the proposed multi-harmonic SNOM system, a systematic analysis on artifact identification and reduction is carried out for three cases. The first case considers a non-tilted tuning fork probe and a constant probe vibration amplitude. An efficient approach is proposed to identify and reduce topographically induced artifacts. It is shown that the second harmonic SNOM signal represents the derivative of the DC SNOM signal along the $z$ direction. By multiplying the second harmonic SNOM signal and the topography signal, topographical artifact is calculated and reduced from the original DC SNOM signal. Similarly, the third harmonic SNOM signal is used to reduce the topographical artifacts embedded in the first harmonic SNOM signal. Furthermore, the proposed approach is also applicable for cross-polarization detection condition. For the second case, we consider a tilted tuning fork probe and a constant probe vibration amplitude, and the proposed approach using multi-harmonic SNOM system is extended to overcome the artifacts when the influence of the tilt angle of the probe is considered. This is achieved via a coordinate transformation such that the physical meaning of the harmonic SNOM signals collected by a tilted tuning fork probe can be derived. Then, a simple method is proposed to characterize the tilt angle based on the approaching curves of both the DC SNOM signal and the first harmonic SNOM signal. Finally, a revised topographical artifact reduction approach is proposed to consider the presence of the tilt angle. For the third case, we analyze the topographical artifact that is induced by a changing probe vibration amplitude.