Design and Fabrication of Sub-wavelength Annular Apertures for Femtosecond Laser Machining
Date Issued
2014
Date
2014
Author(s)
Tung, Yen-Chun
Abstract
The fundamental optical properties of traditional free-space light beam forced the users to make a compromise between the focal spot sizes the depth of focus when adopting optical technique to pursue micromachining. The focal spot size determines the minimum features can be fabricated. On the other hand, the depth of focus influences the ease of alignment in positioning the fabrication light beam. A typical approach to bypass the diffraction limit is to adopt the near-field approach. However, the depth of focus of the emitted light beam will be limited to tens of Nanometers in most cases, which posts a difficult challenge to control the distance between the optical emitted plane and the sample to be machined optically. More specifically, problems remained in this machining approach, which include issues such as residue induced by laser ablation tends to deposit near the optical emitted plane and leads to loss of coupling efficiency. We proposed a method based on illuminating femtosecond laser through a sub-wavelength annular aperture (SAA) on metallic film so as to produce Bessel light beam of sub-wavelength while maintaining large depth of focus first. To further advance the ease of use in one such system, producing sub-wavelength annular aperture with sub-wavelength focusing ability is detailed. It is shown that this method can be applied in material machining with an emphasis to produce high aspect ratio structure. More specifically, the ablation property of femtosecond laser was utilized to eliminate the laser melting induced residue deposition problem associated with traditional laser machining. Throughout the course of this research, we have optimized the parameters associated with the SAA structure for 780 nm light wavelength of the femtosecond laser by using finite difference time domain simulations method. A lateral microscope modified from traditional microscope was developed to facilitate the optical energy distribution of the emitted light beam to be used for optical machining. Finally, the SAA structure designed and the femtosecond laser were integrated to perform the intended optical micromachining.
Subjects
貝索光束
飛秒雷射
次波長圓環孔徑
冷加工
時域有限差分法
Type
thesis
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