Research and Development on the Transmission of Periodic Hole Structure based Sub-wavelength Annular Aperture
Date Issued
2014
Date
2014
Author(s)
Weng, Chun-Hung
Abstract
In traditional optical system, either shorter wavelength incident light or higher numerical aperture lens can lead to spatial resolution improvement. However, the depth of focus also depends on the above-mentioned two parameters. Subwavelength focus of Bessel beam breaks free the strong interaction among depth of focus and focus spot size. In recent years, Ebbesen et al. discovered that surface plasmon generated by periodic subwavelength aperture on metallic film can lead to unexpected large transmission and directional beaming effect. Our research group proposed different configuration of subwavelength apertures. Preliminary studies include using tapered capillary tube to produce annular aperture on optical head, which generated subwavelength size spot by the aperture. However, due to the difficulty in controlling the size of the optical head, we also tried to create the optical head by first sputtering gold on SiO2 substrate and then manufactured subwavelength annular aperture (SAA) by using focus ion beam (FIB). This SAA structure was found to produce Bessel beam and was applied to produce high aspect ratio structure. However, the problems associated with high side lobes energy and low transmission efficiency remain in the above-mentioned design approaches. It is to be noted that the part of the side lobes with energy higher than that of the machining threshold will be recorded. The concentric circles apart from main lobe caused by the high side lobes energy thus caused imprecise machining, which limited further advancement of Bessel-beam based laser micromachining. Based on the understanding generated from prior researches, it is confirmed that reducing the energy ratio between the side lobes and the main lobe can be achieved either by reducing the side lobes energy or by enhancing the Bessel beam main lobe transmission energy. Based on these preliminary studies on different structures, constructing pseudo-subwavelength annular aperture (PSAA) using hole array arranged in a way to mimic SAA on simple gold film was adopted as the main focus of this thesis with an attempt to improve transmission efficiency and lower the side lobes of the Bessel beam emitted. Taking the goal of using PSAA to lower side lobes and to improve transmission, we started from choosing parameters related to the size and distance of the holes and distance of rings of PSAA. Gold film was sputtered on one side of silicon wafer, and the other side of wafer used back-etch method to manufacture square holes. The non-etched remaining area supported the thin gold film. This study used finite-difference time-domain (FDTD) method to do the electromagnetic simulation. Different parameters associated with subwavelength annular aperture were identified to fabricate the structure. Moreover, light intensities and Bessel beam profiles at different distances from the PSAA was measured by using a modified microscope so as to verify the simulation and inspect the properties enhancement in Bessel beam. Finally, possibilities of using these PSAA structures to create a subwavelength laser machining system were examined. The experimental results obtained from the Bessel beam generated by the PSAA structures developed in this thesis confirmed the superiority of the newly created Bessel beam for laser micromachining. The PSAA structures developed appeared to form a good advancement platform for next-generation laser micromachining needs.
Subjects
Bessel beam
subwavelength annular aperture
subwavelength focus
laser machining
photon sieves
Type
thesis
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