Using Multi-layer Alignment and Nano-imprinting Technologies to Design and Fabricate Sub-wavelength Grating Based Wavelength Selected Polarizers
Date Issued
2009
Date
2009
Author(s)
Lu, Chiao-Yun
Abstract
The fabrication of sub-wavelength metal grating polarizers using e-beam direct lithography and multi-layered alignment technologies. As a non-polarized light is impinging into a metal grating, it will absorb the TE-mode whose electric field oscillation direction is parallel to the wire-grid. It will then perpendicularly transmit the TM-mode out of the wire. This thesis investigates the transmission of polarized light and its extinction ratio by designing the parameters of the grating structure. Besides from considering the relationship between structure size and the wavelength of the incident light, the resonant cavity can also filter out any undesired wavelength. Therefore, the metal grating has the ability to both polarize and filter.irst, this paper discusses the characteristics of polarization concerning the parameters of single layer aluminum film structure. By modulating the thickness of the aluminum, the width of the slits, the period of the grating and the filling factor, the project has been able to find and produce a substantially more effective grating structure. Once the thickness of aluminum film reaches 4000Å and the slits are smaller, the filter polarization becomes more effective. Although it will lower the TM transmission, this then is compensated by the suppression of the TE mode in the process creating a desired extinction ratio / filling factor of 1:3.owever, single layer grating systems are unable to contain a resonant cavity and therefore cannot filter out any undesired part of the incident light. To resolve this problem, we used multi-layered alignment technologies to construct a double layered metal grating system. Here, a silicon oxide layer was inserted between the metal layers to form a resonant cavity for the polarized light. The amounts of the shift between the two layers are 0.25 and 0.5 of the slits. By modulating the thickness of the middle oxide layer, we can control wavelength effectively so much so that we can suppress the transmission of longer wavelengths. We also use the finite-difference time-domain method (FDTD) to simulate the phenomenon of near-field optics and to analyze the mechanism of transmission. To confirm our result of the experiment, we used a rigorous-couple-wave-analysis (RCWA) simulation.n conclusion, we modulated the optical path of the incident light in the structure by adjusting the amount of shift or by changing the length of the resonant cavity. These two processes allowed us to control the transmissible wavelength effectively while also enabling the double layered metal grating to become a filtered-polarizer for communication wavelength.n the thesis, there is 60% of TM transmission at 1500nm of the double layers system the thickness control of [3000Å vs 1000Å] and the period is only 600nm with filling factor 1:3, and the extinction ratio is almost 3000. It is a high quality color-filtered polarizer.n order to lower the cost of the process, we also used nano-imprinting and reversal imprinting lithography technologies to fabricate the sub-wavelength polarizer. We substituted the substrate as polymer material to increase its flexibility and feasibility. By using an imprint condition of 19MPa 100nm gold film, we obtained the extinction ratio of more than 70. Any increase in the degree of the oblique incident angle produces a red-shift on the surface plasma resonant wavelength. The TE-mode is much suppressed at the longer wavelength because it is far away from the characteristic wavelength of gold. Therefore, the ability of polarization can be improved dramatically.
Subjects
sub-wavelength metal grating
polarizers
color filters
surface plasma
nano-imprinting techonologies.
Type
thesis
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