A color filter based on a subwavelength patterned grating in poly silicon was proposed and realized on a quartz substrate. It was produced by utilizing the laser interference lithography technique to feature wide effective area compared to the costly e-beam lithography. An oxide layer was introduced on top of the silicon grating layer as a mask to facilitate the silicon-etching and to enhance the filtering selectivity as well. The structural parameters for the device include the grating pitch and height of 450 nm and 100 nm respectively, the silicon stripe width of 150 nm, and the oxide thickness of 200 nm. The fabricated device offered a spectral response suitable for a blue color filter, exhibiting the center wavelength of approximately 460 nm, the bandwidth approximately 90 nm and the peak transmission 40%. The positional dependence of its performance was examined to find the effective area of 3 x 3 mm(2), where the variation in the relative transmission efficiency and in the center wavelength was less than 10% and 2 nm respectively. Finally, the influence of the angle of the incident beam upon the transfer characteristics of the device was investigated to reveal that the rate of change in the relative transmission was equivalent to about 1.5%/degree.
Two optical devices with nano-scale subwavelength structures have been fabricated by using nanoimprint lithography (NIL). (1) Wire grid polarizer (WGP) is one of key optical components for projection displays with liquid crystal microdisplay. Although WGP with 140 nm pitch is commercially available now, it still poses a problem with low extinction ratio (ER) for blue color. Since the ER can be increased by reducing the pitch, fabrication of a WGP with 100 nm pitch was attempted by NIL. We successfully developed thermal nanoimprint and aluminum dry etching processes. Fabricated WGPs showed twice higher ER than 140 nm pitch one. (2) Photonic crystal (PC) structures on LED have been known to enhance the light extraction significantly. Although e-beam lithography has been used for the proof of principle, it is far from real production method. We applied thermal NIL to fabricate PC structures in p-GaN layer of green LED. To identify the PC effect, two structures were fabricated and compared. One structure makes the green light of 525 nm wavelength fall within the photonic band gap (PBG) while the other puts it outside of PBG. The former structure showed 9-fold increment of photoluminescence compared to LED without PC structures, while the latter showed only 6-fold increment.
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