Vol.1 No.1 2008

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Research paper : A challenge to the low-cost production of highly functional optical elements (J. Nishii)−27−Synthesiology - English edition Vol.1 No.1 (2008) When one-dimensional microstructure with a period smaller than the wavelength is formed on the surface of transparent material, so-called “structural birefringence” appears, where the refractive index is different depending on the direction of electric field of passing light[5]. Even in optically isotropic glass, structural birefringence can be realized if anisotropic sub-wavelength structure is formed on the surface. Theoretical optimization of the structure is possible using the calculation methods such as the effective medium theory or the rigorous coupled wave analysis. Important parameters are period, groove width, and height of the periodic structure, and refractive index of the material. Especially, the optimization of groove width is an essential point in order to minimize the wavelength dependency of phase retardation. Moreover, height of structure can be lessened by increasing refractive index of the material. Glass material is advantageous to attain higher refractive index than resin, resulting in the lower height of structure. However, there was no report on whether such structure can be formed on the surface of glass or not, using the molding process.In this study, as the first step, the period was fixed at 500 nm, and various heat resistant molds with various groove widths were fabricated to study the molding characteristics of glasses quantitatively. The results are shown in Figure 4. In the case of mold with groove width 330 nm, the structural height of the molded periodic structure reached 730 nm, which means an extremely high precision molding was possible. It is the first quantitative investigation on the correlation between the shapes of mold and the molded periodic structure obtained by imprinting. The most important point here was the demolding condition. When the mold was released from the glass at high temperature, microstructure formed on the glass surface was deformed by heat. When the demolding was carried out at low temperature, a mechanical damage was caused either in mold or glass or in both by the difference in thermal expansion coefficients between the mold and the glass. Optical glass that could be molded at relatively lower temperature than 500 °C was advantageous because the deterioration of the mold can be prevented. However, the determination of demolding temperature became extremely difficult, because, in general, the viscosity of glass changed rapidly near molding temperature. To solve this issue, the researchers of companies, who have plenty of experience in the molding process, and the researchers of AIST, who are knowledgeable in material property and microfabrication, collaborated successfully. The demolding the periodic structure with the highest aspect ratio in the world was successfully accomplished in a short period. In this study, large surface area of 6 mm x 6 mm, as shown in Figure 5, was confirmed. The phase retardation attained by this structure was 0.1 [6], which was the first result to realize the phase retardation caused by the periodic structure via glass molding process. Molding of the structure with 300 nm period was also successfully achieved. The future goal is to attain the phase retardation of 0.25 in the wavelength region between 400 and 800 nm by optimizing the dimension of periodic structure, which is the requirement for practical use in the next generation optical disc drive.4.2 Development of sub-wavelength antireflective structureThe improvement of the transmission efficiency of light by minimizing the unnecessary reflection is desired for glass optical components used in wide-ranging products such as home electronics, lighting, etc. Currently, antireflection film is coated on the optical elements for imaging and display panels. However, in principle, such films can not respond to demand for the antireflection independent against wavelength or incident angle. On the other hand, it was known that advanced antireflection could be achieved if periodic conical shape structure with the sub-wavelength period could be formed on the surface of elements.Important point was to arrange the conical shape units two-dimensionally with a period smaller than the wavelength. The antireflection effect could not be achieved when the conical structure with the period comparable to wavelength モールド表面ガラス成型体Mold surfaceGlass compactMolding temperature : 500 ℃Press pressure : 0.4 kN/cm2Groove width1 µmFig. 4 SEM photographs of one-dimensional periodic structure mold and molded glassFig. 5 One-dimensional periodic structure with large area showing retardation.

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