Ko Ishizuka

Fabrication of Half-Wave Plates with Subwavelength Structures

Various optical elements with subwavelength structures are currently being developed. Because phase plates require a deep grating structure in which the line width is narrow and the aspect ratio is high, it has been difficult to develop half-wave plates. We have designed and developed λ/2 plates, and evaluated their optical characteristics. The fabricated phase plates have subwavelength gratings the depth of which is 1170 nm with a period of 260 nm. The phase plates achieved a phase retardation of 180 deg. at a wavelength of approximately 850 nm, and 90 deg. at the optical communication wavelength of approximately 1550 nm. The transmittance was approximately 70% around 850 nm and approximately 80% around 1550 nm.

Improvement of phase retardation of wave plate with subwavelength structures by heat treatment

Wave plates with subwavelength structures are fabricated by applying semiconductor processes to realize a high-aspect-ratio grating structure with a small linewidth and a large grating step. We have fabricated wave plates with a large phase retardation using a titanium dioxide (TiO2) film. However, the plates have a problem in that a slight deviation in grating step will bring about a large difference in phase retardation because TiO2 has a large refractive index. To solve this problem, heat treatment of the fabricated wave plates was carried out to change the refractive index. As a result, the relationship between the heating temperature and the increasing ratio of phase retardation was determined, which led to the successful control of the optical properties. The effective temperature was 672 to 1072 K through the characterization of the heat-treated plates. By utilizing this process, the yield of manufactured wave plates can be increased greatly.