Makoto Umetani

Fabrication of Antireflective Subwavelength Structure on Spherical Glass Surface Using Imprinting Process

For the first time, an antireflective structure (ARS) on a convex spherical glass surface was fabricated using an imprinting process. The inverted pattern of the ARS with a 250-nm period was fabricated on a SiC mold with a concave surface using electron beam (EB) direct writing and reactive ion etching. The sample surface height was adjusted to the EB focal position along the mold curvature during the step-and-repeat EB lithography. The imprinted lens with the ARS exhibited surface reflectance of 0.2% at a wavelength of 530 nm and a spherical error magnitude of less than 500 nm.

Formation of antireflective structure on the surface of optical glass by molding

Formation of a surface having a periodic subwavelength structure (SWS) is a well-known technique for reducing the Fresnel reflection of transmissive optical elements. We fabricated an anti-reflective structured (ARS) surface with a twodimensional periodic structure with the period of 300 nm on an optical glass by a precision molding process using a silica glass mold coated with a thin carbon film. The surface structure was formed on the mold using reactive ion etching (RIE) with fluorocarbon plasma. A thin chromium film was patterned using electron beam lithography and a wet etching process. The anti-reflective surface was formed on a phosphate glass with a deformation point of 412°C and a refractive index of 1.60 at a wavelength of 462 nm. The phosphate glass was molded at 420°C for 510 s under a pressure of 5 MPa. The height of the periodic structure on the mold was 550 nm and the height of that on the formed glass was 480 nm. Therefore, the filling rate of the phosphate glass to the mold was 87%. The surface reflectance of the glass was estimated as 0.56% at a wavelength of 462 nm, which was approximately 1/10 that of the optically polished surface.