Optical element and method for manufacturing optical element

A method of producing an optical element, the method comprising:

• forming a first mask layer onto an optical waveguide layer, the optical waveguide layer having a layer-thickness equal to or greater than 1.5 μm and being made of a material selected from a group consisting of Ta2O5, Al2O3, LiNbO3, LiTaO3, AlN, GaN, SiC, and Yttrium aluminum garnet (YAG);
• forming, on to the first mask layer, a second mask layer that is made of a plastic material;
• forming an arrangement of first openings in the second mask layer based on pressing a mold to the second mask layer and irradiating with an energy ray, the mold including a pressing surface provided with a periodic structure of walls where walls elongated in a first direction are arranged at a constant pitch along a second direction orthogonal to the first direction;
• forming an arrangement of second openings in the first mask layer by dry-etching through the first openings in the second mask layer;
• and
• forming a periodic structure of grooves in the optical waveguide layer by dry-etching through the second openings in the first mask layer, wherein
• (D/0.5Λ)≥2.5 is satisfied
• where
• D indicates a depth of the grooves in the periodic structure of grooves;
• and
• Λ indicates a pitch of the grooves in the periodic structure of grooves, and the units of D and Λ are the same,
• wherein L≤100 μm is satisfied where L indicates an effective length of the periodic structure of grooves, and
• wherein a reflectance of the optical element is greater than 20%.