A two-dimensional photonic crystal comprising a plate-shaped body (11) provided with a periodic array of modified refractive index regions (12; 22; 32) whose refractive index differs from that of the body (11), wherein: on a first surface (131) which is one of the surfaces of the body (11), the modified refractive index regions (12; 22; 32) are arranged in a triangular grid pattern (141) and at least three times rotational symmetry with respect to each grid point (121 ) exhibit; on a second surface (132), which is the other surface of the body (11), the modified refractive index regions (12; 22; 32) are arranged in a triangular grid pattern (142) whose position is complementary to the triangular grid (141) the first surface (131), and having at least a three-fold rotational symmetry with respect to each grid point (122); and where the modified refractive index region (12; 22; 32A) comprises pillars (12A; 22A; 32A) extending from a lattice point (121) on the first surface (131) to each of three lattice points (122) on the second surface (132). which are closest to the aforementioned lattice point (121) at the first surface (131), each pillar (12A; 22A; 32A) being formed so that its in-plane sectional shape at each position in the thickness direction of the Body (11) is the same and has a triple rotational symmetry, and is shaped like an oblique cylinder.
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