Optical member with high and low refractive index layers

An optical member, comprising:

• high refractive index layers having a relatively high refractive index and low refractive index layers having a relatively low refractive index disposed alternately in a lateral direction to an optical axis, the refractive index of the high refractive index layers being greater than the refractive index of the low refractive index layers, and the high refractive index layers and the low refractive index layers each relatively thin as compared with an optical length, the optical member being configured to provide a concave lens function,
• wherein:
• (a) each width of the high refractive index layers and the low refractive index layers is equal to or smaller than a wavelength order of incident light,
• (b) the high refractive index layers are each disposed symmetrically with respect to a mechanical center of the optical member so as to be disposed non-densely at the mechanical center of the optical member and disposed densely farther away from the mechanical center,
• (c) the high refractive index layers are disposed such that respective widths of the high refractive index layers decrease gradually toward the mechanical center of the optical member, and
• (d) the low refractive index layers are disposed such that respective widths of the low refractive index layers increase gradually toward the mechanical center of the optical member.