Method for designing diffractive device and method for manufacturing diffractive device

A method for designing a diffractive element for modulating a phase of incident light, using a computer, the method comprising:

• calculating an electric field distribution of an emission light on an emission plane of the diffractive element with respect to the incident light, the incident light being a Gaussian beam;
• calculating an electric field distribution obtained by multiplying an electric field distribution of emission light from the emission plane by a Gaussian window in a plane parallel to the emission plane located at a predetermined distance from the emission plane, as an electric field distribution of a beam approximated by a Bessel Gaussian beam;
• calculating a first electric field distribution as an electric field distribution on the emission plane of the diffractive element with respect to the electric field distribution of the emission light on the plane, on the basis of a principle of Kirchhoff's diffractive integral;
• and
• determining a depth of an unevenness on a surface of the diffractive element, on the basis of the electric field distribution on the emission plane of the diffractive element.