Diffractive optical device and method for producing same

Practical diffractive optical elements are made available efficiently and at low-cost. A refractive-index-modulated diffractive optical element includes a transparent DLC (diamond-like carbon) film (2) formed on a transparent substrate (1), with the DLC film (2) containing a diffraction grating having local regions (2a) of a relatively high refractive index and local regions (2) of a relatively low refractive index. The DLC film (2) can readily be deposited by plasma CVD onto the substrate (1), and the high refractive-index regions (2a) within the DLC film can readily be formed by irradiating it with an energy beam (4) such as an ion beam.