Diffractive optical element and method of its formation

aaPractical 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 formed on a transparent substrate, with the DLC film containing a diffraction grating having local regions of a relatively high refractive index and local regions of a relatively low refractive index. The DLC film can readily be deposited by plasma CVD onto the substrate, and the high refractive-index regions within the DLC film can readily be formed by irradiating it with an energy beam such as an ion beam.