"Dry" method for the formation of reflective phase DOEs using direct laser writing on thin Zr films

A completely "dry" one-step method for the formation of reflective phase diffractive optical elements (DOE) using direct laser writing on thin Zr film without subsequent liquid or plasma treatment has been demonstrated. The method is based on effect of nanostructure formation during the thermochemical oxidation of the zirconium film deposited on a fused silica substrate. Direct laser writing of diffractive structures is performed by scanning a laser beam with a wavelength of 405- 532 nm, focused into a spot with a diameter of 400-700 nm. When line-by-line scanning with a step of 200-500 nm, the formation of nanogratings from tracks with a width of 70-100 nm and with a period equal to the scanning step was observed. It was experimentally established that lines in the form of cracks or deformations arise along the contour of the spatial distribution of temperature induced by the laser beam heating. Cracks or deformations occur under the influence of thermomechanical stresses at the boundary between the metal film and the oxide line, the thickness of which increases sharply during the oxidation of the metal. The formed nanogratings significantly change the optical properties of the film surface. Measurements by a white light interferometer show the presence of a relief with a depth of -200 to +500 nm, although when oxidizing a 100-nm thick Zr film, the increase in thickness cannot exceed 50-60 nm. Binary diffractive structures with a period of 0.9 - 10 mu m, the diffraction efficiency of which exceeded 30%, were made by the new method. The one-step method can be useful for the manufacture of computer-generated holograms used in testing of aspherical surfaces.