Fabrication of multilevel phase computer-generated hologram elements basedon effective medium theory

A conventional method to synthesize diffractive optical elements and comput er-generated holograms (CGH's) with high diffraction efficiency relies on a n increase of phase levels. To fabricate such a device, one should perform electron-beam (e-beam) lithography with multiple-dose exposures or multiple -step photolithography. Here we describe a one-step method, which is based on the effective medium theory, for the fabrication of a multilevel phase C GH. The phase modulations required in cells of a CGH are constructed by mea ns of dividing these cells into fine (subwavelength) structures. The surfac e features of these fine structures control their corresponding indices, an d their values can be calculated according to the effective medium theory. By proper selection of the fine structures, based on the requirements of th e phase modulation of the cells, a CGH with multilevel phases is synthesize d when a binary structure is relieved on the dielectric material. Then the CGH can be fabricated,by direct e-beam Lithography or one-step photolithogr aphy through an amplitude mask followed by an ion-etching treatment. The ex perimental results showed that the reconstructed wave field is in good agre ement with that simulated by a computer, indicating the effectiveness of th e proposed method. (C) 2000 Optical Society of America. OCIS codes: 220.400 0, 050.1970.