The Cu dopant effect on the optical properties of zinc-blende phase CdS in the infrared range has been investigated, based on calculations using the first principle density functional theory with Hubbard U on site correction. Four concentrations of Cu dopant are considered. The results show that the Cu-doped CdS demonstrates a significantly enhanced behavior of absorption and conductivity in the infrared range, implicating that it could be a promising candidate for the infrared switch materials. The mechanism of the enhancement has also been explored. It is found that the Cu dopant induces a hole state above the Fermi energy level (E-F) of CdS, and provides a possible intraband transition with about 0.7 eV energy difference, which is responsible for the enhanced absorption of CdS in the infrared range. (C) 2016 Elsevier B.V. All rights reserved.