Optical second harmonic generation (SHG) from the clean and nitrogen adsorbed Cu(001) surfaces has been studied. The observed SHG signal was largely enhanced by the nitrogen adsorption. Surface SH responses of the Cu(001) surfaces were also calculated based on an ab initio approach. The calculations qualitatively reproduced the experimental results. It is suggested that the N-derived surface states are the initial states of the optical transitions for the enhanced SHG. (C) 2008 Elsevier B.V. All rights reserved.