Terahertz emission from n-type (100), (110) and (111) InAs crystals have been measured as a function of the sample orientation. Emission was excited using 120 fs Ti:Sapphire laser pulses at an incident angle of 45° with fluences of approximately 1-2mJ /cm 2. The data is shown to match the behavior expected for optical rectification at the surface, with small contributions from bulk optical rectification and photo-carrier diffusion. Thus, at fluences employed in the present study, it appears that the dominant mechanism for generating THz radiation is optical rectification at the surface.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.