A better understanding of terahertz emission from semiconductor surfaces with a phased-array effect

We studied the phased-array effects in terahertz emission from semiconductor surfaces upon femtosecond laser illumination. A finite-difference time-domain simulation and experimental observation of the radiation patterns were utilized to examine the optical excitation at normal to the semiconductor surface and oblique angles of 30° and 45° with diameters of ∼10 and 275 μm. The results revealed that there exists a clear phased-array effect for the defocusing conditions. The larger diameter induced a pronounced directivity of the emission owing to the constructive interference of multiple point sources obeying the law of linear superposition, whereas the radiation patterns at smaller diameters were explained as the dipole point source. This finding, in addition to previous studies, will provide a better understanding and contribute to applications of terahertz emission spectroscopy in the field of semiconductor research and development.