We found that irradiation by a single femtosecond laser pulse produced a surface wave structure on a (100) silicon surface. The surface wave structure bearing a concentric shape was produced only in the edge regions in the direction of the electric field from the center. The increase in the number of irradiating pulses allowed the concentric structure gradually to align perpendicularly to the laser beam's electric field. The period of the surface wave structure was nearly equal to the wavelength of the irradiating laser pulse. In this accord, we propose that this structure is a precursor of ripple formation.
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