We present direct experimental observation of the morphological evolution during the formation of nanogratings with sub-100-nm periods with the increasing number of pulses. Theoretical simulation shows that the constructive interference of the scattering light from original nanoplanes will create an intensity maximum located between the two adjacent nanoplanes, resulting in shortening the nanograting period by half. The proposed mechanism explains the formation of nanogratings with periods beyond those predicted by the nanoplasmonic model. (C) 2015 Optical Society of America