  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 of the nanograting period by half. The proposed mechanism enables
explaining the formation of nanogratings with periods beyond that predicted by
the nanoplasmonic model.
