  Irradiation of intense ultrafast laser pulses in glasses can lead to
formation of nanogratings whose periods are significantly smaller than the
incident irradiation wavelength. The mechanism of the exotic phenomenon is
still under debate. Here, we access the snapshots of morphologies in the laser
affected regions in a porous glass which reveal the evolution of the formation
of nanogratings with increasing number of laser pulses. Combined with further
theoretical analyses, our observation provides important clues which suggest
that excitation of standing plasma waves at the interfaces between areas
modified and unmodified by the femtosecond laser irradiation plays a crucial
role for promoting the growth of periodic nanogratings. The finding indicates
that the formation of volume nanogratings induced by irradiation of femtosecond
laser pulses is initiated with a mechanism similar to the formation of surface
nanoripples.
