Abstract

We investigate the formation process of femtosecond laser pulse induced nanogratings in fused silica. Three distinct stages of the growth from randomly arranged nanostructures to uniform periodic gratings are identified. These stages of the formation process are controlled mainly by the number of pulses applied to the sample. During the first formation stage, the exposed volume contains nonperiodic with an irregular shape. After several hundred pulses, stage II is reached. Here, periodicity starts to emerge, but there are still a lot of vacancies between the isolated modifications. Stage III occurs after roughly 1,000 laser pulses. The vacancies have been filled and a periodic nanograting is formed. The three different stages can be seen in Figure 1 a). The number of pulses required for each formation stage depends slightly on the pulse energy E, in agreement with the nanoplasmonic model for the formation of nanogratings [1,2]. Furthermore, in agreement with previous measurements [3] we observed a decrease of the nanograting period with increasing number of pulses. In addition, for a fixed number of pulses the period also decreases with increasing pulse energy. This is due to the fact that stage III of the formation process is reached at a smaller pulse number, and thus the decrease of the grating period has progressed further. With a wavelength of 515 nm, periods as low as 150 nm are obtained which is significantly smaller than the prediction of the nanoplasmonic model.

© 2011 Optical Society of America