Evolution of self-organized nanograting from the pre-induced nanocrack-assisted plasma-laser coupling in sapphire

The period (Lambda of nanograting in sapphire varied from 320 to 398 nm with increasing the laser fluence, which is similar to the change trend of period of the near-subwavelength ripples previously observed on the material surface (0.4 <Lambda/lambda < 1) (Huang et al. in ACS Nano 3:4062, 2009). The result shows that the interference of the incident laser with the plasma could take place at the high-excited state of internal modified interface and leads to a spatial modulation of the local energy (fluence) distribution. The initial plasma-laser interference and the subsequent nanocrack-assisted plasma-laser coupling were used to explain the growth of nanograting what we have observed experimentally. Using a finite-difference time-domain method, we simulated the redistribution of laser fluence about the nanocracks, which derived from the pre-induced periodic refractive index changes in the focal volume after the acid etching. The experimental result and theoretical simulation can be in good agreement. In addition, we realized the erasing and rewriting of nanograting by using two beams of orthogonally polarized femtosecond laser pulses. This study can provide new proof for the physical mechanism of laser-induced nanograting and offer a reference for the fabrication of nanodevices in the substrate of sapphire.