Formation of nanogratings on the surface of a ZnSe crystal irradiated by femtosecond laser pulses

Two collinear femtosecond laser pulses, one at wavelength of $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and the other at $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ (double frequency), simultaneously irradiated the surface of ZnSe crystal, which resulted in regular nanograting with period of $180\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ on the whole ablation area. We attribute the formation of the nanograting to be due to the interference between the surface scattered wave of $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ lasers and the $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ light. The period of the nanograting $\ensuremath{\Lambda}$ is about $\ensuremath{\lambda}∕2n$, where $n$ is refractive index of the sample, and $\ensuremath{\lambda}$, the laser wavelength. This mechanism is supported by observation of rotation of the nanograting with the polarization of $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ light, and by the dependence of $\ensuremath{\Lambda}\ensuremath{\sim}\ensuremath{\lambda}$ of the nanoripples on the surface of semiconductors and dielectrics.