By varying the fluence and exposure of focused femtosecond laser pulses in the mid-IR range (1.0–2.5 μm), a variety of periodic surface structures (gratings) were formed for the first time on the (111) surface of crystalline silicon in the scanning mode. The structures observed by electron microscopy vary in their formation process (e.g., amorphous phase, capillary flow of melt, or ablation removal of material) and orientation of strips (parallel or perpendicular to laser polarization) over a wide range of periods (0.4 to 1.8 μm). The observed variety of structures can be described subject to the combined effect of exposure by their gradual evolution at a constant laser wavelength of 1.95 μm depending on the laser fluence and related electron-hole plasma density, as well as on volumetric energy density that controls the structural phase modification of the material. The possibility of using nano- and microgratings with a specified period as Bragg filters or input/output gratings in silicon planar photonic integrated circuits is discussed.
Kudryashov et al. (Mon,) studied this question.