Deep-subwavelength single grooves prepared by femtosecond laser direct writing on Si

It is well known that, femtosecond-laser pulses are easy to spontaneously induce deep-subwavelength periodic surface structures on transparent dielectrics, but not on non-transparent semiconductors. Nevertheless, in the study we demonstrate that by high-NA 800-nm femtosecond-laser direct writing with controlling the pulse energy and scanning speed in the near-damage-threshold regime, polarization-dependent deep-subwavelength single grooves with linewidths of ~180 nm can be controllably prepared on Si. Generally, the single-groove linewidth increases slightly with the increase of pulse energy and the decrease of scanning speed, whereas the single-groove depth significantly increases from ~300 to ~600 nm with the decrease of scanning speed, or even over 1 μm with the multi-processing, indicating the characteristics of transversal clamping and longitudinal growing of such deep-subwavelength single grooves. Then, EDS composition analysis of the near-groove region confirms that the single-groove formation tends to be an ultrafast, non-thermal ablation process, and the oxidized deposits nearby the grooves are easy to clean up. Furthermore, the results showing the strong dependence of groove orientation on laser polarization, and the occurrence of double-groove structures due to the interference of pre-formed orthogonal grooves, both indicate that the extraordinary field enhancement of strong polarization sensitiveness in the deep-subwavelength groove plays an important role for single-groove growth with high stability and collimation.