关键词: femtosecond-laser direct writing of Si, deep-subwavelength single grooves, polarization dependence, high numerical aperture, ultrafast non-thermal ablation

Abstract: It is well known that femtosecond laser pulses can easily spontaneously induce deep-subwavelength periodic surface structures on transparent dielectrics but not on non-transparent semiconductors. Nevertheless, in this study, we demonstrate that using high-numerical-aperture 800 nm femtosecond laser direct writing with controlled pulse energy and scanning speed in the near-damage-threshold regime, polarization-dependent deep-subwavelength single grooves with linewidths of $\sim 180 $ nm can be controllably prepared on Si. Generally, the single-groove linewidth increases slightly with increase in the pulse energy and decrease in the scanning speed, whereas the single-groove depth significantly increases from $\sim 300$ nm to $\sim 600$ nm with decrease in the scanning speed, or even to over 1 μm with multi-processing, indicating the characteristics of transverse clamping and longitudinal growth of such deep-subwavelength single grooves. Energy dispersive spectroscopy composition analysis of the near-groove region confirms that single-groove formation tends to be an ultrafast, non-thermal ablation process, and the oxidized deposits near the grooves are easy to clean up. Furthermore, the results, showing both 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, indicate that the extraordinary field enhancement of strong polarization sensitivity in the deep-subwavelength groove plays an important role in single-groove growth with high stability and collimation.

Key words: femtosecond-laser direct writing of Si, deep-subwavelength single grooves, polarization dependence, high numerical aperture, ultrafast non-thermal ablation