We irradiated silicon with a train of femtosecond pulses in the presence of SF
6, H
2S, H
2, SiH
4, and amixture of Ar and SF
6 in order to analyze the role of the background gas in determining the morphologyand the optical properties of the resultant surfaces. We discuss factors that affect the surface morphologycreated during irradiation and show that the presence of sulfur in these gases is important in creatingsharp microstructures. We also show that the presence of sulfur is necessary to create the near-unityabsorptance for both above-band and below-band gap radiation (0.25-2.5
m) by silicon; only sampleswith sulfur concentrations higher than 0.6% absorb 95% for above-band gap radiation and have a flat,featureless absorptance of 90% for below-band gap radiation.