Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses

We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF6 produces an absorptance of 0.9 for radiation in the, 1.2-2.5 mum wavelength range, higher than any of the other gases. Use of Cl-2, N-2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat crystalline silicon, for which the absorptance is roughly 0.05-0.2 for a 260 mum thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration. (C) 2003 American Institute of Physics.