Interband and intraband (Drude) contributions to femtosecond laser absorption in aluminum

Theoretical and experimental investigations of the absorption in metallic aluminum of femtosecond-laser radiation pulses with peak, intensity I-o less than or similar to 10(15) W/cm(2) are reported. Energy balance equations are solved for electron and phonon subsystems, together with Helmholtz equation for the laser radiation. Expressions for the relaxation times as functions of electron and phonon temperatures are obtained, with no free parameters, Contrary to the assumption made in published studies, we find that the interband rather than the intraband (Drude) absorption plays the dominant role in the near infrared and throughout the risible region at low and moderate intensities. For 50 fs, 800 nm laser pulses the absorption in interband transitions dominates for intensities up to few times 10(13) W/cm(2). For such pulses, broadening of the parallel-band interband absorption line with the increase in electron and phonon temperatures results, for I-o less than or equal to 5 x 10(13) W/cm(2), in the decrease of the absorption coefficient compared to the room-temperature value. In this paper, we present both the first theoretical prediction and the first experimental observation of this phenomenon. Dielectric permittivity gradients within the skin layer also contribute to the decrease in absorption. The mechanisms of the lattice disordering are considered quantitatively, and it is shown that for I-o < 10(14) W/cm(2) melting does not occur in the laser-pulse duration. Experimental results are presented for 800 and 400 nm wavelengths. The agreement between the theory and the experiment is very good.