Ultrafast transport-mediated homogenization of photoexcited electrons governs the softening of the A1g phonon in bismuth

In order to determine the role of non-thermal transport of hot carriers which\nis decisive for the dissipation of energy in condensed matter we performed\ntime-resolved broadband femtosecond transient reflectivity measurements on\n$7-197 \\mathrm{nm}$ thick Bi(111) films epitaxially grown on Si(111). We\nmonitored the behavior of the Fourier amplitude and the central frequency of\nthe coherent $A_\\mathrm{1g}$ phonon mode as function of the incident fluence,\nfilm thickness, and probe wavelength in the range of $580 -700 \\mathrm{nm}$.\nThe frequency redshift that follows photoexcitation was used as a robust\nquantity to determine the effective distribution of excited carriers that\ngoverns the displacive excitation mechanism of coherent $A_\\mathrm{1g}$ phonons\nin Bi. For Bi films up to $50 \\mathrm{nm}$ thickness a homogeneous excitation\ndue to the ultrafast transport of hot charge carriers is observed, limited by a\ncarrier penetration depth of $60 \\mathrm{nm}$ independent of the totally\ndeposited laser energy.\n