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The reversible phase transition induced by femtosecond laser excitation ofGallium has been studied by measuring the dielectric function at 775 nm with ~200 fs temporal resolution. The real and imaginary parts of the transientdielectric function were calculated from absolute reflectivity of Gallium layermeasured at two different angles of incidence, using Fresnel formulas. Thetime-dependent electron-phonon effective collision frequency, the heatconduction coefficient and the volume fraction of a new phase were restoreddirectly from the experimental data, and the time and space dependent electronand lattice temperatures in the layer undergoing phase transition werereconstructed without ad hoc assumptions. We converted the temporal dependenceof the electron-phonon collision rate into the temperature dependence, anddemonstrated, for the first time, that the electron-phonon collision rate has anon-linear character. This temperature dependence converges into the knownequilibrium function during the cooling stage. The maximum fraction of a newphase in the laser-excited Gallium layer reached only 60% even when thedeposited energy was two times the equilibrium enthalpy of melting. We havealso demonstrated that the phase transition pace and a fraction of thetransformed material depended strongly on the thickness of the laser-excitedGallium layer, which was of the order of several tens of nanometers for thewhole range of the pump laser fluencies up to the damage threshold. Thekinetics of the phase transformation after the laser excitation can beunderstood on the basis of the classical theory of the first-order phasetransition while the duration of non-thermal stage appears to be comparable tothe sub-picosecond pulse length.Comment: 28 pages, including 9 figs. Submitted to Phys. Rev. B 14 March 200

Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting