Unravelling ultrashort laser excitation of nickel at 800 nm wavelength

The optical response of nickel is studied in a wide range of laser fluence, below and above the ablation threshold, by selfreflectivity measurements of ultrashort 800nm single laser pulses. At the ablation threshold, the reflectivity remains unchanged with respect to its unperturbed value irrespective of the pulse duration, from 15 to 100 fs, consistently with the steadiness of the laser-induced ablation threshold fluence Fth for all pulse durations tested. Until the ablation threshold ($F\le F_{th}$) and whatever the pulse duration, the disturbances caused to the initial structure of the electron gas distribution by the laser energy deposition are limited, having no significant impact on the transient optical response of nickel and on its ablation threshold. At higher laser fluences ($F> F_{th}$), the reflectivity becomes rapidly dominated by the contribution to the optical response of the fast-thermalized free electrons (4s-band) with energy largely above the Fermi energy level. In these conditions, the reflectivity decreases for all pulse durations enhancing laser energy coupling and larger optical absorption at the surface of nickel. The optical response of nickel under ultrashort (15-100 fs) irradiation is thus fully elucidated on a wide range of fluence (0.3 $F_{th}$ - 30 $F_{th}$) and for pulse duration down to few-optical-cycle pulse duration. As a key parameter for benchmarking laser-matter interaction in poorly known conditions yet, the evolution of the effective electron collision rate is determined as a function of fluence and pulse duration in very good consistency with experiments.