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The optical response of nickel is studied in a wide range of laser fluence,below and above the ablation threshold, by selfreflectivity measurements ofultrashort 800nm single laser pulses. At the ablation threshold, thereflectivity remains unchanged with respect to its unperturbed valueirrespective of the pulse duration, from 15 to 100 fs, consistently with thesteadiness of the laser-induced ablation threshold fluence Fth for all pulsedurations tested. Until the ablation threshold ($F\le F_{th}$) and whatever thepulse duration, the disturbances caused to the initial structure of theelectron gas distribution by the laser energy deposition are limited, having nosignificant impact on the transient optical response of nickel and on itsablation threshold. At higher laser fluences ($F> F_{th}$), the reflectivitybecomes rapidly dominated by the contribution to the optical response of thefast-thermalized free electrons (4s-band) with energy largely above the Fermienergy level. In these conditions, the reflectivity decreases for all pulsedurations enhancing laser energy coupling and larger optical absorption at thesurface 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 knownconditions yet, the evolution of the effective electron collision rate isdetermined as a function of fluence and pulse duration in very good consistencywith experiments.

Unravelling ultrashort laser excitation of nickel at 800 nm wavelength