 The study investigates the efficient energy coupling of ultra-intense biker-second laser pulses with solid surfaces as a function of the plasma density scale length at the irradiated surface. It shows that there is an optimum density gradient for efficient energy absorption and that this arises due to strong self-focusing and channeling. At longer density gradients, laser filaments result in significantly lower overall energy coupling. The results demonstrate that it is possible to enhance laser energy absorption and coupling to fast electrons by dynamically controlling the plasma density gradient. This article was authored by I. J. Gray, D. C. Carroll, X. H. Yuan, and others.