Dynamical Mass Renormalization and Fermi Momentum in the Normal State of the Cuprate $Bi_{2} Sr_{2} CaCu_{2} O_{8+x} $ as Instigated and Observed by Two-Photon ARPES

The dressing of quasiparticles in solids is investigated by observing changes of the electronic structure $E$(\textbf{\textit{k}}) driven by femtosecond laser pulses. Employing time- and angle-resolved photoemission on the optimally doped cuprate $Bi_{2} Sr_{2} CaCu_{2} O_{8+x} $, just above $T_{c} $, we observe two effects with different characteristic temporal evolutions and, therefore, different microscopic origins. The experiment was carried out using amplified ultrafast laser pulses and a novel time of flight laser-ARPES setup. Both of the effects observed thusly are driven by the relatively high fluences of our amplified near-infrared pump laser and indicate that non-trivial, dynamical changes of the normal state cuprate band structure may be induced by ultrafast laser pulses over time scales at least as short as 150 fs. First, a 10{\%} change of the effective mass due to the 70~meV kink in $E$(\textbf{\textit{k}}) is found to occur during the experiment's 100~fs temporal resolution. Second, a time- and fluence-dependent change in $k_{F} $ is observed. The causes and ramifications of these disparate processes will be discussed.