Optical perturbation of the "hole-pockets" in the underdoped high T c superconducting cuprates

The high-${T}_{c}$ superconducting cuprates are recognized as doped Mott insulators. Several studies indicate that as a function of doping and temperature, there is a crossover from this regime into a phase characterized as a marginal Fermi liquid. Several calculations of the doped Mott insulating phase indicate that the Fermi surface defines small pockets which at the higher doping levels switch to a full closed Fermi surface, characteristic of a more metallic system. Here we use femtosecond laser-based pump-probe techniques to investigate the structure of the Fermi surface in the underdoped region of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{r}}_{2}\mathrm{CaC}{\mathrm{u}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and compare it with that associated with the optimally doped material. We confirm the concept of a small pocket in the underdoped system consistent with theoretical predictions in this strongly correlated state.