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

The high T_c superconducting cuprates are considered doped Mott insulators. As a function of doping and temperature there is a crossover from this regime into a phase characterized a Marginal Fermi Liquid. Several calculations of the doped Mott insulating phase indicate that the Fermi surface defines small pockets which at 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 underdoped Bi₂Sr₂CaCu₂O_8+δ 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.