  The in-plane optical conductivity of Bi2Sr2CaCu2O8+d thin films with small
carrier density (underdoped) up to large carrier density (overdoped) is
analyzed with unprecedented accuracy. Integrating the conductivity up to
increasingly higher energies points to the energy scale involved when the
superfluid condensate builds up. In the underdoped sample, states extending up
to 2 eV contribute to the superfluid. This anomalously large energy scale may
be assigned to a change of in-plane kinetic energy at the superconducting
transition, and is compatible with an electronic pairing mechanism.
