  We report on direct detection of excited states absorption cross-section
using dual wavelength pump-probe technique. Also, we experimentally demonstrate
using porphyrin composite molecules (porphyrin derivatives such as
5,10,15,20-meso-tetrakis phenyl porphyrin (H2TPP), 5,10,15,20 -
meso-tetrakis(4-hydroxyphenyl) porphyrin (H2TPP(OH)4)). The cw laser at 761 nm
wavelength is used as a pump to maintain excited state population. Changes in
the population of excited states lead to the change in transmission are
monitored using femto-second probe pulses of 130 fs width and repeated at a
1kHz rate with central wavelength around 800 nm. Transmittance changes due to
excited state population are modeled using rate equation approach. The effect
of the absorption on the transmitted pulse shape has been discussed as a
function of fluence. Obtained excited state absorption cross-sections of H2TPP
and H2TPP(OH)4 doped boric acid glass (BAG) films are 4.9X10-18 cm2 and
1.2X10-17 cm2 respectively.
