Ultrafast measurement of two-photon absorption by loss modulation

We demonstrate a direct and sensitive technique for measuring two-photon absorption (TPA). An intensity-modulated femtosecond laser beam passes through a sample exhibiting TPA. A TPA signal at twice the modulation frequency is then generated and subsequently measured by a lock-in amplifier. The absolute TPA cross section of Rhodamine 6G at 800 mn is found to be (15.3 +/- 2.0) X 10(-50) cm(4) s/photon and agrees well with previously published results obtained with much higher intensity [J. Chem. Phys. 112, 9201 (2000)]. Our method may be especially useful in measuring nonlinear absorptions of nonfluorescent materials. (C) 2002 Optical Society of America.