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 800nm is found to be (15.3±2.0)×10<sup>-50</sup> cm<sup>4</sup> 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.
© 2002 Optical Society of America
Peifang Tian and Warren S. Warren, "Ultrafast measurement of two-photon absorption by loss modulation," Opt. Lett. 27, 1634-1636 (2002)