A Sensitive Technique for Two-Photon Absorption Measurements: Towards Higher Resolution Microscopy.

High repetition rate (HRR) lasers are essential in multiphoton microscopy for satisfactory signal to noise at low average powers. However, HRR lasers generate thermal distortions in samples even with the slightest single photon absorption. Using an optical chopper with HRR lasers ("blanking") we demonstrate a femtosecond z-scan setup that effectively eliminates thermal as well as small linear absorption effects and precisely measures two-photon absorption (TPA) cross-sections of chromophores. Accurate measurement of TPA cross-sections in biologically relevant chromophores is especially important since the TPA spectrum is considerably different in regions with even minute linear absorption. Such blanking measurements with chopper also show enhanced fluorescence efficiency of the chromophores.