Z-Scan Method Using the Flat-Topped Beam and Its Application to the Evaluation of the Two-Photon-Absorbing Material

Abstract

It was studied the method to evaluate the two-photon-absorbing material from the measurement of the z-scan curve using flat-topped (FT) beam. Theoretically, I obtained the analytical expression for the focused beam profile of FT beam by using the Rayleigh diffraction formula, and calculated the transmittance dip as a function of two-photon absorption (TPA) coefficient. It was found that the TPA coefficient could be obtained from the magnitude of the transmittance dip in the z-scan curve with the FT beam. Experimentally, the z-scan curves were measured for three different octupolar molecules (O1, O2 and O3). Each molecule has same molecular skeleton but the lengths of the dipolar subunits or the donor strengths are different each other. The z-scan measurements were performed using FT beam in tetrahydrofuran solutions at 800 nm in the femtosecond regime. Their TPA cross-sections were obtained from the magnitudes of the transmittance dip in the measured z-scan curves: 25, 38 and 117 cm4/(photon/s) for O1, O2 and O3, respectively. These results are ascribed to the fact that TPA cross-sections of the octupolar molecules are raised by increasing the number of the alternating double bonds in the dipolar subunits or by enlarging the electron-donating strength of the substituent.