Two-photon-absorption properties of multipolar molecules studied using femtosecond z-scan method

Two-photon-absorption (TPA) properties of multipolar molecules were studied using femtosecond z-scan method. We measured the z-scan curves at 800 nm, 1 kHz using 150 fs-laser pulse. The z-scan curves were measured for two kinds of multipolar molecules: three different quadrupole molecules (Q1, Q2, Q3) and three different octupolar molecules (O1, O2, O3). Each multipolar molecule has same molecular skeleton but the lengths of the dipolar subunits or the donor strengths are different each other. Their TPA cross-sections were obtained from the magnitudes of the transmittance dip in the measured z-scan curves: 233 (Q1), 127 (Q2), 164 (Q3) and 381 (O1), 230 (O2), 85 (O3) cm4/(photon/s) for quadrupole and octupolar molecules, respectively. These results are ascribed to the fact that TPA cross-sections of the multipolar 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.