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Opposite signs of nonlinear refraction in chloroaluminum (ClAl) phthalocyanine (Pc) dissolved in methanol are observed using the Z-scan technique with two output modes of a frequency-doubled, Q-switched, mode-locked Nd:YAG laser operating at 10 Hz. Using the single picosecond (ps) pulses switched out of the pulse trains, positive nonlinear refraction is observed and attributed to population transitions among ClAlPc’s singlet states. Using the full nanosecond pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed, and is attributed to the heat-induced temperature rise and the population relaxing to ClAlPc’s triplet states.

We report a method of determining the lifetimes of the high‐lying excited singlet states of dye molecules in solution. The method is based on the Z‐scan technique and the observation of saturation of excited state absorption in dye solutions using laser pulses having Gaussian temporal and spatial distributions. This leads to the important finding that the use of laser pulses as long as tens of picosecond can resolve subpicosecond lifetimes. The method is demonstrated by measuring an upper excited singlet state lifetime of 900 fs using 30 ps full width at half maximum input pulses on a chloroaluminum phthalocyanine dye.

We studied the carrier dynamics in a series of SnOx thin films using femtosecond transient absorption (TA) spectroscopy. The observed carrier relaxation was found to be strongly dependent on thin film stoichiometry. The TA spectra corresponding to free carriers, trapped carriers, and state filling were observed in the picosecond time region for SnO2, SnOx, and SnO film, respectively. The TA decay kinetics of all films were best fit with a tri-exponential decay model with fast (1 ps), medium (∼10 ps), and slow (ns) components. Our results revealed the carrier relaxation and recombination processes in SnOx thin films, identifying the critical role of stoichiometry in photo-induced phenomena.

Using the pump-probe with phase object technique with 20 ps laser pulses at 532 nm, we investigated the carrier relaxation process subsequent to two-photon absorption (TPA) in ZnO. As a result, we found that an additional subnanosecond decay pathway is activated when the pump beam intensity surpasses 0.4 GW/cm2. We attributed this intensity-dependent pathway to a TPA induced bulk defect state and our results demonstrate that this photo induced defect state has potential applications in ZnO based optoelectronic and spintronic devices.