| Basic spectroscopy, fluorescence, kinetics and nonlinear optical properties of several classes of optical limiting molecules, such as donor-acceptor azulenic, carbocyanine, stilben and rhodamine dyes, in solution and in polymer have been studied. In order to study these molecules a new transient spectroscopy experimental system, which generates ∼1 picosecond pulses and allows us to measure absorption differences as small as ΔA = 0.005, was designed and built. Using this experimental system, we have measured the spectra, kinetics and cross sections of the transients, which appeared after light absorption by the molecules studied. Some of the molecules exhibit strong reverse saturable absorption in a wide segment of the visible spectrum, with subpicosecond response time and fast decay rate. Others showed strong two-photon absorption. Both reverse saturable absorption and two-photon absorption may be utilized for optical limiting of strong laser irradiation. We have also developed a new two-dimensional Z-scan technique, which can be used to measure nonlinear refraction and absorption of materials using arbitrarily beam shaped laser pulse and any sample thickness. In addition, the beam profile evolution anywhere inside the optical limiting material can be extracted. We have tested this technique using Gaussian and non-Gaussian beam distributions, thin and thick samples, and confirmed that it is more accurate than the conventional one-dimensional Z-scan method.; We also describe a picosecond time-resolved x-ray diffraction and extended x-ray absorption fine structure (EXAFS) spectroscopy experimental system, suitable for the study of structures evolved in the curse of ultrafast processes in liquids and solids. This system uses a laser pulse to excite the sample and a delayed ultrashort hard x-ray pulse, generated in an x-ray diode driven by the same laser, to probe the structure of intermediate species produced during the photo-excitation. Several time-resolved x-ray diffraction studies on laser induced heat and strain propagation in metal and semiconductor crystals are described. Crystal lattice changes as small as 5 × 10 −4 Å and temperature changes of 20°C were detected with 10 picosecond time resolution. We have also measured the structures of the initial and final products of carbon tetrabromide photodissociation reaction in ethanol solution by means of time-resolved EXAFS spectroscopy. |
